Microbial activity promotes the enrichment of cobalt over nickel on hydrogenetic ferromanganese crusts

<p>The different mineral phases of the ferromanganese (Fe–Mn) crusts stem from the interaction of biotic and abiotic components. It is therefore vital to study the activity of these components to decipher their contribution to the enrichment/depletion of metals in the crust. Thus, the present study examined sorption and release of Co and Ni by Fe-Mn crusts with associated microbial communities in the presence and absence of the metabolic poison sodium azide (15 mM). The study was conducted in the presence (G⁺) and absence (G⁻) of added glucose (0.1%) at temperatures of 4 ± 1°C and 28 ± 2°C. Results showed that the microbial community had maximal sorption of Co of 66.12 µg g⁻¹ at 4 ± 1°C in the absence of added glucose and 479.75 µg g⁻¹ at 28 ± 2°C in the presence of added glucose. Maximum sorption of Ni in the absence of added glucose was 1.89 µg g⁻¹ at 4 ± 1°C and release of Ni was 51.28 µg g⁻¹ in the presence of added glucose. Under abiotic conditions with 15 mM sodium azide as a metabolic inhibitor, significant amounts of Co and Ni were released in the G⁺ medium. Total cell counts on the Fe-Mn crust in the presence of added glucose increased by an order of magnitude from 10⁶ to 10⁷ cells g⁻¹ and in the absence of added glucose remained within the order of 10⁶ cells g⁻¹ irrespective of temperature of incubation. Microscopic observation of the samples from biotic incubations showed numerous bacterial cells, exopolysaccharides, and structures resembling secondary minerals formed by bacteria. The results indicate that bacteria promote the enrichment of Co and Ni on the hydrogenetic Fe-Mn crusts by sorption processes and release of Ni by reductive dissolution of the oxides. The higher enrichment of Co than Ni is attributed to the way in which microbes interact with the metals.</p