20.4: Evolution of Photosynthesis - Biology

Plant Protection

Plants have evolved a great ability to absorb light over the entire visible range of the spectra. Hence both heat dissipation and inhibition of formation of ROS (by such molecules as vitamin E) are both mechanism of defense of excessive solar energy

Given that both plants and animals must be protected from ROS, antioxidant molecules made by plants may prove to protect humans from diseases such as cancer, cardiovascular disease, and general inflammatory diseases, all of which have been shown to involve oxidative damage to biological molecules. Humans, who can't synthesize the variety and amounts of antioxidants that are found in plants, appear to be healther when they consume large amounts of plant products. These phytomolecule also have other properties, including regulation of gene transcription which can also have a significant effect on disease propensity.


20.4 Photosynthesis mechanisms

Biological energy conversions can be categorised into two groups: (i) photosynthesis (the process whereby solar energy is fixed to yield energy useful to organisms and industry), and (ii) biomass conversion (the product of photosynthesis) into energy. Photosynthesis occurs in plants, algae and photosynthetic bacteria, while biomass conversion reactions often occur in non-photosynthetic micro-organisms. This section focuses on photosynthetic processes.

20.4.1 Plant photosynthesis

Photosynthesis is often regarded as a CO 2 anabolic reaction, whereby glucose is formed from CO2 and water. CO2 anabolism is an energy-consuming reaction in that it utilises chemical energy produced by photosynthesis. In its narrowest sense, photosynthesis can be regarded as a process whereby energy is supplied for CO2 anabolism. In a broader sense, photosynthesis, including CO2 anabolism, can be divided into several steps: (i) photoelectric charge isolation using photon energy (conversion to electrical energy), (ii) fixation of electrical energy in the form of chemical energy (ATP synthesis), and (iii) chemical reactions involving ATP (fixation of CO2, and hydrogen production).

The supply of energy for CO2 anabolism is common to all photosynthetic organisms which exhibit photosynthesis. Energy conversion, ATP synthesis and the production of both CO2 and hydrogen on the other hand, are not unique to photosynthetic organisms, but occur in all types of micro-organisms, and are in fact similar to the respiratory processes which occur in mitochondria of higher organisms.

Two types of photosynthesis are distinguishable on the basis of source of the electrons used as energy carriers. In plants such as green algae, and cyanobacteria (blue-green algae), water is the electron source, while in photosynthetic bacteria, organic or sulphur compounds provide electron sources.

Photosynthetic mechanisms which occur within plant photosynthetic membranes are schematically presented in Fig. 20.1 . Two photosystem II water molecules are initially decomposed by four incident photons, to yield one oxygen molecule and four excited electrons. Excited electron energy is subsequently utilised in ATP synthesis. Unlike in the case of ordinary chemical reactions, ATP synthesis cannot be stoichiometrically analysed.

Figure 20.1 . Schematic representation of mechanisms involved in plant photosynthesis

The ratio of excited photons to ATP produced is still a somewhat debatable issue. Although it has generally been thought that two photons give rise to the formation of two ATP molecules, some researchers claim that three photons are involved. Furthermore, other researchers have suggested a loose coupling between proton transport and ATP synthesis:

Subsequent to their energy release in ATP production, photosystem II electrons are transported to photosystem I, where they are again excited to a higher energy level, allowing them to be utilised for NADP reduction. NADP serves both as an electron carrier and an oxidising and reducing agent in vivo. Two photons are utilised per molecule of NADP reduced:

Photosystem I may also be involved in ATP synthesis. In cases where it is involved, excited photosystem I electrons are recycled:

Fixation of one molecule of CO2, involves the following equation:

If two ATP molecules are obtained through Photosystem II excitation ( Eq. 20.1 ), the net reaction following Eqs. 20.1 to 20.4 is:

Experimental data indicates that between 8 and 12 photons are required for fixation of one molecule of CO2. Since the energy equivalent of one photon (700 nm) is approximately 170 kJ/E, and the change in free energy during the fixation of CO2 is approximately 450 U/mol, the energy efficiency of this process for monochromatic light of a wavelength of 700 nm is estimated to be approximately 21–33 per cent. However, owing to the quantum nature of photosynthetic reactions, energy efficiency decreases if light of shorter wavelengths (i.e. higher quantum energy) is used. Additionally, energy losses, energy requirements for plant growth, and the distribution of solar energy wavelengths need to be considered.

Plant photosynthesis takes place only in the presence of visible light (400–700 nm). However, solar light contains both visible and infrared components. Since visible light accounts for about 45 per cent of all solar energy, the maximum achievable energy efficiency for CO2 fixation using solar radiation is approximately 13 per cent.

20.4.2 Bacterial photosynthesis

Bacterial photosynthesis is thought to be a relatively old form ofphotosynthesis. It incorporates the use of either organic or sulphur compounds as electron donors in photosystem I ( Fig. 20.2 ). Unlike in the case of plant photosynthesis, cyclic photophosphorylation takes place in bacterial photosynthesis, i.e. electrons are repeatedly excited in a cyclic manner, with ATP being generated in each cycle. Photosynthetic bacteria are also capable of reducing electron carriers such as NAD, via a linear reaction similar to the electron transmission which occurs during plant photosynthesis ( Fig. 20.2 ).

Figure 20.2 . Schematic representation of mechanisms involved in bacterial photosynthesis

CO2-fixing reactions do not produce energy during bacterial photosynthesis (i.e. equimolar amounts of organic compounds are produced through decomposition of organic compounds), except when sulphur compounds serve as electron carriers.

Electrons are donated as follows:

The structure of the photosynthetic reaction centre (RC), involved in the early steps of photosynthesis, has been elucidated for certain photosynthetic bacteria ( Fig. 20.3 ). Such chlorophyll-containing bacteria, which include Rhodopseudomonas viridis and Rhodobacter sphaeroides, show similarities with respect to the arrangement of chlorophyll, and the three-dimensional structures of major portions of the proteins possessing that pigment. Such structural similarities between photosynthetic bacteria, seem to suggest the acquisition of an optimal structure by these bacteria, over a long evolutionary period.

Figure 20.3 . Initial steps of photosynthesis in bacterial photosynthetic membranes

Pigments such as bacteriochlorophyll are also present within the RC. Photoelectric charge isolation takes place within dimers of these bacteriochlorophyll pigments, resulting in the release of high-energy electrons, via the action of bacteriochlorophyll monomers such as bacteriopheophytin, quinone A, and quinone B. These high-energy electrons are subsequently conjugated with proton transportation in the cytochrome b/c1 complex. A noteworthy feature of the RC function is that photon involvement in photoelectric charge isolation, resembles that which occurs in photo-semiconductors. These RC centres can thus be regarded as molecular elements produced by nature. The fact that photoelectric charge isolation is observed in protein molecules will greatly influence future research relevant to molecular elements and solar batteries.

