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 types of freshwater environments from the southernmost region of South America (Tierra del Fuego). Water bodies were grouped in four categories: humic lakes, clear oligotrophic lakes, beaver ponds and steppe shallow lakes, which differed in their nutrient and dissolved organic carbon (DOC) contents. We tested if microbial planktonic communities were different among lakes with dissimilar nutrient and DOC concentrations, analysing to what extent the known large-scale patterns of lake trophic structure applies to a diverse but localized set of lakes. We found that mixotrophs dominated over strict autotrophs in both humic and clear oligotrophic systems, whereas in eutrophic lakes autotrophy was a successful strategy. The functional phytoplankton approach also allowed the separation between oligotrophic (clear and humic) and eutrophic systems, with different functional groups. The lowest abundances of picoplankton were found in oligotrophic lakes, picoeukaryotes being more abundant than picocyanobacteria in beaver ponds and humic lakes. Our results show that in low nutrient environments, mixotrophic strategies thrive over strict autotrophs suggesting the paramount importance of the microbial loop when compared with high trophic status systems where the prevalence of autotrophy indicates that the energy flux depends on phytoplankton.