Diversity of cyanobacteria and heterotrophic bacteria in cyanobacterial blooms in Lake Joutikas, Finland

Water samples were collected in August 2001 and 2002 from the eutrophic Lake Joutikas during cyanobacterial blooms. DNA and RNA were isolated from size fractionated samples and the diversity of the bacteria present in each fraction was studied by PCR amplification of partial 16S rRNA and denaturing gradient gel electrophoresis (DGGE) fingerprinting. Major bands from the gels were sequenced for further identification. Cyanobacteria were also identified and counted under the microscope. Anabaena/Aphanizomenon were the most abundant cyanobacteria in both years, although the dominant species was different each year. When comparing the 2 techniques, equal numbers of abundant Anabaena/Aphanizomenon morphotypes were detected by microscopy and phylotypes by DGGE. The genera Microcystis and Synechococcus appeared more abundant in the DGGE analysis than under the microscope. In the heterotrophic bacterial community variation was observed between the bloom samples from the 2 years. Verrucomicrobia was the most abundant group in both years in both DNA- and RNA-derived profiles. Otherwise the patterns based on DNA- and RNA-derived DGGE-profiles differed, especially in 2002. The presence of Actinobacteria and Chloroflexi was less pronounced in RNA-based than in DNA-based analysis. This indicates that their relative biomass was smaller than estimated by DNA-analysis. It might also indicate that they were metabolically inactive. In contrast, in 2002, the CFB group (Cytophaga-Flavobacterium-Bacteroides) and delta -Proteobacteria were more prominent in the RNA-based than in the DNA-based profiles. Thus they probably formed a substantial fraction of biomass and/or were active members in the blooms.