ADAPTATION TO SULFIDE AND TO THE UNDERWATER LIGHT-FIELD IN 3 CYANOBACTERIAL ISOLATES FROM LAKE ARCAS (SPAIN)

Three strains of cyanobacteria isolated from karstic Lake Areas were t ested for photosynthetic adaptations to soluble sulfide. One of them, AO11, was identified as Oscillatoria cf. ornata, and forms dense popul ations in the sulfide-rich anoxic hypolimnion of this lake. This cyano bacterium was able to perform sulfide-dependent anoxygenic photosynthe sis and its oxygenic photosynthesis was relatively insensitive to sulf ide. The other strains studied were AP1 and AO21, identified respectiv ely as Pseudanabaena sp, and Oscillatoria cf. tenuis, populations of w hich were present only in epilimnetic waters at low population densiti es. Pseudanabaena sp. also carried out anoxygenic photosynthesis, but oxygenic photosynthesis was totally inhibited by 0.5 mM sulfide. Oscil latoria cf, tenuis lost most of its oxygenic photosynthetic capacity w hen submitted to 0.1 mM sulfide and anoxygenic photosynthesis accounte d for less than 20% of sulfide-free controls. In addition to different photosynthetic capabilities, the three cyanobacteria exhibited differ ences in light-harvesting photosynthetic accessory pigments. Pigment a nalysis of cultures grown under different light conditions showed the capacity of Oscillatoria cf. ornata AO11 to produce phycoerythrin unde r low light intensity or under predominantly green light, while neithe r Pseudanabaena sp. AP1 nor Oscillatoria cf. tenuis AO21 produced this pigment. The complementary chromatic adaptation of Oscillatoria cf. o rnata correlates well with its summertime distribution under the dim l ight field of the hypolimnion. The distribution and abundance of speci fic cyanobacterial populations in Lake Areas can thus be explained by the interplay of light regime and presence of sulfide as some of the m ost determinant ecological parameters.