Answer :
Cyanobacterial species are thought to be the first photosynthetic organisms. These unicellular bacteria occur in Maine environments, the presence of water acted as a sun-screen, preventing the denaturation of photosynthetic hardware by exposure to high uv. This high volume of Cyanobacteria in the oceans produced vast quantities of oxygen - this allowed organisms to grow in size a their metabolic demands could now be met. At one point the large concentrations of oxygen lead to gigantism. following the first endosymbiotic event, where a cyanobacterial cell became enslaved in a bacterial cell, photosynthetic eukaryotes were 'born'. This lead to the formation of both terrestrial and marine chloroxygenic organisms.
