Ocean manganese nodules: Biogenesis and bioleaching possibilities

Marine manganese nodules harbor a mixed population of manganese-oxidizing a nd -reducing bacteria, as well as bacteria that do not act on manganese. Th ey also host phagotrophic protozoa. Out of several laboratory studies, one line of investigation indicates that some kinds of bacteria found on nodule s can catalyze the oxidation of Mn2+ bound to Mn(IV) oxide at the surface o f nodules at in situ temperature and pressure. In the absence of appropriat e bacteria, the rate of oxidation of the surface-bound Mn2+ is significantl y slower. How Fe(lll) is incorporated is still unclear. Base metals such as Cu, Ni and Co, which are usually found in marine nodules in various quanti ties, may be incorporated through scavenging by Mn(IV) oxide. Laboratory ev idence shows that some other bacteria on nodules can reduce the Mn(IV) oxid e to Mn2+ in the presence of an appropriate source of reducing power in the form of organic carbon. The reduction of Mn(IV) oxide mobilizes manganese and trace metals such as Cu, Ni, and Co in the nodules. These bacteria redu ce Mn(IV) whether oxygen is present or not. They could be harnessed in the bioleaching of nodules. The manganese(lv)-reducing bacteria and the bacteri a that neither oxidize nor reduce manganese are thought to keep the organic matter in the nodule environment sufficiently low to favor bacterial manga nese-oxidation, which in laboratory tests is inhibited by excess organic ca rbon. The preying by the protozoa on the different kinds of bacteria is tho ught to control the bacterial population on nodules and ensure continued gr owth of the manganese-oxidizing bacteria and the nodules. To eliminate inte rference from seawater salts in the bioleaching of Mn, Cu, Ni and Co, it ma y be more advantageous to use terrestrial Mn(IV)-reducing bacteria than the Mn(IV)-reducing bacteria resident on nodules. The terrestrial bacteria wou ld allow bioleaching in fresh water. Unlike the marine bacteria, most terre strial bacteria studied to date reduce Mn(IV) oxide only anaerobically: A g eneral outline for an anaerobic bioleach process of marine nodules with fre shwater bacteria is discussed.