[Participation of extracellular polysaccharides in detoxication of cadmium ions by cyanobacteria Nostoc muscorum].

Using the methods of absorption and fluorescence spectroscopy, histochemistry and micrography, the influence of cadmium in different concentrations on the exudation of exopolysaccharides by Nostoc muscorum was studied to elucidate the possibility of distance detoxication of cadmium ions by the cyanobacterium with the participation of its exometabolites. The toxic action of Cd was controlled by changes in biomass, trichomes morphology, and the photosynthetic apparatus of the cyanobacterium. The content of exopolysaccharides in the extracellular medium was 0.7 +/- 0.01 gram glucose units per gram dry biomass at the culture density of 2 gram dry biomass per liter. In the initial phase of growth, the exopolysaccharide content decreased to 0.4 +/- 0.1 gram glucose units per gram dry biomass, while upon incubation with cadmium ions it increased several times during a week, after a short adaptation period. The kinetics of exopolysaccharide accumulation and their concentration depended on cadmium content in the medium. After the incubation of N. muscorum with Cd2+, the exometabolites and the mucous coat colored with ditizone, which was indicative of cadmium accumulation. The decomposition of the absorption spectra of the hydrolyzed extracellular metabolites showed that in the presence of cadmium ions, the dominant monosaccharide became glucosamine, which readily binds to Cd2+. It was concluded that the system of N. muscorum protection against cadmium includes the binding of the metal by cell structures, primarily, by the mucilaginous sheath and the distance detoxication by exopolysaccharides in the cultural medium. Cadmium induces the activation of the mucilaginous sheath protective function by changing its composition and the rate of renewal. To provide the distance protection of N. muscorum, the rate of polysaccharide release of the altered primary structure increased.