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Many examples of convergence relating to photosynthesis exist, from patterns of plastid gene transfer to repeated evolution of specific carbon dioxide concentration mechanisms in plants. A few highlights are mentioned here, but for more details and cases explore the full list of topics below!
Evolution of photosynthesis
In the process of photosynthesis, photons of light energy are captured to convert carbon dioxide (CO2) and - in most cases - water (H2O) into carbohydrate and oxygen (O2). Carbon is "fixed" as H2O molecules donate electrons, resulting in reduction of CO2 to carbohydrate. Photosynthetic organisms use the fixed carbon to build complex carbohydrates as well as other essential compounds such as lipids and amino acids. Virtually all living things depend on photosynthesis, from the plants and algae that use it directly to organisms that breathe oxygen and feed on plants and algae for energy to fuel their own life processes.
Photosynthesis appears to have evolved at the origin of bacterial life around 3,500 million years ago (Ma). Early photosynthetic 'eobacteria' such as Chlorobacteria and certain flagellate 'eubacteria' (e.g. purple or proteobacteria and sphingobacteria) do not produce oxygen. This is because they do not use water as an electron donor but rather hydrogen, sulfur or a variety of organic molecules. Oxygenic photosynthesis using water appears to have evolved only once, in the cyanobacteria (also known as blue-green algae), perhaps as far back as 2,500 Ma. As cyanobacteria diversified, increasing oxygen production changed the Earth's atmosphere radically, and this was a likely trigger for the development of new forms of life (e.g. around 1,800 Ma eukaryotic cells appear, with DNA held in a double-membraned nucleus).
Photosynthetic eukaryotes evolved by 'endosymbiosis', in which a eukaryotic 'host' cell engulfed a cyanobacterium. Over time the genome of the captured cyanobacterium was reduced from several thousand to a few hundred genes (carried on circular DNA), as most genes carrying information for photosynthetic machinery were transferred to the host genome. Much evidence supports the hypothesis that only one ancestral endosymbiosis event occurred, from which three 'primary plastid' groups evolved: the glaucocystophytes, green algae (or chlorophytes, including the land plants) and red algae (or rhodophytes). Further secondary or even tertiary endosymbioses resulted in subsequent algal derivatives with complex plastid membranes and molecular signatures, including coccoliths, diatoms, dinoflagellates, euglenoids and many others.
Photosynthesis and convergence
Recent discoveries have encouraged caution about the 'single origin of plastids' hypothesis and provided fresh examples of convergent evolution. A few living eukaryotes (e.g. the diatom Rhopaloidea and amoeba Paulinella) have stable photosynthetic endosymbionts that could be on the way to becoming novel plastids, suggesting that the convergent acquisition of plastids from different cyanobacterial lineages may still be a possible explanation for observed algal diversity. Gene locations shared between all three primary plastids, in addition to parallels in patterns of gene loss can be taken as strong support for plastid monophyly (shared ancestry) but some authors suggest that patterns of gene transfer to the host nucleus may in fact be constrained, resulting in convergent patterns of loss in independent lineages.
Many animals and even some protists sequester algae (or just their photosynthetic organelles such as chloroplasts) in order to gain energy from photosynthesis for themselves. Enchanting instances of convergence occur, for example various species of 'zooxanthellae' (photosynthetic dinoflagellate algae) live as endosymbionts within marine organisms, including corals, tropical sea anemones, jellyfish, the giant clam Tridacna and protists such as foraminifera, radiolaria and ciliates. The acoel flatworm Symsagittifera roscoffensis consumes thousands of green algae (Tetraselmis convolutae) when juvenile and subsequently loses its mouth, as adults come to rely entirely on their algal symbionts. Elysia sea slugs consume chromophyte algae, selectively retaining chloroplasts in photosynthetic vacuoles. This symbiosis has resulted in the transfer of key chloroplast plasmid genes to the sea slug genome, in an intriguing parallel to the transfer of genes from plastid to host nucleus early in eukaryote evolution.
Many plants can thrive in hot, bright and dry conditions through adaptations that prevent water loss (e.g. closing stomata during the day) as well as CO2 concentration mechanisms (CCMs) that maintain effective carbon fixation and gas exchange in spite of daily stomatal closure. 'C4 photosynthesis' and 'Crassulacean Acid Metabolism' (or 'CAM photosynthesis') are CCMs that evolved independently in many distinct lineages of land plants. C4 plants include many crops such as maize and sugarcane, while CAM plants include many succulent desert angiosperms (e.g. cacti, Euphorbia, Hoodia, Stapelia, Aloe and Agave) plus the aquatic lycophyte Isoetes. Notably, C4 and CAM photosynthesis both rely on a phase of carbon fixation where CO2 is added to a compound called PEP (phosphoenylpyruvate) through the enzyme PEP carboxylase. In C4 plants CO2 fixation is controlled by restriction to the leaf mesophyll whereas in CAM a temporal separation occurs, as CO2 can only enter the leaf when stomata open at night. CCMs occur not only in plants but also in algae that need them; at least two algal groups independently evolved C4 photosynthesis and many cyanobacteria concentrate CO2 via organelle-like carboxysomes.
Succulent desert plants offer further windows onto convergence. Two groups of cacti and certain species of Euphorbia have independently arrived at the same solution to desert life, namely complete loss of leaves and development of a succulent, fully photosynthetic stem with stomata protected within stem placations or furrows.
Light sensitivity based on opsins is well documented throughout the tree of life, and notably in the cyanobacterium Anabaena it is involved with photosynthesis and in particular the production of key pigments. The cyanobacterium Leptolyngbya even possesses an actual 'eye-spot'. This pigmented sensor is involved in phototactic behaviour and represents an interesting convergence on the eye-spots of various protistans.
Convergent evolution is striking among lichens, which comprise fungi in symbiotic association with cyanobacteria or green algae (chlorophytes). Thousands of species and several independent groups of ascomycete fungi form lichens with various green algal or cyanobacterial partners, with convergences at the level of growth form, structure and biochemistry. Furthermore, lichen-like associations have also been observed in three basidiomycetes and in a Glomus-like zygomycete that associates with the cyanobacterium Nostoc.