Ancient European Lakes: Reservoirs of Hidden Microbial Diversity? The Case of Lake Pamvotis (NW Greece)

Ancient European lakes are clustered within a radius of 300 km around Lake Ohrid. Information concerning microbial diversity in these lakes is limited. We studied diversity of the dominant prokaryotic phylotypes in the sediments in one of these lakes, known as Lake Pamvotis. The analysis was performed in samples from two stations for four seasons of the same year. DNA extraction followed by PCR amplification (16S rDNA), Denaturing Gradient Gel Electrophoresis, cloning and sequencing was applied in order to reveal the sequence signatures of the dominant bacterial and archaeal phylotypes. Bacterial and archaeal cell numbers were quantified by real-time PCR. Several environmental variables measured in parallel, including pH, Nickel, Chromium, Arsenic, Calcium, Total Nitrogen and Total Carbon, were found to affect strongly the prokaryotic abundances. Most of the identified sequences of Bacteria belong to Proteobacteria and most of the sequences of Archaea belong to Euryarchaeota. The great majority of these bacterial (84.21%) and archaeal sequences (95.65%) have no cultivated counterparts in the databases. In addition, many of these bacterial (50.88%) and archaeal sequences (20.65%) correspond to potentially new species. Six of the bacterial sequences constitute a new class of Cyanobacteria which we have named “Lake Pamvotis cluster” (LPC). Our findings highlight Lake Pamvotis as a habitat for several previously unidentified species of Bacteria and Archaea.