Factors That Affect Oxygen Binding

The oxygen-carrying capacity of hemoglobin determines how much oxygen is carried in the blood. In addition to PO2, other environmental factors and diseases can affect oxygen carrying capacity and delivery.

Carbon dioxide levels, blood pH, and body temperature affect oxygen-carrying capacity (Figure 20.20). When carbon dioxide is in the blood, it reacts with water to form bicarbonate (HCO − 3) and hydrogen ions (H + ). As the level of carbon dioxide in the blood increases, more H + is produced and the pH decreases. This increase in carbon dioxide and subsequent decrease in pH reduce the affinity of hemoglobin for oxygen. The oxygen dissociates from the Hb molecule, shifting the oxygen dissociation curve to the right. Therefore, more oxygen is needed to reach the same hemoglobin saturation level as when the pH was higher. A similar shift in the curve also results from an increase in body temperature. Increased temperature, such as from increased activity of skeletal muscle, causes the affinity of hemoglobin for oxygen to be reduced.

Diseases like sickle cell anemia and thalassemia decrease the blood’s ability to deliver oxygen to tissues and its oxygen-carrying capacity. In sickle cell anemia, the shape of the red blood cell is crescent-shaped, elongated, and stiffened, reducing its ability to deliver oxygen (Figure 20.21). In this form, red blood cells cannot pass through the capillaries. This is painful when it occurs. Thalassemia is a rare genetic disease caused by a defect in either the alpha or the beta subunit of Hb. Patients with thalassemia produce a high number of red blood cells, but these cells have lower-than-normal levels of hemoglobin. Therefore, the oxygen-carrying capacity is diminished.

Figure 20.21.
Individuals with sickle cell anemia have crescent-shaped red blood cells. (credit: modification of work by Ed Uthman scale-bar data from Matt Russell)

20.4 Aquatic and Marine Biomes

Like terrestrial biomes, aquatic biomes are influenced by abiotic factors. In the case of aquatic biomes the abiotic factors include light, temperature, flow regime, and dissolved solids. The aquatic medium—water— has different physical and chemical properties than air. Even if the water in a pond or other body of water is perfectly clear (there are no suspended particles), water, on its own, absorbs light. As one descends deep enough into a body of water, eventually there will be a depth at which the sunlight cannot reach. While there are some abiotic and biotic factors in a terrestrial ecosystem that shade light (like fog, dust, or insect swarms), these are not usually permanent features of the environment. The importance of light in aquatic biomes is central to the communities of organisms found in both freshwater and marine ecosystems because it controls productivity through photosynthesis.

In addition to light, solar radiation warms bodies of water and many exhibit distinct layers of water at differing temperatures. The water temperature affects the organisms’ rates of growth and the amount of dissolved oxygen available for respiration.

The movement of water is also important in many aquatic biomes. In rivers, the organisms must obviously be adapted to the constant movement of the water around them, but even in larger bodies of water such as the oceans, regular currents and tides impact availability of nutrients, food resources, and the presence of the water itself.

Finally, all natural water contains dissolved solids, or salts. Fresh water contains low levels of such dissolved substances because the water is rapidly recycled through evaporation and precipitation. The oceans have a relatively constant high salt content. Aquatic habitats at the interface of marine and freshwater ecosystems have complex and variable salt environments that range between freshwater and marine levels. These are known as brackish water environments. Lakes located in closed drainage basins concentrate salt in their waters and can have extremely high salt content that only a few and highly specialized species are able to inhabit.

Marine Biomes

The ocean is a continuous body of salt water that is relatively uniform in chemical composition. It is a weak solution of mineral salts and decayed biological matter. Within the ocean, coral reefs are a second type of marine biome. Estuaries, coastal areas where salt water and fresh water mix, form a third unique marine biome.

The ocean is categorized by several zones (Figure 20.28). All of the ocean’s open water is referred to as the pelagic realm (or zone). The benthic realm (or zone) extends along the ocean bottom from the shoreline to the deepest parts of the ocean floor. From the surface to the bottom or the limit to which photosynthesis occurs is the photic zone (approximately 200 m or 650 ft). At depths greater than 200 m, light cannot penetrate thus, this is referred to as the aphotic zone . The majority of the ocean is aphotic and lacks sufficient light for photosynthesis. The deepest part of the ocean, the Challenger Deep (in the Mariana Trench, located in the western Pacific Ocean), is about 11,000 m (about 6.8 mi) deep. To give some perspective on the depth of this trench, the ocean is, on average, 4267 m or 14,000 ft deep.


The physical diversity of the ocean has a significant influence on the diversity of organisms that live within it. The ocean is categorized into different zones based on how far light reaches into the water. Each zone has a distinct group of species adapted to the biotic and abiotic conditions particular to that zone.

The intertidal zone (Figure 20.28) is the oceanic region that is closest to land. With each tidal cycle, the intertidal zone alternates between being inundated with water and left high and dry. Generally, most people think of this portion of the ocean as a sandy beach. In some cases, the intertidal zone is indeed a sandy beach, but it can also be rocky, muddy, or dense with tangled roots in mangrove forests. The intertidal zone is an extremely variable environment because of tides. Organisms may be exposed to air at low tide and are underwater during high tide. Therefore, living things that thrive in the intertidal zone are often adapted to being dry for long periods of time. The shore of the intertidal zone is also repeatedly struck by waves and the organisms found there are adapted to withstand damage from the pounding action of the waves (Figure 20.27). The exoskeletons of shoreline crustaceans (such as the shore crab, Carcinus maenas) are tough and protect them from desiccation (drying out) and wave damage. Another consequence of the pounding waves is that few algae and plants establish themselves in constantly moving sand or mud.

The neritic zone (Figure 20.28) extends from the margin of the intertidal zone to depths of about 200 m (or 650 ft) at the edge of the continental shelf. When the water is relatively clear, photosynthesis can occur in the neritic zone. The water contains silt and is well-oxygenated, low in pressure, and stable in temperature. These factors all contribute to the neritic zone having the highest productivity and biodiversity of the ocean. Phytoplankton, including photosynthetic bacteria and larger species of algae, are responsible for the bulk of this primary productivity. Zooplankton, protists, small fishes, and shrimp feed on the producers and are the primary food source for most of the world’s fisheries. The majority of these fisheries exist within the neritic zone.

Beyond the neritic zone is the open ocean area known as the oceanic zone (Figure 20.28). Within the oceanic zone there is thermal stratification. Abundant phytoplankton and zooplankton support populations of fish and whales. Nutrients are scarce and this is a relatively less productive part of the marine biome. When photosynthetic organisms and the organisms that feed on them die, their bodies fall to the bottom of the ocean where they remain the open ocean lacks a process for bringing the organic nutrients back up to the surface.

