Seasonal cyanobacterial dynamics in a mesoeutrophic reservoir: microscopic counts and DGGE (Denaturing Gradient Gel Electrophoresis)

The seasonal planktic cyanobacterial dynamics was assessed during the year 2000 by microscopic and DGGE techniques, on the basis of 22 samples collected from the Haute-Sûre reservoir (Grand-Duchy of Luxembourg). Microscopic investigations were carried out according to the standard Utermöhl procedure while 16S rRNA gene fragments obtained by semi-nested PCR were subsequently separated by DGGE. Sequencing of selected excised bands was performed to genotypically define the cyanobacterial assemblages. The dynamics of cyanobacterial communities obtained by both approaches were compared. Several discordances
were pointed out. The counting procedure failed to detect cyanobacteria with small dimensions or in very low abundances, whereas DGGE had a lower detection limit when cyanobacteria were scarce (e.g. in spring) and performed better for the study of picosized forms. Generally, only the dominant cyanobacteria were revealed by these two methods. Actually, both techniques appeared to be complementary rather than equivalent. This study underlines the necessity to use multidisciplinary approaches to obtain a more complete view of the microbial diversity and of the
community structure