Ferromanganese oxide deposits: Geochemical and microbiological perspectives of interactions of cobalt and nickel

Bacterial participation in the formation of ferromanganese (Fe-Mn) mineral deposits is highly debated, as the precipitation of metals could occur by both chemical and biologically mediated mechanisms. Besides, the enrichment of trace metals like Co and Ni in Fe-Mn deposits is further complicated by the aging process of the crust, scavenging by the Fe-Mn oxide surfaces and co-precipitation by bacterial activity. The concentration of Co in Fe-Mn crusts varies from 0.1 to 1.9% and Ni from 0.2 to 1.1% in individual bulk crusts implying marked heterogeneity in their composition. Several reviews address different aspects like the molecular mechanisms of Co and Ni uptake in bacteria, enzymes containing Ni and Co and their relative toxicity. Nonetheless, only limited data are available on the interactions of Co and Ni with bacteria and how they contribute to the enrichment of these metals in the deep-sea mineral deposits. Hence, this review focuses on the geochemical and microbiological perspective of the interactions of Co and Ni, different modes of their enrichment and requirement for several physiological and biogeochemical processes in bacteria. The detailed survey of the studies indicates that both chemical and biological mechanisms promote the accretion and dissolution of minerals with bacteria acceler-ating the net oxidation-reduction reactions in their directions. Combined geochemical and microbiological in-teractions in conjunction with detailed molecular analysis would further confine processes affecting Fe-Mn mineral formation.