New Light on Aerobic Anoxygenic Phototrophs

Abstract

Discovered 30 years ago, aerobic anoxygenic phototrophs (AAP) represent an entirely new bacterial functional group that was surprisingly found to constitute nearly 10% of microbial cells in the world’s biggest surface ecosystem, the ocean. These intriguing and colorful descendents of anaerobic anoxygenic phototrophs possess a fully functional photosynthetic apparatus that is paradoxically operative only under oxic conditions. An obviously ancient group, the AAP display numerous extensive evolutionary modifications to their photosynthetic machinery from that of their ancestors, such as different suites of light-harvesting 2 complexes and, in some species, the only zinc-based chlorophyll pigments found anywhere in nature. Whereas AAP are incapable of photoautotrophy and rely on heterotrophy for 80% or more of their cellular energetics, sunlight can double organic carbon assimilatory efficiency over that of strict heterotrophs, making AAP key players in the marine carbon cycle. The AAP inhabit not just soil, rivers and oceans, but also hypersaline waters, thermal springs and even the dark realm of deep ocean hydrothermal vents. Ubiquity and atypical photosynthetic nature has inspired an ever-increasing scientific interest in the AAP, for which there are more exceptions than rules.