Diversity and seasonal dynamics of the photoautotrophic picoplankton in Lake Balaton (Hungary)

Lake Balaton, in Hungary, is the largest shallow lake in Central Europe. This fresh water ecosystem has been well studied, including investigations of its photoautotrophic picoplankton (PPP). Previous studies revealed that picoeukaryotes could reach an extremely high abundance in winter, while picocyanobacteria are the predominant picoplankters in warmer periods, as in other lakes in the temperate zone. In addition to epifluorescence microscopy-which allows discrimination only between phycocyanin-rich picocyanobacteria, phycoerythrin-rich picocyanobacteria and picoeukaryotic algae-we used PCR-based molecular methods to reveal the detailed genetic diversity and seasonal dynamics of the PPP in Lake Balaton for the first time. Our results show that a single integrated pelagic sample, taken vertically from the whole water column, may harbor a large number of picocyanobacterial genotypes including previously unidentified groups. Based on length polymorphism analysis of the phycocyanin operon (which contains a non-coding region of variable size), the composition of picocyanobacterial communities showed significant seasonal changes and spatial variation. The relative importance of some of the operational taxonomic units (OTUs) we detected was correlated with environmental factors, such as temperature and the concentration of available nitrogen forms. The picoeukaryotic algal community of winter PPP was dominated by chlorophytes related to the group Trebouxiophyceae. The results of this study highlight the fine internal structure and dynamics of the PPP community in freshwater ecosystems-a view that is usually blurred by the lower resolution of commonly used methods, such as epifluorescence microscopy and flow cytometry.