Time-depth distribution of the microbial anaerobic assemblage of Lake Cisó was analyzed by microscopy, pigment composition, and electrophoretic analysis of 5S rRNAs. Purple (Amoebobacter-like and Thiocystis minor-like cells) and green (Chlorobium-like) sulfur bacteria were very abundant. Both groups coexisted in depth and in time despite the fact that they compete for the same natural resources (e.g., light and sulfide). Cell abundance, group-specific pigment content, and group-specific 5S rRNA content did not change in parallel with depth. This was due to variations in the specific content of both RNA and pigments. Specific content of RNA was systematically higher in purple than in green sulfur bacteria. The latter, in turn, displayed a much higher pigment content. Specific content of both RNA and pigments changed with depth and time. Analysis of tRNA band patterns indicated no changes in the populations forming the assemblage. Changes in specific contents, therefore, were the result of physiological adaptations of the populations already present in the system. We concluded that each group of bacteria showed differential adaptations in both RNA and pigment content, and that the specific contents measured were good indicators of the physiological status of these bacteria in situ. The higher content of RNA in purple sulfur bacteria indicates that these organisms are the main contributors to anaerobic carbon fixation and sulfide oxidation processes in Lake Cisó.