Microscopic measurements of fresh biomass and 16S rRNA gene sequences from clonelibraries and denaturing gradient gel electrophoresis (DGGE) were used to investigate cyanobacterialdiversity in Polish water bodies in 2002. In addition, measurements of microcystin (MC) concentrationswere made. Thirty water samples were taken from 11 water bodies; of these samples,18 were obtained from the Sulejow Reservoir during regular monitoring from June to October. Intraandextracellular MC concentrations in Sulejow samples were measured by high performance liquidchromatography (HPLC). The extracellular MC concentration was assessed using a protein phosphataseinhibition assay (PPIA) in additional lakes. Additionally, physicochemical parameters weremeasured (total nitrogen [TN], total phosphorus [TP], TN:TP ratio, chlorophyll a concentration,temperature). In Sulejow, high intracellular MC concentrations corresponded to large cyanobacterialbiovolumes and to low TN:TP ratios. In the other lakes, extracellular MCs were not linked to anymeasured parameters. The combination of the microscopic and molecular data showed that Aphanizomenonand Microcystis were the dominant genera during the summer period in the Sulejow Reservoir.At the genetic level, there was a succession of 2 different operational taxonomic units (OTUs)belonging to the lineage Anabaena/Aphanizomenon. In the other water bodies, the most frequentpopulations were Aphanizomenon, Anabaena, Microcystis and Planktothrix. Small populations ofRomeria, Snowella, Woronichinia, Limnothrix and Pseudanabaena were observed, and an enigmaticcluster affiliated with Prochlorothrix was genetically retrieved. Anabaena and Microcystis werepresumed to be the main genera responsible for the MC production.
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