Sulfate-Reducing Bacteria in Gypsum Karst Lakes of Northern Lithuania

Microbiological studies were performed in three small gypsum karst lakes in northern Lithuania, most typical of the region. Samples were taken in different seasons of 2001. The conditions for microbial growth in the lakes are determined by elevated content of salts (from 0.5 to 2.0 g/l), dominated by SO ₄ ^2? and Ca²⁺ ions (up to 1.4 and 0.6 g/l, respectively). The elevated sulfate concentration is favorable for sulfate-reducing bacteria (SRBs). Summer and winter stratification gives rise to anaerobic water layers enriched in products of anaerobic degradation: H₂S and CH₄. The lakes under study contain abundant SRBs not only in bottom sediments (from 10³ to 10⁷ cells/dm³) but also in the water column (from 10² to 10⁶ cells/ml). The characteristic spatial and temporal variations in the rate of sulfate reduction were noted. The highest rates of this process were recorded in summer: 0.95–2.60 mg S^2?/dm³ per day in bottom sediments and up to 0.49 mg S^2?/l per day in the water column. The maximum values (up to 11.36 mg S^2?/dm³) were noted in areas where bottom sediments were enriched in plankton debris. Molecular analysis of conservative sequences of the gene for 16S rRNA in sulfate-reducing microorganisms grown on lactate allowed them to be identified as Desulfovibrio desulfuricans.