Beneath the pelagic zone is the benthic realm, the deepwater region beyond the continental shelf (Figure 20.28). The bottom of the benthic realm is comprised of sand, silt, and dead organisms. Temperature decreases as water depth increases. This is a nutrient-rich portion of the ocean because of the dead organisms that fall from the upper layers of the ocean. Because of this high level of nutrients, a diversity of fungi, sponges, sea anemones, marine worms, sea stars, fishes, and bacteria exists.

The deepest part of the ocean is the abyssal zone , which is at depths of 4000 m or greater. The abyssal zone (Figure 20.28) is very cold and has very high pressure, high oxygen content, and low nutrient content. There are a variety of invertebrates and fishes found in this zone, but the abyssal zone does not have photosynthetic organisms. Chemosynthetic bacteria use the hydrogen sulfide and other minerals emitted from deep hydrothermal vents. These chemosynthetic bacteria use the hydrogen sulfide as an energy source and serve as the base of the food chain found around the vents.

Visual Connection

In which of the following regions would you expect to find photosynthetic organisms?

  1. The aphotic zone, the neritic zone, the oceanic zone, and the benthic realm.
  2. The photic zone, the intertidal zone, the neritic zone, and the oceanic zone.
  3. The photic zone, the abyssal zone, the neritic zone, and the oceanic zone.
  4. The pelagic realm, the aphotic zone, the neritic zone, and the oceanic zone.

Coral Reefs

Coral reefs are ocean ridges formed by marine invertebrates living in warm shallow waters within the photic zone of the ocean. They are found within 30˚ north and south of the equator. The Great Barrier Reef is a well-known reef system located several miles off the northeastern coast of Australia. Other coral reefs are fringing islands, which are directly adjacent to land, or atolls, which are circular reefs surrounding a former island that is now underwater. The coral-forming colonies of organisms (members of phylum Cnidaria) secrete a calcium carbonate skeleton. These calcium-rich skeletons slowly accumulate, thus forming the underwater reef (Figure 20.29). Corals found in shallower waters (at a depth of approximately 60 m or about 200 ft) have a mutualistic relationship with photosynthetic unicellular protists. The relationship provides corals with the majority of the nutrition and the energy they require. The waters in which these corals live are nutritionally poor and, without this mutualism, it would not be possible for large corals to grow because there are few planktonic organisms for them to feed on. Some corals living in deeper and colder water do not have a mutualistic relationship with protists these corals must obtain their energy exclusively by feeding on plankton using stinging cells on their tentacles.

Concepts in Action

In this National Oceanic and Atmospheric Administration (NOAA) video, marine ecologist Dr. Peter Etnoyer discusses his research on coral organisms.

Coral reefs are one of the most diverse biomes. It is estimated that more than 4000 fish species inhabit coral reefs. These fishes can feed on coral, the cryptofauna (invertebrates found within the calcium carbonate structures of the coral reefs), or the seaweed and algae that are associated with the coral. These species include predators, herbivores, or planktivores. Predators are animal species that hunt and are carnivores or “flesh eaters.” Herbivores eat plant material, and planktivores eat plankton.

Evolution Connection

Global Decline of Coral Reefs

It takes a long time to build a coral reef. The animals that create coral reefs do so over thousands of years, continuing to slowly deposit the calcium carbonate that forms their characteristic ocean homes. Bathed in warm tropical waters, the coral animals and their symbiotic protist partners evolved to survive at the upper limit of ocean water temperature.

Together, climate change and human activity pose dual threats to the long-term survival of the world’s coral reefs. The main cause of killing of coral reefs is warmer-than-usual surface water. As global warming raises ocean temperatures, coral reefs are suffering. The excessive warmth causes the coral organisms to expel their endosymbiotic, food-producing protists, resulting in a phenomenon known as bleaching. The colors of corals are a result of the particular protist endosymbiont, and when the protists leave, the corals lose their color and turn white, hence the term “bleaching.”

Rising levels of atmospheric carbon dioxide further threaten the corals in other ways as carbon dioxide dissolves in ocean waters, it lowers pH, thus increasing ocean acidity. As acidity increases, it interferes with the calcification that normally occurs as coral animals build their calcium carbonate homes.

When a coral reef begins to die, species diversity plummets as animals lose food and shelter. Coral reefs are also economically important tourist destinations, so the decline of coral reefs poses a serious threat to coastal economies.

Human population growth has damaged corals in other ways, too. As human coastal populations increase, the runoff of sediment and agricultural chemicals has increased, causing some of the once-clear tropical waters to become cloudy. At the same time, overfishing of popular fish species has allowed the predator species that eat corals to go unchecked.

Although a rise in global temperatures of 1°C–2°C (a conservative scientific projection) in the coming decades may not seem large, it is very significant to this biome. When change occurs rapidly, species can become extinct before evolution leads to newly adapted species. Many scientists believe that global warming, with its rapid (in terms of evolutionary time) and inexorable increases in temperature, is tipping the balance beyond the point at which many of the world’s coral reefs can recover.

Estuaries: Where the Ocean Meets Fresh Water

Estuaries are biomes that occur where a river, a source of fresh water, meets the ocean. Therefore, both fresh water and salt water are found in the same vicinity mixing results in a diluted (brackish) salt water. Estuaries form protected areas where many of the offspring of crustaceans, mollusks, and fish begin their lives. Salinity is an important factor that influences the organisms and the adaptations of the organisms found in estuaries. The salinity of estuaries varies and is based on the rate of flow of its freshwater sources. Once or twice a day, high tides bring salt water into the estuary. Low tides occurring at the same frequency reverse the current of salt water (Figure 20.30).

The daily mixing of fresh water and salt water is a physiological challenge for the plants and animals that inhabit estuaries. Many estuarine plant species are halophytes, plants that can tolerate salty conditions. Halophytic plants are adapted to deal with salt water spray and salt water on their roots. In some halophytes, filters in the roots remove the salt from the water that the plant absorbs. Animals, such as mussels and clams (phylum Mollusca), have developed behavioral adaptations that expend a lot of energy to function in this rapidly changing environment. When these animals are exposed to low salinity, they stop feeding, close their shells, and switch from aerobic respiration (in which they use gills) to anaerobic respiration (a process that does not require oxygen). When high tide returns to the estuary, the salinity and oxygen content of the water increases, and these animals open their shells, begin feeding, and return to aerobic respiration.

Freshwater Biomes

Freshwater biomes include lakes, ponds, and wetlands (standing water) as well as rivers and streams (flowing water). Humans rely on freshwater biomes to provide aquatic resources for drinking water, crop irrigation, sanitation, recreation, and industry. These various roles and human benefits are referred to as ecosystem services . Lakes and ponds are found in terrestrial landscapes and are therefore connected with abiotic and biotic factors influencing these terrestrial biomes.

