Formation of cadmium sulfide and metallic cadmium crystallites on the surface of cyanobacterium Nostoc muscorum

The mechanism of binding cadmium ions by Nostoc muscorum cells and the effe cts of cadmium on the structural-functional organization of this cyanobacte rium were investigated. Particles of various dimension and shape were revea led with light and electron microscopy on the mucilaginous sheath of cyanob acterium incubated with Cd(NO3)(2). The chemical composition of the particl es was determined from the energy-dispersive X-ray spectra, and the phase s tate of the particles was assessed from the electron diffraction patterns. It is shown that crystallites of cadmium sulfide (CdS), metallic cadmium (C d-0), calcite, and metallic iron are formed on the surface of trichomes. Th e CdS crystallites of variable dimensions and shapes were formed in the pre sence of cadmium upon incubation of N. muscorum both in darkness and in lig ht, whereas the Cd-0 crystallites were only formed under illumination. In t he presence of cadmium, the colonies of N. muscorum gradually became more c ompact, the trichomes shortened, and the cells reduced in size. Furthermore , the pigment composition was altered, and the energy transfer between the pigments was disturbed. Cyanobacteria died after 3-4 weeks of incubation wi th cadmium. According to atomic absorption spectrophotometry, the concentra tion of cadmium in the medium reduced by 19% within the first minute of inc ubation and did not substantially change upon longer treatment. The cadmium -binding capacity of N. muscorum cells was equal to 130 mg of Cd2+ per gram dry wt. It is concluded that N. muscorum cells can accumulate cadmium ions from aqueous media, binding these ions on the cell surface and transformin g them to less toxic microparticles of CdS and Cd-0. The possible mechanism of CdS and Cd-0 formation is discussed.