Adaptation to sulfide and to the underwater light field in three cyanobacterial isolates from Lake Arcas (Spain)

Three strains of cyanobacteria isolated from karstic Lake Arcas were tested for photosynthetic adaptations to soluble sulfide. One of them, AO11, was identified as Oscillatoria cf. ornata , and forms dense populations in the sulfide‐rich anoxic hypolimnion of this lake. This cyanobacterium was able to perform sulfide‐dependent anoxygenic photosynthesis and its oxygenic photosynthesis was relatively insensitive to sulfide. The other strains studied were AP1 and AO21, identified respectively as Pseudanabaena sp. and Oscillatoria cf. tenuis , populations of which were present only in epilimnetic waters at low population densities. Pseudanabaena sp. also carried out anoxygenic photosynthesis, but oxygenic photosynthesis was totally inhibited by 0.5 mM sulfide. Oscillatoria cf. tenuis lost most of its oxygenic photosynthetic capacity when submitted to 0.1 mM sulfide and anoxygenic photosynthesis accounted for less than 20% of sulfide‐free controls. In addition to different photosynthetic capabilities, the three cyanobacteria exhibited differences in light‐harvesting photosynthetic accessory pigments. Pigment analysis of cultures grown under different light conditions showed the capacity of Oscillatoria cf. ornata AO11 to produce phycoerythrin under low light intensity or under predominantly green light, while neither Pseudanabaena sp. AP1 nor Oscillatoria cf. tenuis AO21 produced this pigment. The complementary chromatic adaptation of Oscillatoria cf. ornata correlates well with its summertime distribution under the dim light field of the hypolimnion. The distribution and abundance of specific cyanobacterial populations in Lake Arcas can thus be explained by the interplay of light regime and presence of sulfide as some of the most determinant ecological parameters.