Lakes and Ponds

Lakes and ponds can range in area from a few square meters to thousands of square kilometers. Temperature is an important abiotic factor affecting living things found in lakes and ponds. During the summer in temperate regions, thermal stratification of deep lakes occurs when the upper layer of water is warmed by the Sun and does not mix with deeper, cooler water. The process produces a sharp transition between the warm water above and cold water beneath. The two layers do not mix until cooling temperatures and winds break down the stratification and the water in the lake mixes from top to bottom. During the period of stratification, most of the productivity occurs in the warm, well-illuminated, upper layer, while dead organisms slowly rain down into the cold, dark layer below where decomposing bacteria and cold-adapted species such as lake trout exist. Like the ocean, lakes and ponds have a photic layer in which photosynthesis can occur. Phytoplankton (algae and cyanobacteria) are found here and provide the base of the food web of lakes and ponds. Zooplankton, such as rotifers and small crustaceans, consume these phytoplankton. At the bottom of lakes and ponds, bacteria in the aphotic zone break down dead organisms that sink to the bottom.

Nitrogen and particularly phosphorus are important limiting nutrients in lakes and ponds. Therefore, they are determining factors in the amount of phytoplankton growth in lakes and ponds. When there is a large input of nitrogen and phosphorus (e.g., from sewage and runoff from fertilized lawns and farms), the growth of algae skyrockets, resulting in a large accumulation of algae called an algal bloom . Algal blooms (Figure 20.31) can become so extensive that they reduce light penetration in water. As a result, the lake or pond becomes aphotic and photosynthetic plants cannot survive. When the algae die and decompose, severe oxygen depletion of the water occurs. Fishes and other organisms that require oxygen are then more likely to die.

Rivers and Streams

Rivers and the narrower streams that feed into the rivers are continuously moving bodies of water that carry water from the source or headwater to the mouth at a lake or ocean. The largest rivers include the Nile River in Africa, the Amazon River in South America, and the Mississippi River in North America (Figure 20.32).

Abiotic features of rivers and streams vary along the length of the river or stream. Streams begin at a point of origin referred to as source water . The source water is usually cold, low in nutrients, and clear. The channel (the width of the river or stream) is narrower here than at any other place along the length of the river or stream. Headwater streams are of necessity at a higher elevation than the mouth of the river and often originate in regions with steep grades leading to higher flow rates than lower elevation stretches of the river.

Faster-moving water and the short distance from its origin results in minimal silt levels in headwater streams therefore, the water is clear. Photosynthesis here is mostly attributed to algae that are growing on rocks the swift current inhibits the growth of phytoplankton. Photosynthesis may be further reduced by tree cover reaching over the narrow stream. This shading also keeps temperatures lower. An additional input of energy can come from leaves or other organic material that falls into a river or stream from the trees and other plants that border the water. When the leaves decompose, the organic material and nutrients in the leaves are returned to the water. The leaves also support a food chain of invertebrates that eat them and are in turn eaten by predatory invertebrates and fish. Plants and animals have adapted to this fast-moving water. For instance, leeches (phylum Annelida) have elongated bodies and suckers on both ends. These suckers attach to the substrate, keeping the leech anchored in place. In temperate regions, freshwater trout species (phylum Chordata) may be an important predator in these fast-moving and colder river and streams.

As the river or stream flows away from the source, the width of the channel gradually widens, the current slows, and the temperature characteristically increases. The increasing width results from the increased volume of water from more and more tributaries. Gradients are typically lower farther along the river, which accounts for the slowing flow. With increasing volume can come increased silt, and as the flow rate slows, the silt may settle, thus increasing the deposition of sediment. Phytoplankton can also be suspended in slow-moving water. Therefore, the water will not be as clear as it is near the source. The water is also warmer as a result of longer exposure to sunlight and the absence of tree cover over wider expanses between banks. Worms (phylum Annelida) and insects (phylum Arthropoda) can be found burrowing into the mud. Predatory vertebrates (phylum Chordata) include waterfowl, frogs, and fishes. In heavily silt-laden rivers, these predators must find food in the murky waters, and, unlike the trout in the clear waters at the source, these vertebrates cannot use vision as their primary sense to find food. Instead, they are more likely to use taste or chemical cues to find prey.

When a river reaches the ocean or a large lake, the water typically slows dramatically and any silt in the river water will settle. Rivers with high silt content discharging into oceans with minimal currents and wave action will build deltas, low-elevation areas of sand and mud, as the silt settles onto the ocean bottom. Rivers with low silt content or in areas where ocean currents or wave action are high create estuarine areas where the fresh water and salt water mix.


Wetlands are environments in which the soil is either permanently or periodically saturated with water. Wetlands are different from lakes and ponds because wetlands exhibit a near continuous cover of emergent vegetation. Emergent vegetation consists of wetland plants that are rooted in the soil but have portions of leaves, stems, and flowers extending above the water’s surface. There are several types of wetlands including marshes, swamps, bogs, mudflats, and salt marshes (Figure 20.33).

Freshwater marshes and swamps are characterized by slow and steady water flow. Bogs develop in depressions where water flow is low or nonexistent. Bogs usually occur in areas where there is a clay bottom with poor percolation. Percolation is the movement of water through the pores in the soil or rocks. The water found in a bog is stagnant and oxygen depleted because the oxygen that is used during the decomposition of organic matter is not replaced. As the oxygen in the water is depleted, decomposition slows. This leads to organic acids and other acids building up and lowering the pH of the water. At a lower pH, nitrogen becomes unavailable to plants. This creates a challenge for plants because nitrogen is an important limiting resource. Some types of bog plants (such as sundews, pitcher plants, and Venus flytraps) capture insects and extract the nitrogen from their bodies. Bogs have low net primary productivity because the water found in bogs has low levels of nitrogen and oxygen.

New study sheds light on evolution of photosynthesis

New Brunswick, N.J. (June 28, 2021) - A Rutgers-led study sheds new light on the evolution of photosynthesis in plants and algae, which could help to improve crop production.

The paper appears in the journal New Phytologist.

The scientists reviewed research on the photosynthetic amoeba Paulinella, which is a model to explore a fundamental question about eukaryote evolution: why was there a single origin of algae and plants? That is, why did photosynthesis by primary plastid endosymbiosis not originate multiple times in the tree of life?

Photosynthesis is the process by which plants and other organisms use sunlight to synthesize foods from carbon dioxide and water, which generates oxygen as a byproduct.

Endosymbiosis is a relationship between two organisms wherein one cell resides inside the other. This interaction, when stable and beneficial for the "host" cell, can result in massive genetic innovation. Despite its critical evolutionary role, there is limited knowledge about how endosymbiosis is initially established.

