The role of light and concentration gradients in the vertical stratification and seasonal development of phototrophic bacteria in a meromictic lake

The seasonal population dynamics of phototrophic sulphur bacteria and their vertical stratification in Lake La Cruz were studied over two consecutive summers. Development of purple sulphur bacteria occurred just below the oxi c-anoxic boundary (the plate located at 14-15 m depth). The principal const ituent was a species of Amoebobacter, containing okenone as its main carote noid and which reached maximal densities at the end of summer (2.2 x 10(6) cells/ml and BChl.-a concentrations of 111 mu g/l in September 1987). Green phototrophic bacteria developed below the Amoebobacter layer at a mean dep th of 16 m. Pelodicyton clathratiforme was the dominant species, reaching m aximal cell concentrations of 3.1x10(6) cells/ml and BChl.-d concentrations of 362 mu g/l, also in September. Growth of these purple and green sulphur bacteria was light-limited, and sulphide was also present in very low conc entrations owing to the low sulphate content of the lake. The two thermal s tratification periods under study differed noticeably as a consequence of a ltered meteorological conditions and structural differences in the water co lumn, with a shallower oxic-anoxic boundary and a major development of alga l populations in the upper oxic layer during the second period. This reduce d the light available to deep phototrophic bacteria, which were unable to a ttain the biomass achieved in the earlier period. The monimolimnion of Lake La Cruz acts as a refuge for phototrophic bacteria during the mixing perio d and this is considered to be a key factor in understanding the ecology of these microorganisms.