AEROBIC ANOXYGENIC PHOTOTROPHIC BACTERIA

The aerobic anoxygenic phototrophic bacteria are a relatively recently discovered bacterial group. Although taxonomically and phylogenetical ly heterogeneous, these bacteria share the following distinguishing fe atures: the presence of bacteriochlorophyll a incorporated into reacti on center and light-harvesting complexes, low levels of the photosynth etic unit in cells, an abundance of carotenoids, a strong inhibition b y light of bacteriochlorophyll synthesis, and the inability to grow ph otosynthetically under anaerobic conditions. Aerobic anoxygenic photot rophic bacteria are classified in two marine (Erythrobacter and Roseob acter) and six freshwater (Acidiphilium, Erythromicrobium, Erythromona s, Porphyrobacter, Roseococcus, and Sandaracinobacter) genera, which p hylogenetically belong to the alpha-1, alpha-3, and alpha-4 subclasses of the class Proteobacteria. Despite this phylogenetic information, t he evolution and ancestry of their photosynthetic properties are uncle ar: We discuss several current proposals for the evolutionary origin o f aerobic phototrophic bacteria. The closest phylogenetic relatives of aerobic phototrophic bacteria include facultatively anaerobic purple nonsulfur phototrophic bacteria. Since these two bacterial groups shar e many properties, yet have significant differences, we compare and co ntrast their physiology, with an emphasis on morphology and photosynth etic and other metabolic processes.