Primary plastid endosymbiosis, which evolved about 1.5 billion years ago, is the process in which a eukaryote -- which are organisms such as plants and algae whose cells have a membrane-bound nucleus and tiny organs called organelles -- engulfs a prokaryote, which are organisms such as bacteria that lack a membrane-enclosed nucleus. The plastid is a membrane-bound organelle within the cells of plants and algae.

"It turns out that photosynthesis results in enormous risks because it produces harmful chemicals and heat as byproducts that can damage the host cell," said senior author Debashish Bhattacharya, a Distinguished Professor in the Department of Biochemistry and Microbiology at Rutgers University-New Brunswick. "Therefore, creating a novel organelle is a highly complex process that makes it fleetingly rare in evolution. Paulinella, which is the only known case of an independent plastid primary endosymbiosis other than in algae and plants, offers many clues to this process that helps explain why it is so rare."

The origin of photosynthesis in algae and plants changed our planet by providing a major source of oxygen and supporting many ecosystems, due to their primary production, of fixed carbon (sugars and lipids). Understanding how this critical process happened will help us potentially engineer it in synthetic systems as well as to improve crop production.

"Because Paulinella is an independent origin of photosynthesis, it provides key clues to how this process occurs and what costs it imposes on the host cell," said lead author Timothy G. Stephens, a postdoctoral researcher at Rutgers. "The genome of Paulinella contains many independently evolved genes involved in photosynthesis and dealing with the associated stresses that can potentially be engineered in algae and plants could help to improve their ability to withstand stresses such as high light levels or salt stress."

The findings are explained in two videos:

The study included researchers from the Carnegie Institution.

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Structures of the cytochrome b6f complex obtained from the thermophilic cyanobacterium Mastigocladus laminosus and the green alga Chlamydomonas reinhardtii, whose appearance in evolution is separated by 10 9 years, are almost identical. Two monomers with a molecular weight of 110 000, containing eight subunits and seven natural prosthetic groups, are separated by a large lipid-containing “quinone exchange cavity”. A unique heme, heme x, that is five-coordinated and high-spin, with no strong field ligand, occupies a position close to intramembrane heme bn. This position is filled by the n-side bound quinone, Qn, in the cytochrome bc1 complex of the mitochondrial respiratory chain. The structure and position of heme x suggest that it could function in ferredoxin-dependent cyclic electron transport as well as being an intermediate in a quinone cycle mechanism for electron and proton transfer. The significant differences between the cyanobacterial and algal structures are as follows. (i) On the n-side, a plastoquinone molecule is present in the quinone exchange cavity in the cyanobacterial complex, and a sulfolipid is bound in the algal complex at a position corresponding to a synthetic DOPC lipid molecule in the cyanobacterial complex. (ii) On the p-side, in both complexes a quinone analogue inhibitor, TDS, passes through a portal that separates the large cavity from a niche containing the Fe2S2 cluster. However, in the cyanobacterial complex, TDS is in an orientation that is the opposite of its position in the algal structure and bc1 complexes, so its headgroup in the M. laminosus structure is 20 Å from the Fe2S2 cluster.

The studies reported in this review were supported by NIH Grant GMS-38323 (W.A.C.) and a fellowship from The Japanese Ministry of Education (G.K.).

To whom correspondence should be addressed. Telephone: (765) 494-4956, Fax: (765) 496-1189. E-mail: [email protected]

Current address: Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Komaba 3-8-1, Meguro, Tokyo 153-8902, Japan.


We provide trait values regarding stand structure, growth, production, and decay of 15 Sphagnum species (aim 1). To some extent, our results support the previously found trade-off between growth and decay (Figs. 7, 10 Turetsky et al. 2008 Laing et al. 2014 ), where hollow species grow and decay faster than hummock species. However, previous research has compared few species, mostly from open bogs. As we broaden this by including a wider range of species and habitats, a more complex picture emerges with intriguing effects of vegetation type, microhabitat, and phylogeny, and we discuss further based on aims 2–4.

Stand structure, growth, and photosynthetic capacity

Photosynthetic capacity (Fig. 6) reflects the species potential photosynthesis, whereas biomass accumulation and LI also reflect the environment over the season. Broadly speaking these two growth measures gave similar responses among species, but deviations from that pattern are of interest.

Many hollow species grew faster than hummock species (aim 3), as has been reported in earlier studies that examined only a few species (reviewed in Gunnarsson 2005 Rydin et al. 2006 ). But in the wet year, in which the species could be expected to grow near their potential, two species with wide niches, S. angustifolium (sect. Cuspidata) and S. magellanicum (sect. Sphagnum), grew more than other hummock species. As a result, all species from sections Cuspidata and Sphagnum grew more than those from sections Acutifolia and Subsecunda. This indicates that phylogeny is more important than microhabitat in explaining potential productivity.

Also for LI, hummock species are in general slow-growing. S. girgensohnii is an example of a hummock species with fast LI, which could be explained by the shaded habitat that promotes elongation but hampers photosynthesis and biomass accumulation. Among the hollow species, S. tenellum (sect. Cuspidata) made little length growth. In the PCA, LI is only weakly related to production (Ga) and may therefore not be a good response measure in environmental research if different species or habitats are compared.

The innate growth rate and the photosynthetic capacity also showed a higher average in sect. Cuspidata, but the variation within the section is large. Species with larger capitula had higher capacity and biomass accumulation per shoot. This effect of size is less important when calculated per unit area (Fig. 7, right panel) due to allometric constraints where shoot mass scales as the inverse of numerical density (Harley et al. 1989 Laing et al. 2014 ). The shade-inhabiting S. girgensohnii stands out in section Acutifolia as a fast photosynthesizer. Also from the spruce forest, S. magellanicum had a markedly higher rate (especially per unit dry weight), compared to other S. magellanicum habitats and other species in section Sphagnum. Laing et al. ( 2014 ) proposed the hypothesis that photosynthetic capacity and length growth are higher in shade growing species, but biomass produced is not necessarily higher than in open habitats. Overall, our data give some support for this hypothesis, especially within S. magellanicum. It would be interesting to investigate whether this is an acclimation or adaptation to be efficient under a tree canopy. Unlike the other species in section Acutifolia, Sphagnum girgensohnii lacks red pigments suggested to have a function in photoprotection (Bonnett et al. 2010 ), and it is noteworthy that the forest samples of S. magellanicum also lacked the red pigmentation so typical of this species. Bryophytes are generally shade tolerant, but sphagna growing in open habitats seem generally less so (Marschall and Proctor 2004 ). As red and brown species lose a lot of these pigments in shade, a possible mechanism is reallocation of nitrogen from photoprotective pigments to chlorophyll and rubisco. However, in these shaded habitats, the high photosynthetic capacity is combined with high length growth, but does not lead to very high biomass growth.


