Abundance, size, and activity of aerobic anoxygenic phototrophic bacteria in coastal waters of the West Antarctic Peninsula

The objective of this study was to examine the abundance, size, and single-cell activity of aerobic anoxygenic phototrophic (AAP) bacteria in summer and fall over 2 yr in coastal waters of the West Antarctic Peninsula. Single-cell incorporation of H-3-leucine was measured using a new microautoradiography approach coupled to infrared epifluorescence microscopy. The relative abundance of these photoheterotrophic bacteria was higher in January (1 to 8%) than in May (0.3 to 1 %) but differed greatly between the 2 yr we sampled, as did several biogeochemical properties. The biovolume of AAP bacteria was nearly 3-fold larger than that of cells in the total bacterial community and ca. 2-fold larger than AAP bacterial biovolumes in low latitude coastal waters. A large fraction of AAP bacteria and of the total community incorporated 3H-leucine in January (ca. 48% for both communities), which then decreased by 4-fold in May. The area of silver grains formed around active cells in microautoradiography preparations decreased nearly 5-fold from January to May. Silver grain area data indicated that AAP bacteria were about 40% more active than the total community in January but there was no difference in May, probably because of the nearly 20-fold decrease in light availability from January to May. These data suggest that AAP bacteria contribute more to biomass production in these polar waters than suggested by their abundance, but only in the austral summer.