Aerobic Anoxygenic Phototrophs

SummaryDuring the last 15 years, more than 20 strains of aerobic bacteria which possess bacteriochlorophyll (BChl) a have been found. They are distinguished from typical anaerobic (anoxygenic) phototrophs in that they synthesize BChl only under aerobic conditions and cannot growwithout O2 or other oxidants, even in the light. In some species, photosynthetic activities have been demonstrated. Reaction centers and light-harvesting complexes isolated from some species were shown to be similar to those of typical purple photosynthetic bacteria. The regulatory mechanism of synthesis of pigments and proteins of the photosynthetic apparatus are apparently opposite with respect to O2 tension to that of typical anoxygenic phototrophs. The low content of BChl, unique composition of carotenoids and presence of non-photosynthetic carotenoids in most strains are other marked characteristics of these aerobic bacteria. Phylogenetically, they are not classified into single group. Species so far described are distributed rather widely within the α-subclass of Proteobacteria (purple bacteria) in which most of the purple nonsulfur bacteria as well as many non-photosynthetic bacteria are included. Apparently, these aerobic BChl-containing bacteria represent an evolutionary transient phase from anaerobic phototrophs to aerobic non-phototrophs. However, some characteristic features distinct from anaerobic phototrophs suggest that most of them are in a evolutionary stable state.