We made laboratory experiments to assess the intrinsic decay resistance of the litter and field experiments to also include the habitat effects. Here, one should bear in mind that the habitat is created by the species and is therefore also a functional trait that can be seen as a reflection of the extended phenotype (Dawkins 1982 ). Decay resistance can be enhanced by leaching of organochemical substances which slow down decomposition (Verhoeven and Toth 1995 ) and by differential capillarity and water-holding capacity of the Sphagnum species that waterlogs the habitat. Waterlogging, in turn, affects aeration and redox potential through many hydrological feedbacks (Waddington et al. 2015 ).

Decomposition in the laboratory was relatively fast during the first 7 months compared to the following 7 months, and the time chosen to measure decay may therefore affect the results. However, the relative decomposition of species was consistent, as in Johnson and Damman's ( 1991 ) field decay study, and measurements after 7 and 14 months were highly correlated. Therefore, we feel confident that our results capture the relative decomposition relationships among species.

Several studies found that hollow species decompose faster than hummock species (Clymo 1965 Johnson and Damman 1991 Belyea 1996 Limpens and Berendse 2003 ). As predicted, we found that species from section Cuspidata, that is, mainly hollow species, generally decay faster than section Acutifolia species where we find most hummock species. This is especially true in laboratory conditions. Reciprocal litter bag experiments have suggested that the decomposition is less dependent on the mire habitat where they are degrading than on the species (Turetsky et al. 2008 ). Our results indicate that this is an oversimplification. With our intention to test whether traits are related mostly to habitat or phylogeny (aim 2), we conclude that there is a higher intrinsic resistance to decay in most Acutifolia species. In contrast, there are greater habitat constraints by wetness in Cuspidata species, as well as in S. rubellum (the wettest growing Acutifolia species) and S. magellanicum (in its wet bog habitat).

Generally, Sphagnum litter decomposes more slowly than vascular plant litter (review in Scheffer et al. 2001 ), but our results show that variation among Sphagnum species should be taken into account in ecological research. Cornwell et al. ( 2008 ) performed a metastudy on vascular plants and found large differences in decay between functional groups (18-fold), but we found large differences within the genus Sphagnum (6.5-fold for laboratory comparison and 15-fold for field, comparing species averages).

Trade-offs, phylogeny, and environments

Both the regression models testing the relationship between growth (G and NP) and decomposition and the PCA illustrate the complexity of trade-offs between traits related to growth and decay (aim 4).

The PCA illustrates relationships and trade-offs among traits. Nitrogen concentration has been pointed out as a main factor influencing decay rate in Sphagnum (Lang et al. 2009 ) and is here seen to be a main contributor in the PCA. Nitrogen concentration could be seen as a result of fast growth and is strongly related to length growth, but length growth does not always reflect the biomass gained. Nitrogen concentration and length growth may also be positively affected by nitrogen concentration in the surroundings, and it is likely that the mire margin sites have more available nitrogen than the bog (Bragazza et al. 2005 ). However, the PCA indicates that high nitrogen content in bog species also leads to higher rates of decay and production.

The decay resistance in Sphagnum is often attributed to higher concentration of polysaccharides (Hájek et al. 2011 ) or phenolics (Verhoeven and Liefveld 1997 Freeman et al. 2001 ). It has been suggested that there is a resource allocation trade-off in Sphagnum between structural and metabolic carbohydrates (Turetsky et al. 2008 ). This implies that faster-growing species invest in easily degradable carbohydrates, which leads to the expected faster decomposition in hollow species. Support for this hypothesis was reported by Laing et al. ( 2014 ). Our results support the direction of this trade-off, but the relationships are rather weak. Our next step must be to measure the organochemical compounds in the very same samples as used in this paper, to further test the mechanisms behind the trade-off.

The distribution of the species in the trait space shows how phylogeny (sections) and habitat (vegetation type) to various degree cluster in the trait space. HWT and shade have been suggested to be the two most important variables in structuring the functional traits in Sphagnum (Hájek et al. 2009 Laing et al. 2014 ). In support of previous studies, light availability was associated with PC1 and the traits driving this separation in the trait space were LI and nitrogen concentration (Figs. 10, 11). The heavily shaded patches differ markedly from the others, and this is reflected in the PCA. The shaded habitat may induce greener shoots with a higher capacity for photosynthesis, and competition for light may trigger length growth and elongation. While most of the total variation along PC1 can be explained by vegetation type and species (58%), there is some evidence that phylogeny plays a role (10%). The large proportion of variance explained by within species (between vegetation types) and within sample (measurement error and species variation nested in vegetation type) highlights the importance of extended sampling in trait sampling campaigns and the dangers of a species mean-centered approach (Albert et al. 2010 ). The contrasting behavior between the most shaded S. girgensohnii and S. magellanicum and their open habitat relatives illustrates this.

Wetness, or HWT, was a less strong explanatory variable for the multivariate trait space than we expected (cf. Laing et al. ( 2014 )) and was only weakly correlated with PC2. The variables for photosynthetic capacity (NPg and NPa) are mainly driving the 2nd axis in the PCA, suggesting that HWT controls the photosynthetic capacity to some extent. The phylogenetic effect may, however, be a much more important component along PC2, but covariation with vegetation type makes it hard to draw firm conclusions. The indications of a phylogenetic effect on trait space are in line with Johnson et al. ( 2015 ) who reported a strong phylogenetic signal in habitat (microtopography and shade) preference in Sphagnum.

Between-year variation

In our study, we were able to compare the traits in two contrasting years, one exceptionally wet and one dry. Due to the stochastic nature of droughts, such events are rarely investigated in natural habitats even though they are ecologically important: Gerdol et al. ( 2008 ) found Sphagnum production to be around 50% lower in a dry year compared to a normal year.

Potentially, the higher rate of decomposition in Cuspidata species could lead to an ever-increasing amount of Acutifolia hummock peat. However, this is not the case. In fact, the structures of hummocks and hollows have been found to be very stable (Belyea and Clymo 2001 ). The microtopographical pattern we observed can be caused by the dry year leading to a larger decrease in growth and increase in decomposition in Cuspidata than in Acutifolia species. This is likely to be the explanation behind the relative microtopographical stability a fluctuating weather and climate will benefit both strategies but under opposite situations (dry vs. wet year). Many species, whether they are typical of hummocks or hollows, grow as well or better in hollows compared to in hummocks, but only hummock species can last on hummocks. Basically, hummock species avoid desiccation (by capillary water transport) rather than tolerating it (references in Rydin et al. 2006 ). Our results show that species from Acutifolia have less biomass accumulation than Cuspidata species in a year with good water availability. But in a dry year, only hummock species (i.e., mostly Acutifolia) can retain enough water for photosynthesis and growth (Fig. 3). For example, Sphagnum fuscum grew about as much in length and biomass in both years (in the fen even more in the dry year). There is evidence that even though hollow species are more susceptible to desiccation, they may be more tolerant (Hájek and Vicherová 2013 ), at least if hardening is induced slowly. In 2013, the drying out was probably too fast and too severe. This indicates that desiccation avoidance is more important than induced desiccation tolerance. However, S. fallax with among the highest growth in both years was not severely affected by the dry spell. This result is difficult to explain it had some shade in the lagg, but other shaded species were severely affected in the dry year.

