Diversity of non-bloom-forming cyanobacteria in a hypertrophic shallow lake

HUBER, M.P.; LLAMES, M.E.; FERRARO, M.; UNREIN, F.

Kastoria

Congreso; 17th Workshop of the International Association of Phytoplankton Taxonomy and Ecology (IAP); 2014

Association of Phytoplankton Taxonomy and Ecology

The Pampa plain of Argentina contains thousands of shallow lakes, characterized by high concentration of chlorophyll-a and nutrients, and low transparency. Unlike other hypertrophic systems, in Chascomús lake, phytoplankton is permanently dominated by non-bloom-forming Chroococcales Cyanobacteria. The temporal dynamic of the phytoplankton community structure was intensely studied during two years using a polyphasic approach which combined microscopy (epifluorescence and inverted microscopes), molecular analysis (Denaturing Gradient Gel Electrophoresis ?DGGE-, sequencing of 16S rDNA gene) and flow cytometry (including sorting and characterization of sorted populations). Four different dominant morphologies comprised the pico- and nanoplanktonic fractions. Autotrophic picoplankton was almost exclusively represented by ficocianine-rich Synechococcus-like sp. (up to 1 x 107 cell mL-1). The nanoplanktonic fraction was dominated by small aggregates of Chroococcales (Aphanocapsa elachista, Chroococcus microscopicus and Cyanodictyon sp.), together with a number of undetermined microaggregates of 3 to 12 Synechococcus-like cells. All these cyanobacteria constituted between 81 and 92 % of the total algal biomass. With the flow cytometry approach, at least 2 or 3 subpopulations of single cell-picocyanobacteria and one aggregated cyanobacteria were repeatedly recognized. In contrast, up to 12 different band positions were obtained from the DGGE fingerprints. Preliminary phylogenetic analysis based on partial 16S sequences revealed that cyanobacteria from Chascomús clustered in a new well-defined separate group. Microscopic counts, cytograms as well as DGGE profiles, revealed only weak temporal fluctuations in community composition, that agree with the steady state condition observed by other researchers and support the idea that the algal assemblage in this lake is mostly structured by the severe light limitation prevailing throughout the year.