Accumulation of heavy metals by individually analyzed bacterial cells and associated nonliving material in polluted lake sediments

The microfloras of sediments control the biogeochemical cycling of heavy metals in natural waters, thereby determining, in large part, the ecological effects of the metals. However, little is known about interactions of metals with individual benthic microbes in nature owing to the technical difficulty of studying such small-scale processes in complex, heterogeneous sedimentary ecosystems. Seeking to overcome this obstacle, we used energy-dispersive X-ray microanalysis and transmission electron microscopy to investigate the accumulation of Cu and other elements by the cell walls, cytoplasm, cytoplasmic inclusions, fibrils, and mineral coatings of individual bacterial cells in the sediments of a lake polluted with heavy metals. Data from statistically meaningful numbers of specimens revealed various associations of elements with each other and with specific microstructures. The results imply (i) common occurrence of Fe and Mn oxyhydroxide coatings on cell walls and fibrils; (ii) strong sorption of Cu by these coatings and the inability of Ca, Mg, and K to displace the Cu from the binding sites; (iii) weaker but more extensive direct sorption of Cu by cell walls from which the Cu is readily displaced by Ca, Mg, and K; (iv) blockage of cell wall ligands by Si-, Al-, and Fe-bearing mineral deposits; and (v) preferential accumulation of Cu in the coatings or in cytoplasmic inclusions enriched in Fe and usually enriched in P as well (with or without measurable enrichment in Mn) with respect to the cytoplasm. Our findings help to elucidate the accumulation of heavy metals by microbes in aquatic. ecosystems and suggest the existence of diverse mechanisms of detoxification.