Major contribution of both zooplankton and protists to the top-down regulation of freshwater aerobic anoxygenic phototrophic bacteria

Abstract

Aerobic anoxygenic phototrophic (AAP) bacteria are photoheterotrophic prokaryotes that use light as a secondary energy source to complement the consumption of organic matter. Despite this metabolic flexibility and their widespread distribution, their low relative abundances suggest that they may be subjected to strong regulatory processes. However, there is still little information on the regulation of AAP abundance, particularly in freshwaters. Here, we used a lake mesocosm experiment to address the top-down regulation of freshwater AAP by protists and zooplankton under 2 contrasting nutrient regimes. Our results support the hypothesis that freshwater AAP are subject to intense top-down regulation, and are selectively removed by grazers. The average gross growth rate of AAP was ca. 1.5 times higher, and grazing loss rates 1.6 times higher than those of the bulk bacterial community. Our results further indicate that whereas protists are the main predators of AAP, zooplankton may account for over a third of AAP losses, and both exhibit a greater selectivity for AAP relative to total bacteria. The mechanistic under - pinning of this selectivity is still unclear, but it may be related to the average larger cell size of AAP, and to their higher potential growth rates relative to the bulk bacterial community. Our results further suggest that AAP may play a disproportionate role in the nutrition of lake zooplankton, and in the trophic transfer of organic carbon in lake food webs.