Aerobic anoxygenic phototrophic bacteria and their roles in marine ecosystems

Aerobic anoxygenic phototrophic bacteria (AAPB) are characterized by the following physiological and ecological features. A mother AAPB cell can unusually divide into 3 daughter cells and looks like a "Y" during the division. AAPB cells sometimes adhere together forming a free-floating population. Most of the known AAPB species are obligately aerobic. Bacteriochlorophyll a (BChl a) is the only photosynthetic pigment in AAPB, and the number of BChl a molecules in an AAPB cell is much less than that in an anaerobic phototrophic bacterial cell, while the accessorial pigments carotenoids in AAPB are abundant in concentration and diverse in species. In addition to the common magnesium containing BChl a, a zinc-containing BChla was also seen in AAPB. AAPB have light harvesting complex I but usually lack light harvesting complex II. Although AAPB featur in photosynthesis, their growth is not necessarily light-dependent. There is a mechanism controlling the photosynthesis approach. AAPB are widely distributed in marine environments especially in oligotrophic oceans accounting for a substantial portion of the total biomass and playing a unique role in the cycle of carbon and other biogenic elements. Besides the contribution to primary production, AAPB also have great potentials in bioremediation of polluted environments. Studies on AAPB would be of great value in understanding the evolution of photosynthesis and the structure and function of marine ecosystems.