Characterization of photoautotrophic picoplankton assemblages in turbid, alkaline lakes of the Carpathian Basin (Central Europe)

The photoautotrophic picoplankton (PPP) of ten shallow, hyposaline soda lakes located in three different geographical regions in the Carpathian Basin (Central Europe) was characterized. These lakes, which frequently dry out completely, are extremely rich in PPP. Epifluorescence microscopy was applied to determine picocyanobacterial and picoeukaryotic cell abundance and PCR-based molecular techniques (denaturing gradient gel electrophoresis and cloning with phylospecies delineation) to identify the members of PPP. Most of these lakes were eu- and hypertrophic with varying contribution of picocyanobacteria to the total PPP cell number. We found an unusually high PPP abundance with peaks of 8.16 × 106 cells mL-1 for picoeukaryotes and 1.78 × 107 cells mL-1 for picocyanobacteria. The majority of the retrieved PPP sequences belonged to picocyanobacteria (nonmarine Synechococcus/ Cyanobium), while others showed similarity to eukaryotic algal plastids (close to Trebouxiophycean isolates). Molecular analysis revealed significant genetic diversity in the PPP fraction of these lakes and showed that the closest relatives of our picocyanobacterial clones were recovered from different habitats, indicating seemingly no correlation between the 'saline' ecotypes and their phylogenetic position. Our results also confirmed that PPP might exploit different aquatic ecosystems and be successful even in the case of abrupt changes of environmental parameters (in our case, salinity). According to our knowledge, this is the first survey focusing on the identification of the PPP community members in turbid and alkaline lakes with extraordinarily high picoplankton productivity.