Cyanobacterial community composition in shallow, eutrophic Lake Tuusulanjarvi studied by microscopy, strain isolation, DGGE and cloning
Rajaniemi-Wacklin, Pirjo; Rantala, Anne; Kuuppo, Pirjo; Haukka, Kaisa; Sivonen, Kaarina
Cyanobacterial community composition was followed over a two-year period in a shallow boreal Lake Tuusulanjärvi (Finland). The occurrence and abundance of cyanobacteria in the lake were studied by denaturing gradient gel electrophoresis (DGGE), cloning and subsequent sequencing of 16S rRNA gene fragments from the DGGE bands and clones as well as by strain isolation and microscopic counting. During the monitoring period, the cyanobacterial community in Lake Tuusulanjärvi was mainly comprised of heterocytous cyanobacteria (Anabaena / Aphanizomenon), Microcystis and Synechococcus. Both potentially hepatotoxic and non-toxic Anabaena genotypes co-occurred in late summer. The abundance of the main groups was related to nutrient concentrations and temperature / global radiation. A long period of low concentration of inorganic nitrogen as well as decreasing temperatures / radiation and phosphorus concentrations seemed to switch the dominance from Microcystis in mid summer to heterocytous cyanobacteria in late summer. Although, the major cyanobacterial groups were generally detected by all three methods (DGGE, cloning and microscopy) used, the proportions of the groups varied between methods. The differences can be related to the high 16S rRNA gene copy number in heterocytous cyanobacteria and difficulties in the morphological identification of simple unicellular cyanobacteria.