Microbial mediation of Mn(II) and Co(II) precipitation at the O2/H2S interfaces in two anoxic fjords1

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Abstract
We studied the potential for bacterial catalysis of Mn(II), Co(II), and Fe(III) precipitation at the O2/H2S interfaces in Saanich Inlet, British Columbia, and in Framvaren Fjord, Norway. In Saanich Inlet the interface is below the photic zone, but in Framvaren Fjord it is within the region of significant light penetration. Biological catalysis of metal removal was measured as the binding of radioactive isotopes as a function of time in the presence and absence of poisons which do not measurably interfere with the solution chemistry of manganese. In a region just above the oxic/ anoxic interface in Saanich Inlet, the binding of Co(II) and Mn(II) was inhibited by the bacterial poisons, but Fe(III) precipitation was not. Measurements of the oxidation state of the particulate material indicated a slightly higher Mn(IV):Mn(II) ratio in the region of dissolved Mn(II) precipitation, confirming that manganese was being oxidized and not simply bound. In both fjords, binding was either stronger or more rapid under air saturation than under conditions of oxygen limitation, suggesting that O2 is the electron acceptor during manganese oxidation. In Framvaren Fjord a threefold increase in Mn(II) concentration [to 10 µM Mn(II)] severely inhibited both Mn(II) and Co(II) binding. A 25‐ to 250‐fold increase in Co(II) concentration [to 0.18 or 1.8 µM Co(II)], on the other hand, completely inhibited Co(II) binding, but the highest concentration inhibited Mn(II) binding by only 40%. The results suggest microbial mediation of Co(II) and Mn(II) precipitation in these environments and that the mechanisms of these processes are not the same.