ABSTRACT: The meromictic lake 'El Tobar' was sampled at 3 time points during the annual cycle, coinciding with the thermal stratification period. The photosynthetic microbial community was composed of mixolimnetic oxygenic phototrophs, which were distributed through the mixolimnion, phycoerytrine-containing unicellular cyanobacteria and sulphur photo- trophic bacteria placed at the halocline. During the stratification period anoxic conditions, formerly confined to the monimolimnion, moved 1 m upwards, reaching freshwater positions. Consequently, photosynthetic bacteria were re-arranged into a double purple-green structure placed just above and embedded in the chemocline. In the freshwater layers we found a population of the purple sulphur bacterium Chromatium minus placed just above a population of the brown-colored green sulphur bacterium Chlorobium phaeobacteroides. A few centimeters below we found Lamprocystis modestohalophilus overlying Chlorobium vibrioforme. This peculiar structure was probably due to anoxic conditions reaching upper positions at the end of the stratification period, thus allowing the growth of freshwater phototrophic bacteria. The distribution of the phototrophic bacterial populations reveals a vertical structure which is strongly influenced by salinity, whereas light intensity only determines the relative position of purple and green bacteria.

KEY WORDS: Meromictic lake · Phototrophic sulphur bacteria · Halocline · Population dynamics · Chromatium · Chlorobium

Published in AME Vol. 20, No. 3 (1999) on December 30
ISSN: 0948-3055. Copyright © Inter-Research, Oldendorf/Luhe, 1999