Plants, algae and cyanobacteria which carry out oxygenic photosynthesis use water as the source of electrons and produce oxygen as a product. The earliest photosynthesizing organisms were likely anoxygenic phototrophs. These bacteria uses hydrogen sulfide or organic compounds rather than water as a source of electrons. They produce an oxidized organic compound or an oxidized form of sulphur, usually elementary sulphur, usually elemental sulphur as an end product. Most of the anoxygenic phototrophic bacteria found in aquatic habitats such as bogs, lakes and the upper layer of muds.
The photosynthetic systems of the anoxygenic phototrophs are fundamentally different form those of plants, algae, and cyanobacteria. They have a unique type of chlorophyll called bacteriochlorophyll. This pigment absorbs wave length of light that penetrate to greater depths and are not used by other photosynthetic organisms.
By producing elemental sulphur as a by-product of their photosynthesis, anoxygenic phototropic bacteria probably caused the huge deposits of elemental sulphur that occur in various parts of the world.
These bacteria are deeply colored – red, orange, purple and bright green, because of the chlorophyll and accessory photosynthetic pigments they contain. Water samples from deep regions of lakes with an abundance of these organisms are also intensely colored.