Wide-ranging abundances of aerobic anoxygenic phototrophic bacteria in the world ocean revealed by epifluorescence microscopy and quantitative PCR

Reports that aerobic anoxygenic phototrophic bacteria (AAnPB) containing bacteriochlorophyll a (BChla) constitute 10% or more of total bacteria cell counts in seawater prompted us to quantify AAnPB abundances in local waters 15 kin off the coast of Southern California and at 18 sites around the globe using infrared epifluorescent microscopy (IREM) and quantitative polymerase chain reaction (QPCR). IREM-estimated AAnPB comprised 1.66% +/- 0.55% (mean +/- SD) of total bacterial counts in the euphotic zone between April and September 2001 off Southern California. IREM counts of AAnPB need to be interpreted with caution due to detection sensitivity concerns and overlap between chlorophyll a and bacteriochlorophyll a autofluorescence at wavelengths > 850 nm. QPCR-estimated AAnPB were 1.17% +/- 1.56% of total bacteria counts at 5-m depth between January 2001 and July 2003 off the coast of Southern California (assuming one pufM gene cell(-1) and 2.5 fg total DNA cell(-1)). Global QPCR surveys of AAnPB conducted in tropical to polar waters yielded low estimates (< 2.5% of total bacteria counts) of AAnPB in most marine environments surveyed, but estimates > 10% were observed in estuarine waters of Long Island Sound and Chesapeake Bay. AAnPB abundances are lower in most marine systems than previously reported from the Eastern Pacific and demonstrate that AAnPB abundances are more variable than previously realized. Our results do not support the notion AAnPB are more abundant in oligotrophic environments and, in fact, suggest the opposite. These results raise questions about where aerobic anoxygenic photosynthesis is advantageous in marine ecosystems.