Vertical distribution of photosynthetic sulphur bacteria linked to saline gradients in Lake 'El Tobar' (Cuenca, Spain)
L. J. Garcia-Gil1,*, E. Vicente2, A. Camacho2, C. M. Borrego1, X. Vila1, X. P. Cristina1, J. Rodriguez-Gonzalez1
1 Institute of Aquatic Ecology, University of Girona, Campus de Montilivi, 17071 Girona, Spain
2 Dept of Microbiology and Ecology, Faculty of Sciences, University of Valencia, Burjassot, 46100 Valencia, Spain
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
