Distribution of Planktonic Aerobic Anoxygenic Photoheterotrophic Bacteria in the Northwest Atlantic

Aerobic anoxygenic photoheterotrophic (AAP) bacteria can use both dissolved organic matter and light for energy production, but their photosynthesis does not produce oxygen. We measured AAP bacterial cell and bacteriochlorophyll distributions in the northwest Atlantic, from the coast of the Gulf of Maine to the Sargasso Sea, in October 2001 and March 2002. The abundance of AAPs ranged from$7 \times 10^3 to 9.8 \times 10^4 cells mL^{-1}$(mean,$2.9 \times 10^4 mL^{-1}$) in surface waters, or between 1% and 9% (mean, 2.3%) of total bacteria. Mean abundances in October in the Gulf of Maine ($6.6 \times 10^4 mL^{-1}$) were about five times higher than those measured in March ($1.3 \times 10^4 mL^{-1}$), whereas the mean Sargasso Sea values were not different between October and March. AAP cells were larger than other bacteria, so AAP biomass ranged from 2% to 13% of total bacterial biomass. AAP cells were higher in abundance, biomass, and proportion of total bacteria in productive coastal and shelf waters than in the Sargasso Sea. Cell quotas of bacteriochlorophyll were low and quite variable, ranging from$0.02 to 0.17 fg cell^{-1}$(mean,$0.08 fg cell^{-1}$). Our results indicate possible control by temperature and organic and inorganic nutrients on the distribution of planktonic AAPs, but they do not support the idea that they are specifically adapted to oligotrophic conditions.