Ecology of a deep-living Oscillatoria (=Planktothrix) population in the sulphide-rich waters of a Spanish karstic lake

In a three-year field survey which included some ecophysiological studies, the ecology of a deep living Oscillatoria (= Planktothrix) cf. ornata popul ation at the chemocline and in the hypolimnion of the sulphate-rich karstic Lake Areas was studied. This cyanobacterium, together with the phototrophi c sulphur bacterium, Chromatium weissei, develops dense populations in the anoxic waters of the lake during the summer stratification (maximal cyanoba cterial population densities were around 1,300 filaments/ml), both being ab sent from the water column during the mixing period. O. cf. ornata from Lak e Areas shows several adaptative features which help to explain the develop ment of these dense populations. Phycoerythrin, the main light-harvesting a ccessory pigment, allows this cyanobacterium to collect efficiently the yel low-green light reaching the chemocline, which can only be used via photosy stem II. Oxygen-evolving photosynthesis in this species is sulphide-resista nt, it can be sustained at the sulphide concentrations found at the chemocl ine of the lake. In addition, buoyancy regulation mediated by gas-vesicle f ormation and carbohydrate accumulation substantially overcomes sinking loss es in the population.