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 3 H-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.The role of bacteria in ecosystem functioning, as a link between dissolved organic matter (DOM) and higher trophic levels, appears to strengthen in more oligotrophic conditions, where bacterial : algal biomass and bacterial : primary production ratios tend to be higher than in eutrophic ecosystems (Gasol et al. 1997;Biddanda et al. 2001;Cotner and Biddanda 2002).Within an oligotrophic gradient, high mountain lakes over the tree line are exposed to low temperatures, high ultraviolet irradiation, long ice and snow cover period, and fluctuating hydrology. These abiotic factors may influence the development and dynamics of bacterioplankton (Straskrabová et al. 1999). The generally low concentration of dissolved organic carbon (DOC) in clear-water high mountain lakes (commonly Ͻ1 mg L Ϫ1 , see Laurion et al. 2000), and its potentially low quality or bioavailability due to photoreactivity and photoalteration processes (Benner and Biddanda 1998;Reche et al. 2001), may make carbon the main limiting nutrient to bacterial growth. Moreover, the interaction of low mineral nutrient and/or DOC concentrations with 1 Corresponding author (jmmedina@ugr.es). AcknowledgmentsWe sincerely acknowledge L. Cruz-Pizarro, R. Morales-Baquero, P. Sánchez-Castillo, and I. Reche for their contribution to the database. We thank M. J. Villalba and J. A. Delgado for assistance in the field and Richard Davies for...

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