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.

The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen rather than water. There are three major metabolic pathways by which photosynthesis is carried out: C₃ photosynthesis, C₄ photosynthesis, and CAM photosynthesis. C₃ photosynthesis is the oldest and most common form. A C₃ plant uses the Calvin cycle for the initial steps that incorporate CO₂ into organic material. A C₄ plant prefaces the Calvin cycle with reactions that incorporate CO₂ into four-carbon compounds. A CAM plant uses crassulacean acid metabolism, an adaptation for photosynthesis in arid conditions. C₄ and CAM plants have special adaptations that save water.

Available evidence from geobiological studies of sedimentary rocks from the Archean eon, more than 2500 million years ago ("Ma"), 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, consistent with recent studies of photosynthesis. 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. It is suggested that photosynthesis likely originated at low-wavelength geothermal light from acidic hydrothermal vents, that Zn-tetrapyrroles were the first photochemically active pigments, and that 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.

The first oxygenic photosynthetic organisms are proposed to be H₂S-dependent. It is also suggested photosynthesis originated under sunlight, using H₂S emitted by volcanoes and hydrothermal vents which ended the need for scarce H₂ emitted by alkaline hydrothermal vents. Oxygenic photosynthesis uses water as an electron donor, which is oxidized to molecular oxygen (O
₂) in the photosynthetic reaction center. The biochemical capacity for oxygenic photosynthesis evolved in a common ancestor of extant cyanobacteria. The first appearance of free oxygen in the atmosphere is sometimes referred to as the oxygen catastrophe. The geological record indicates that this transforming event took place during the Paleoproterozoic era at least 2450–2320 million years ago (Ma), and, it is speculated, much earlier. A clear paleontological window on cyanobacterial evolution opened about 2000 Ma, revealing an already-diverse biota of blue-greens. Cyanobacteria remained principal primary producers throughout the Proterozoic Eon (2500–543 Ma), in part because the redox structure of the oceans favored photoautotrophs capable of nitrogen fixation. Green algae joined blue-greens as major primary producers on continental shelves near the end of the Proterozoic; but only with the Mesozoic (251–65 Ma) radiations of dinoflagellates, coccolithophorids, and diatoms did primary production in marine shelf waters take modern form. Cyanobacteria remain critical to marine ecosystems as primary producers in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as the plastids of marine algae. Modern photosynthesis in plants and most photosynthetic prokaryotes is oxygenic.

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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. 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. 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.

An even closer form of symbiosis may explain the origin of chloroplasts. Chloroplasts have many similarities with photosynthetic bacteria, including a circular chromosome, prokaryotic-type ribosomes, and similar proteins in the photosynthetic reaction center. The endosymbiotic theory suggests that photosynthetic bacteria were acquired (by endocytosis) by early eukaryotic cells to form the first plant cells. Therefore, chloroplasts may be photosynthetic bacteria that adapted to life inside plant cells. Like mitochondria, chloroplasts still possess their own DNA, separate from the nuclear DNA of their plant host cells and the genes in this chloroplast DNA resemble those in cyanobacteria. DNA in chloroplasts codes for redox proteins such as photosynthetic reaction centers. The CoRR Hypothesis proposes that this Co-location is required for Redox Regulation.

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 enzymes and co-enzymes. The enzyme RuBisCO is responsible for "fixing" CO₂ – that is, it attaches it to a carbon-based molecule to form a sugar, which can be used by the plant, releasing an oxygen molecule along the way. However, the enzyme is notoriously inefficient, and just as effectively will also fix oxygen instead of CO₂ in a process called photorespiration. This is energetically costly as the plant has to use energy to turn the products of photorespiration back into a form that can react with CO₂.^{[citation needed]}