Microbial planktonic communities of freshwater environments from Tierra del Fuego: dominant trophic strategies in lakes with contrasting features

Abstract

We analysed the structure of the microbial plankton communities of different typesof freshwater environments from the southernmost region of South America (Tierradel Fuego). Water bodies were grouped in four categories: humic lakes, clear oligo-trophic lakes, beaver ponds and steppe shallow lakes, which differed in their nutrientand dissolved organic carbon (DOC) contents. We tested if microbial planktoniccommunities were different among lakes with dissimilar nutrient and DOC concen-trations, analysing to what extent the known large-scale patterns of lake trophicstructure applies to a diverse but localized set of lakes. We found that mixotrophsdominated over strict autotrophs in both humic and clear oligotrophic systems,whereas in eutrophic lakes autotrophy was a successful strategy. The functionalphytoplankton approach also allowed the separation between oligotrophic (clear andhumic) and eutrophic systems, with different functional groups. The lowest abun-dances of picoplankton were found in oligotrophic lakes, picoeukaryotes being moreabundant than picocyanobacteria in beaver ponds and humic lakes. Our resultsshow that in low nutrient environments, mixotrophic strategies thrive over strictautotrophs suggesting the paramount importance of the microbial loop when com-pared with high trophic status systems where the prevalence of autotrophy indicatesthat the energy flux depends on phytoplankton.