Neither with nor without you: A complex algal control on bacterioplankton in a high mountain lake

Abstract

The literature proposes that the microbial loop is a key link for ecosystem functioning, particularly in oligotrophic conditions. From original and published data for a period spanning 1986 to 1999, we examined the food web structure of a clear‐water, oligotrophic, high mountain lake. The heterotrophic microbial food web was weakly developed in a grazing food chain dominated by copepods and phytoplankton, the latter mainly composed of mixotrophic flagellates. Bacteria constituted a minor component of the plankton community in terms of abundance, biomass, and production, in contrast to the situation usually reported in oligotrophic conditions. Abiotic and biotic factors that regulate bacterioplankton biomass and production were assessed, including the bacterivory capability of mixotrophic flagellates experimentally quantified by using 3H‐thymidine as a tracer. Algae were the main factor controlling bacterioplankton. Their regulatory effect has a dual nature: (1) a resource‐based control, through the dependence of the bacteria on photosynthetic carbon released by algae, i.e., a commensalistic interaction (“without you I cannot live”) and (2) a predatory control, with bacteria as prey for mixotrophs (“with you I die”). Mixotrophic metabolism can constitute an adaptive strategy for algae to overcome ultraviolet (UV) stress, by using bacteria as a source of carbon and mineral nutrients in conditions of inhibited photosynthesis and mineral nutrient uptake. Mixotrophy acts as a bypass of carbon flux toward the grazing food chain, explaining the scarce development of the heterotrophic microbes in this and other high mountain lakes.