There should be a similar effect on decomposition in dry versus wet years, that is, decay should be low in hollows in wet years and high in dry years when they are not waterlogged. For hummock species, the water content may not change that much in the upper part of the hummock even though the water table was lowered due to capillary rise (water table depth – moss productivity feedback (Waddington et al. 2015 )). In a dry period, the looser upper layers of the hollows cannot sustain a hydrological connection with the water table and will face desiccation rapidly. This feedback will promote decomposition, thereby making the peat denser in drier years. Here, we cannot separate the effects of the dry and the wet years, but it seems that the effect of the drought was not severe enough for the hollows to reach the same decay rate as in the laboratory. The differences between species between years indicate, as formerly suggested in Rice et al. ( 2008 ), that there is a trade-off in Sphagnum between tolerating environmental stress and fast biomass production. Our results also highlight the importance of considering temporal variation in traits (Violle et al. 2012 ).

Variation within species

The S. magellanicum patches from the pine bog and open bog had similar intrinsic decay resistance. We interpret the lower field decay in the open bog as an effect of habitat – the species grow much closer to the water table in the open bog and consequently will be less aerated. In contrast, the forest samples had lower intrinsic decay resistance. While this too could be a habitat effect, it has recently been suggested that mire margin populations of S. magellanicum may differ genetically (perhaps even taxonomically) from bog populations (Kyrkjeeide et al. 2016 ). Open, ombrotrophic bog is the main habitat of S. fuscum, and the low decay in bog compared to fen is compatible with this. With similar biomass growth, the potential to form hummocks seems smaller in the fen. So, even if traits are relatively constant within species, we should expect them to be modified when species grow outside their main habitat. Reciprocal transplants of the litter would be necessary to disentangle the role of genetic variation versus plasticity.

Appendix A

Voucher information and GenBank numbers (psbA-trnH, trnL-rpl32F, trnG-trnS, ITS + 5.8S – = sequence not obtained) for all accessions sampled for the phylogenetic analysis. Taxa are arranged phylogenetically from outgroups through Blepharis.

Outgroups. Acanthopsis hoffmannseggiana (Nees) C.B. Clarke – KM034032, KM033945, DQ059217, KM034004, South Africa, Balkwill et al., 11763 (J) Crossandra greenstockii S. Moore – KM034033, trnL-rpl32: KM033946, DQ059250, DQ028427, South Africa, McDade & Balkwill 1241 (J) Sclerochiton harveyanus Nees – –, KM033947, DQ059244, KM034005, South Africa, Balkwill 12274 (J).

Blepharis. Blepharis acuminata Oberm. –KM034034, KM033948, DQ059227, KM034006, South Africa, McDade et al., 1272 (J) Blepharis aspera Oberm. –KM034035, KM033949, KM033984, KM034007, Zimbabwe, O. West 7549 (K) Blepharis asteracanthus C.B. Clarke – KM034036, KM033950, DQ059228, KM034008, Faden et al., 96/204 (K) Blepharis attenuata Napper– KM042911, KM033951, KM033985, KM034009, Jordan, Muhaidat 401 (cultivated in greenhouse, Toronto) Blepharis boranensis Vollesen– KM034037, KM033952, KM033986, KM034010, Ethiopia, M.G. Gilbert & Demissew Sebsebe 8555 (K) Blepharis buchneri Lindau– KM034038, KM033953, DQ059229, KM034011, Faden et al., 96/307 (K) Blepharis calcitrapa Benoist– KM034039, KM033954, DQ059230, DQ028422, Malagasy Republic, Daniel et al., 10403 (CAS) Blepharis ciliaris (L.) B.L. Burtt– KM034040, KM033955, KM033987, KM034012, Oman, Radcliffe-Smith 3897 (K) Blepharis dhofarensis A.G. Mill.– KM034041, KM033957, DQ061158, KM034014, Oman, A.G. Miller 2552 (K) Blepharis dhofarensis A.G. Mill.– KM034042, KM033958, DQ059231, DQ028413, new, Yemen, Thulin et al., 9715 (K) Blepharis dhofarensis A.G. Mill.– –, KM033956, KM033988, KM034013, Thulin 11435 (UPS) Blepharis diversispina C.B. Clarke– KM034043, KM033960, DQ059232, KM034015, McDade et al., 1725 (J) Blepharis edulis (Forssk.) Pers.– KM034044, KM033961, DQ059233, DQ028418, Friis 6735 (K) Blepharis espinosa Phillips– KM034045, KM033962, KM033989, KM034016, South Africa, Free State Province, Scheepers 1549 (S) Blepharis furcata (L. f.) Pers.– KM034046, KM033963, KM033990, KM034017, South Africa, Cape, M.J.A. Werger 153 (K) Blepharis gigantea Oberm.– KM034047, KM033964, KM033991, KM034018, Namibia, Kers 2646 (S) Blepharis glinus Fiori– KM034048, KM033965, KM033992, KM034019, Somalia, J.B. Gillett, C.F. Hemming, R.M. Watson 22122 (K) Blepharis grossa (Nees) T. Anderson– KM034049, KM033966, KM033993, KM034020, Namibia, H.M. Tölken, Hardy 796 (K) Blepharis inopinata Vollesen– KM034050, KM033967, KM033994, –, Zambia, Pope 2155 (K) Blepharis integrifolia (L. f.) E. Mey. ex Schinz– KM034051, KM033968, KM033995, KM034021, Balkwill et al., 11656 (J) or Balkwill et al., 11814 (J) Blepharis katangensis De Wild.– KM034052, KM033969, DQ059236, DQ028421, Bidgood et al., 3521 (K) Blepharis kuriensis Vierh.– KM034053, KM033970, KM033996, KM034022, Yemen, A.R. Smith and J. Lavranos 702 (K) Blepharis linariifolia Pers.– KM034054, KM033971, KM033997, KM034023, Tanzania, Hedrén et al., 739 (UPS) Blepharis macra (Nees) Vollesen– KM034055, KM033972, KM033998, KM034024, South Africa, Cape, Port Nolloth District, van Breda 1347 (K) Blepharis maderaspatensis (L.) B. Heyne ex Roth– KM034056, KM033973, DQ059237, DQ028423, McDade et al., 1292 (PH) Blepharis mitrata C.B. Clarke– KM034057, KM033974, KM033999, KM034025, Namibia, Seydel 3981 (M) Blepharis natalensis Oberm.– KM034058, KM033975, DQ059239, DQ028420, South Africa, Balkwill et al., 11667 (J) Blepharis noli-me-tangere S. Moore– KM034059, KM033976, KM034000, KM034026, Angola, C. Henriques 454 (K) Blepharis obmitrata C.B. Clarke– KM034060, KM033977, KM034001, KM034027, Congo-Brazzaville, E. Kami 4132 (K) Blepharis pruinosa Engl.– KM034061, KM033978, KM034002, KM034028, Namibia, Kers 622 (S) Blepharis scindica Stocks ex T. Anderson– KM034062, KM033979, KM034003, KM034029, Pakistan Viraura-Nagar Parkar Rd., Garser & Ghafoor 4153 (B) Blepharis sinuata (Nees) C.B. Clarke– KM034063, KM033980, DQ059240, DQ028419, South Africa, McDade & Dold 1193 (PH) Blepharis subvolubilis C.B. Clarke– KM034064, KM033981, DQ059241, DQ028417, Balkwill et al., 10850 (J) Blepharis tenuiramea S. Moore– KM034065, KM033982, DQ059242, KM034030, Tanzania, Bidgood et al., 3869 (K) Blepharis trispina Napper– KM034066, KM033983, DQ059243, KM034031, Tanzania, Bidgood et al., 1102 (K).

