<P> 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 . </P> <P> 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 ribosome, 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 possess their own DNA, separate from the nuclear DNA of their plant host cells and the genes in this chloroplast DNA resemble those found in cyanobacteria . DNA in chloroplasts codes for redox proteins such as those found in the photosynthetic reaction centers . The CoRR Hypothesis proposes that this Co-location is required for Redox Regulation . </P> <P> The biochemical capacity to use water as the source for electrons in photosynthesis evolved once, in a common ancestor of extant cyanobacteria . The geological record indicates that this transforming event took place early in Earth's history, at least 2450--2320 million years ago (Ma), and, it is speculated, much earlier . Because the Earth's atmosphere contained almost no oxygen during the estimated development of photosynthesis, it is believed that the first photosynthetic cyanobacteria did not generate oxygen . Available evidence from geobiological studies of Archean (> 2500 Ma) sedimentary rocks indicates that life existed 3500 Ma, but the question of when oxygenic photosynthesis evolved is still unanswered . A clear paleontological window on cyanobacterial evolution opened about 2000 Ma, revealing an already - diverse biota of blue - green algae . Cyanobacteria remained the principal primary producers of oxygen 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 - green algae as the major primary producers of oxygen on continental shelves near the end of the Proterozoic, but it was only with the Mesozoic (251--65 Ma) radiations of dinoflagellates, coccolithophorids, and diatoms did the primary production of oxygen in marine shelf waters take modern form . Cyanobacteria remain critical to marine ecosystems as primary producers of oxygen in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as the plastids of marine algae . </P> <P> Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the 19th century . </P>

What are the reactions of the first stage of photosynthesis called