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 into organic material. A C4 plant prefaces the Calvin cycle with reactions that incorporate 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.
Origin
Available evidence from geobiological studies of sedimentary rocks from the Archean eon, more than 2.5Gy ago, suggests that life existed at 3.5Gy. Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4Gy,Photosynthesis got a really early start, New Scientist, 2 October 2004Revealing the dawn of photosynthesis, New Scientist, 19 August 2006 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.Tang, K.-H., Tang, Y. J., Blankenship, R. E. (2011). "Carbon metabolic pathways in phototrophic bacteria and their broader evolutionary implications" Frontiers in Microbiology 2: Atc. 165. http://dx.doi.org/10.3389/micb.2011.00165 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 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 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 -dependent. It is also suggested photosynthesis originated under sunlight, using 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 () 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.
Timeline of photosynthesis on Earth 4.6 billion years agoEarth forms3.4 billion years agoFirst photosynthetic bacteria appear2.7 billion years agoCyanobacteria become the first oxygen producers2.4 – 2.3 billion years agoEarliest evidence (from rocks) that oxygen was in the atmosphere1.2 billion years agoRed and brown algae become structurally more complex than bacteria0.75 billion years agoGreen algae outperform red and brown algae in the strong light of shallow water 0.475 billion years agoFirst land plants – mosses and liverworts0.423 billion years ago Vascular plants evolve Source:
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. 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.
Evolution of photosynthetic pathways
In its simplest form, photosynthesis is adding water to 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" – 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 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 .
Concentrating carbon The C4 metabolic pathway is a valuable recent evolutionary innovation in plants, involving a complex set of adaptive changes to physiology and gene expression patterns. About 7600 species of plants use carbon fixation, which represents about 3% of all terrestrial species of plants. All these 7600 species are angiosperms.
C4 plants evolved carbon concentrating mechanisms. These work by increasing the concentration of around RuBisCO, thereby facilitating photosynthesis and decreasing photorespiration. The process of concentrating around RuBisCO requires more energy than allowing gases to diffuse, but under certain conditions – i.e. warm temperatures (>25 °C), low concentrations, or high oxygen concentrations – pays off in terms of the decreased loss of sugars through photorespiration.
One type of C4 metabolism employs a so-called Kranz anatomy. This transports through an outer mesophyll layer, via a range of organic molecules, to the central bundle sheath cells, where the is released. In this way, is concentrated near the site of RuBisCO operation. Because RuBisCO is operating in an environment with much more than it otherwise would be, it performs more efficiently. In C4 photosynthesis, carbon is fixed by an enzyme called PEP carboxylase, which, like all enzymes involved in C4 photosynthesis, originated from non-photosynthetic ancestral enzymes.
A second mechanism, CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions.C.Michael Hogan. 2011. Respiration. Encyclopedia of Earth. Eds. Mark McGinley & C. J. Cleveland. National council for Science and the Environment. Washington DC The most important benefit of CAM to the plant is the ability to leave most leaf stomata closed during the day. This reduces water loss due to evapotranspiration. The stomata open at night to collect , which is stored as the four-carbon acid malate, and then used during photosynthesis during the day. The pre-collected is concentrated around the enzyme RuBisCO, increasing photosynthetic efficiency. More is then harvested from the atmosphere when stomata open, during the cool, moist nights, reducing water loss.
CAM has evolved convergently many times. It occurs in 16,000 species (about 7% of plants), belonging to over 300 genera and around 40 families, but this is thought to be a considerable underestimate. It is found in quillworts (relatives of club mosses), in ferns, and in gymnosperms, but the great majority of plants using CAM are angiosperms (flowering plants).
Evolutionary record
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. Whole-genome and individual gene duplication are also associated with C4 evolution. Many potential evolutionary pathways resulting in the phenotype are possible and have been characterised using Bayesian inference, confirming that non-photosynthetic adaptations often provide evolutionary stepping stones for the further evolution of .
The C4 construction is most famously used by a subset of grasses, while CAM is employed by many succulents and cacti. The trait appears to have emerged during the Oligocene, around ; however, they did not become ecologically significant until the Miocene, . Remarkably, some charcoalified fossils preserve tissue organised into the Kranz anatomy, with intact bundle sheath cells, allowing the presence C4 metabolism to be identified without doubt at this time. Isotopic markers are used to deduce their distribution and significance.