Vertical distribution of photosynthetic sulphur bacteria linked to saline gradients in Lake 'El Tobar' (Cuenca, Spain)

The meromictic lake 'El Tobar' was sampled at 3 time points during the annu al cycle. coinciding with the thermal stratification period. The photosynth etic microbial community was composed of mixolimnetic oxygenic phototrophs, which were distributed through the mixolimnion, phycoerytrine-containing u nicellular cyanobacteria and sulphur phototrophic bacteria placed at the ha locline. During the stratification period anoxic conditions, formerly confi ned to the monimolimnion, moved 1 m upwards. reaching freshwater positions. Consequently, photosynthetic bacteria were re-arranged into a double purpl e-green structure placed just above and embedded in the chemocline. In the freshwater layers we found a population of the purple sulphur bacterium Chr omatium minus placed just above a population of the brown-colored green sul phur bacterium Chlorobium phaeobacteroides. A few centimeters below we foun d Lamprocystis modestohalophilus overlying Chlorobium vibrioforme. This pec uliar structure was probably due to anoxic conditions reaching upper positi ons at the end of the stratification period, thus allowing the growth of fr eshwater phototrophic bacteria. The distribution of the phototrophic bacter ial populations reveals a Vertical structure which is strongly influenced b y salinity, whereas light intensity only determines the relative position o f purple and green bacteria.