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

Abstract

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 3 H-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 3 H-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.

Single-cell activity of freshwater aerobic anoxygenic phototrophic bacteria and their contribution to biomass production

“…In addition, the average specific rate of leucine uptake, based on the SGA around individual cells, was almost twofold higher for active AAP bacteria than for the average active bacterial cell in the community. This difference was slightly higher than that of the two previous MAR-AAP studies (1.6-and 1.4-fold, Kirchman et al, 2014;Stegman et al, 2014), and is also in agreement with previous studies showing that AAP bacteria tend to grow faster than the bulk bacterial community across a diverse range in aquatic habitats (Koblízek et al, 2007;Liu et al, 2010;Ferrera et al, 2011). We have further shown that AAP bacteria were on average larger than the average cell size in the community, a pattern that has been reported before (Sieracki et al, 2006;Lamy et al, 2011;Fauteux et al, 2015).…”

“…This result supports the hypothesis that the small enhancement of AAP bacterial activity due to phototrophy can be accumulated and evidenced over the lifetime of an AAP cell (see Kirchman and Hanson, 2013;Kirchman et al, 2014). Thus, it may be possible that light does increase AAP growth rates but that the enhancement is too small to be measurable in a short incubation experiment.…”

“…The larger cell sizes, higher activity and growth rates and preferential grazing of AAP bacteria have repeatedly led to the hypothesis that this group plays a disproportionately large role in the cycling of carbon and nutrients in aquatic food webs Kirchman et al, 2014;Koblížek, 2015). So far, however, no study had actually quantified their contribution to total bacterial activity in natural ecosystems.…”

“…Available data indicate that despite their low relative abundances and the apparently low amount of energy potentially gained through phototrophy (Kirchman and Hanson, 2013), AAP bacteria are more active, larger and grow faster than the bulk bacterial community (Koblízek et al, 2007;Ferrera et al, 2011;Hojerová et al, 2011;Kirchman et al, 2014). It is not clear, however, whether these higher apparent growth rates and potential competitiveness of AAP bacteria can be directly attributed to a light-driven effect.…”

Global genetic capacity for mixotrophy in marine picocyanobacteria

“…According to Hartmann et al (2012), the majority of bacterivory in the Atlantic may be carried out by phototrophs. Similarly, the ocean is populated by up to 11% aerobic anoxygenic photoheterotrophic bacteria that combine phototrophy and carbon fixation with organic compound uptake (Kolber et al, 1999;Sieracki et al, 2006;Jiao et al, 2007;Kirchman et al, 2014). Heterotrophic bacteria that utilize light to pump protons via rhodopsin are abundant globally as well (Béjà et al, 2000(Béjà et al, , 2001Rusch et al, 2007), making up an estimated 13% of the photic zone bacteria in the Mediterranean and Red Seas (Sabehi et al, 2005), 50% in the Sargasso Sea (Campbell et al, 2008), and 48% in 116 marine and terrestrial samples examined by Finkel et al (2013).…”