Bacteriology of manganese nodules: III. Reduction of MnO(2) by two strains of nodule bacteria.

MnO(2) reduction by aerobic growing cultures of Bacillus 29 and coccus 32, isolated from ferromanganese nodules, was assessed for 7 days. A 1-day lag was observed before the onset of MnO(2) reduction by either culture. Addition of HgCl(2) to a final concentration of about 10 M caused a rapid cessation of MnO(2) reduction by the growing cultures. Neither culture reduced MnO(2) when grown under continued anaerobiosis from the start of an experiment. However, if conditions were made anaerobic after MnO(2) reduction was initiated, reduction continued at a rate only slightly lower than that under aerobic conditions. Resting-cell cultures reduced MnO(2) equally well aerobically and anaerobically, provided that ferricyanide was present to serve as electron carrier. These findings showed that oxygen is needed for culture adaptation to MnO(2) reduction, and that oxygen does not interfere with microbial MnO(2) reduction itself. Both cultures caused sharp drops in the pH of the medium during MnO(2) reduction: with coccus 32, during the entire incubation time; with Bacillus 29, for the first 3 days. The E(h) of the medium fluctuated with either culture and never fell below 469 mv with Bacillus 29 and below 394 mv with coccus 32. The rates of glucose consumption and Mn release by Bacillus 29 and coccus 32 were fairly constant, but the rates of lactate and pyruvate production were not. Although acid production undoubtedly helped in the reduction of pyrolusite (MnO(2)) by the bacteria, it did not appear to be important in the reduction of manganese oxide in ferromanganese nodules, as shown by the results with a nodule enrichment.