Evidence for abundant Chlorobium populations is provided by chemical fossils found in sediments deposited at the Cretaceous mass extinction.
These two gradients promote the growth of different microorganisms such as Clostridium, Desulfovibrio, Chlorobium, Chromatium, Rhodomicrobium, and Beggiatoa, as well as many other species of bacteria, cyanobacteria, and algae.