Evolution of photosynthesis

The evolution of photosynthesis refers to the origin and subsequent evolution of photosynthesis, the process by which light energy is used to assemble sugars from carbon dioxide and a hydrogen and electron source such as water. It is believed that the pigments used for photosynthesis initially were used for protection from the harmful effects of light, particularly ultraviolet light. The process of photosynthesis was discovered by Jan Ingenhousz, a Dutch-born British physician and scientist, first publishing about it in 1779.^[1]

The first photosynthetic organisms probably

Available evidence from geobiological studies of Archean (>2500 Ma) sedimentary rocks indicates that life existed 3500 Ma. Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old,^[4][5] consistent with recent studies of photosynthesis.^[6][7] Early photosynthetic systems, such as those from green and purple sulfur and green and purple nonsulfur bacteria, are thought to have been anoxygenic, using various molecules as electron donors. Green and purple sulfur bacteria are thought to have used hydrogen and hydrogen sulfide as electron and hydrogen donors. Green nonsulfur bacteria used various amino and other organic acids. Purple nonsulfur bacteria used a variety of nonspecific organic and inorganic molecules.^[8] It is suggested that photosynthesis likely originated at low-wavelength geothermal light from acidic hydrothermal vents, Zn-tetrapyrroles were the first photochemically active pigments, the photosynthetic organisms were anaerobic and relied on H₂S without relying on H₂ emitted by alkaline hydrothermal vents. The divergence of anoxygenic photosynthetic organisms at the photic zone could have led to the ability to strip electrons from H₂S more efficiently under ultraviolet radiation. There is geochemical evidence that suggests that anaerobic photosynthesis emerged 3.3 to 3.5 billion years ago. The organisms later developed a Chlorophyll F synthase. They could have also stripped electrons from soluble metal ions although it is unknown.^[9]

The first oxygenic photosynthetic organisms are proposed to be H₂S-dependent.

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Symbiosis and the origin of chloroplasts

Several groups of animals have formed symbiotic relationships with photosynthetic algae. These are most common in corals, sponges and sea anemones. It is presumed that this is due to the particularly simple body plans and large surface areas of these animals compared to their volumes.^[17] In addition, a few marine mollusks Elysia viridis and Elysia chlorotica also maintain a symbiotic relationship with chloroplasts they capture from the algae in their diet and then store in their bodies. This allows the mollusks to survive solely by photosynthesis for several months at a time.^[18][19] Some of the genes from the plant cell nucleus have even been transferred to the slugs, so that the chloroplasts can be supplied with proteins that they need to survive.^[20]

An even closer form of symbiosis may explain the origin of chloroplasts. Chloroplasts have many similarities with

Evolution of photosynthetic pathways

In its simplest form, photosynthesis is adding water to CO₂ to produce sugars and oxygen, but a complex chemical pathway is involved, facilitated along the way by a range of

C₄ plants evolved carbon concentrating mechanisms. These work by increasing the concentration of CO₂ around RuBisCO, thereby facilitating photosynthesis and decreasing photorespiration. The process of concentrating CO₂ around RuBisCO requires more energy than allowing gases to diffuse, but under certain conditions – i.e. warm temperatures (>25 °C), low CO₂ concentrations, or high oxygen concentrations – pays off in terms of the decreased loss of sugars through photorespiration.^{[citation needed]}

One type of C₄ metabolism employs a so-called

These two pathways, with the same effect on RuBisCO, evolved a number of times independently – indeed, C₄ alone arose 62 times in 18 different plant families. A number of 'pre-adaptations' seem to have paved the way for C₄, leading to its clustering in certain clades: it has most frequently developed in plants that already had features such as extensive vascular bundle sheath tissue.^[34] Whole-genome and individual gene duplication are also associated with C₄ evolution.^[35] Many potential evolutionary pathways resulting in the C₄ phenotype are possible and have been characterised using Bayesian inference,^[25] confirming that non-photosynthetic adaptations often provide evolutionary stepping stones for the further evolution of C₄.

It is troublesome procuring original fossil material in sufficient quantity to analyse the grass itself, but fortunately there is a good proxy: horses. Horses were globally widespread in the period of interest, and browsed almost exclusively on grasses. There's an old phrase in isotope palæontology, "you are what you eat (plus a little bit)" – this refers to the fact that organisms reflect the isotopic composition of whatever they eat, plus a small adjustment factor. There is a good record of horse teeth throughout the globe, and their δ¹³C has been measured. The record shows a sharp negative inflection around 6 to 7 million years ago, during the Messinian, and this is interpreted as the rise of C₄ plants on a global scale.^[37]

When is C₄ an advantage?

While C₄ enhances the efficiency of RuBisCO, the concentration of carbon is highly energy intensive. This means that C₄ plants only have an advantage over C₃ organisms in certain conditions: namely, high temperatures and low rainfall. C₄ plants also need high levels of sunlight to thrive.^[40] Models suggest that, without wildfires removing shade-casting trees and shrubs, there would be no space for C₄ plants.^[41] But, wildfires have occurred for 400 million years – why did C₄ take so long to arise, and then appear independently so many times? The Carboniferous period (~300 million years ago) had notoriously high oxygen levels – almost enough to allow spontaneous combustion^[42] – and very low CO₂, but there is no C₄ isotopic signature to be found. And there doesn't seem to be a sudden trigger for the Miocene rise.^{[citation needed]}

During the Miocene, the atmosphere and climate were relatively stable. If anything, CO₂ increased gradually from 14 to 9 million years ago before settling down to concentrations similar to the Holocene.^[43] This suggests that it did not have a key role in invoking C₄ evolution.^[36] Grasses themselves (the group which would give rise to the most occurrences of C₄) had probably been around for 60 million years or more, so had had plenty of time to evolve C₄,^[44][45] which, in any case, is present in a diverse range of groups and thus evolved independently. There is a strong signal of climate change in South Asia;^[36] increasing aridity – hence increasing fire frequency and intensity – may have led to an increase in the importance of grasslands.^[46] However, this is difficult to reconcile with the North American record.^[36] It is possible that the signal is entirely biological, forced by the fire- and grazer-^[47] driven acceleration of grass evolution – which, both by increasing weathering and incorporating more carbon into sediments, reduced atmospheric CO₂ levels.^[47] Finally, there is evidence that the onset of C₄ from 9 to 7 million years ago is a biased signal, which only holds true for North America, from where most samples originate; emerging evidence suggests that grasslands evolved to a dominant state at least 15Ma earlier in South America.^{[citation needed]}

See also

• Photorespiration
• Evolution of plants

References