Define &lsquot&rsquo Test (With Example) | Biostatistics

t-Test is a method to find out whether the difference is quite significant to conclude that the two samples are really different, or the difference in two samples is simply because of some fluctuation, or due to some error or lapses in the experiment. In some cases, it is observed that measurements of two different samples vary.

It can be find out by the following formula:

t = Mean difference/Standard error of differences

In this the “mean difference” and the “standard error of differences” can be determined by the following formulae:

Mean difference = Sum of difference/Number of trials (n)

Standard error of differences = √Sum of square of differences/(n – 1) n – √(Sum of differences) 2 /n

Two varieties of maize (variety X and Y) were collected from four different localities. They showed differences in yield in Kg/hectare as mentioned in Table 2.2. With the help of t-test, find out whether yield of variety Y is significantly superior to the yield of variety X.

Table 2.2 Data showing yield of maize varieties, their yield differences and square of differences:

The details of Table 2.2 may be calculated as under:

Mean differences = Sum of differences/Number of trials (n) = 80/4 = 20

In case of some measurements, the average is mentioned as ± standard error.

Photosynthesis could be as old as life itself

Researchers find that the earliest bacteria had the tools to perform a crucial step in photosynthesis, changing how we think life evolved on Earth.

The finding also challenges expectations for how life might have evolved on other planets. The evolution of photosynthesis that produces oxygen is thought to be the key factor in the eventual emergence of complex life. This was thought to take several billion years to evolve, but if in fact the earliest life could do it, then other planets may have evolved complex life much earlier than previously thought.

The research team, led by scientists from Imperial College London, traced the evolution of key proteins needed for photosynthesis back to possibly the origin of bacterial life on Earth. Their results are published and freely accessible in BBA -- Bioenergetics.

Lead researcher Dr Tanai Cardona, from the Department of Life Sciences at Imperial, said: "We had previously shown that the biological system for performing oxygen-production, known as Photosystem II, was extremely old, but until now we hadn't been able to place it on the timeline of life's history. Now, we know that Photosystem II show patterns of evolution that are usually only attributed to the oldest known enzymes, which were crucial for life itself to evolve."

Photosynthesis, which converts sunlight into energy, can come in two forms: one that produces oxygen, and one that doesn't. The oxygen-producing form is usually assumed to have evolved later, particularly with the emergence of cyanobacteria, or blue-green algae, around 2.5 billion years ago.

While some research has suggested pockets of oxygen-producing (oxygenic) photosynthesis may have been around before this, it was still considered to be an innovation that took at least a couple of billion years to evolve on Earth.

The new research finds that enzymes capable of performing the key process in oxygenic photosynthesis -- splitting water into hydrogen and oxygen -- could actually have been present in some of the earliest bacteria. The earliest evidence for life on Earth is over 3.4 billion years old and some studies have suggested that the earliest life could well be older than 4.0 billion years old.

Like the evolution of the eye, the first version of oxygenic photosynthesis may have been very simple and inefficient as the earliest eyes sensed only light, the earliest photosynthesis may have been very inefficient and slow.

On Earth, it took more than a billion years for bacteria to perfect the process leading to the evolution of cyanobacteria, and two billion years more for animals and plants to conquer the land. However, that oxygen production was present at all so early on means in other environments, such as on other planets, the transition to complex life could have taken much less time.

The team made their discovery by tracing the 'molecular clock' of key photosynthesis proteins responsible for splitting water. This method estimates the rate of evolution of proteins by looking at the time between known evolutionary moments, such as the emergence of different groups of cyanobacteria or land plants, which carry a version of these proteins today. The calculated rate of evolution is then extended back in time, to see when the proteins first evolved.

They compared the evolution rate of these photosynthesis proteins to that of other key proteins in the evolution of life, including those that form energy storage molecules in the body and those that translate DNA sequences into RNA, which is thought to have originated before the ancestor of all cellular life on Earth. They also compared the rate to events known to have occurred more recently, when life was already varied and cyanobacteria had appeared.

The photosynthesis proteins showed nearly identical patterns of evolution to the oldest enzymes, stretching far back in time, suggesting they evolved in a similar way.

First author of the study Thomas Oliver, from the Department of Life Sciences at Imperial, said: "We have used a technique called Ancestral Sequence Reconstruction to predict the protein sequences of ancestral photosynthetic proteins. These sequences give us information on how the ancestral Photosystem II would have worked and we were able to show that many of the key components required for oxygen evolution in Photosystem II can be traced to the earliest stages in the evolution of the enzyme."

Knowing how these key photosynthesis proteins evolve is not only relevant for the search for life on other planets, but could also help researchers find strategies to use photosynthesis in new ways through synthetic biology.

Dr Cardona, who is leading such a project as part of his UKRI Future Leaders Fellowship, said: "Now we have a good sense of how photosynthesis proteins evolve, adapting to a changing world, we can use 'directed evolution' to learn how to change them to produce new kinds of chemistry. We could develop photosystems that could carry out complex new green and sustainable chemical reactions entirely powered by light."

Watch the video: Evolution of Photosynthesis (January 2022).