VERTICAL MODELS OF PHOTOTROPHIC BACTERIAL DISTRIBUTION IN THE METALIMNETIC MICROBIAL COMMUNITIES OF SEVERAL FRESH-WATER NORTH-AMERICAN KETTLE LAKES

The composition and the structure of the metalimnetic communities of p hototrophic microorganisms were studied in 24 lakes of Wisconsin and M ichigan (USA), during the period of summer stratification, and related to environmental parameters. The presence of phototrophic bacteria wa s reported for the first time in some lakes. Different types of photot rophic microorganisms were separated in three different clusters, by d ecreasing values of photosynthetic available radiation (PAR) and redox potential (Eh): (1) cyanobacteria and eukaryotic phytoplankton, (2) C hromatiaceae and multicellular filamentous green bacteria, (3) green s ulfur bacteria. Each cluster conformed one different layer of the vert ical distribution of the community. At each layer, usually one type of microorganisms outcompeted the others, but at times they were found t ogether in competition. The three layers were mixed at the bottom of t he lake at the beginning of the stratification period, before widening and colonising the water column. At the end of summer, the contractio n of the metalimnion resulted in overlapping or suppressing some layer s. (C) 1998 Federation of European Microbiological Societies. Publishe d by Elsevier Science B.V.