| Abstract: | Model experiments on the mechanism of bacterial iron-reduction in water-logged soils In model experiments, iron-reduction with Bacillus polymyxa (nit+) and Clostridium butyricum (nit?) was followed in relation to the development of pH, Eh and glucose fermentation. In parallel tubes, the influence of NO, (KNO,) and/or Mn02-Powder (Merck) on Fe(I1) formation and rH (calculated from pH and Eh) was measured. All tubes were incubated anaerobically (N2/C02 = 9/1) at 30°C. Both with Bacilluspolymyxa (nit+) as well as with Clostridium butyricum (nit?), the reduction-intensity rH decreased rapidly reaching entirely reducing conditions (rH = 0) within 2–3 days. In both cases iron-reduction and glucose utilization developed sigmoidal and miror symmetrically (Fig. 1) In the case of B. polymyxa (nit+) the addition of NO, and/or Mn02 suppressed iron-reduction nearly entirely. With C. butyricum, only Mn02 (but not nitrate) affected iron-reduction significantly. Nitrate remained unchanged throughout the incubation period, although completely reducing conditions (rH = 0) were obtained (Fig. 2). Apparently, nitrate, Mn02 or Fe203 are reduced directly and specifically rather than indirectly as a consequence of reducing metabolites and/or by a lowered redox potential in the environment. If nitrate, Mn02 and Fe203 are reduced chemically by reducing condition, the reduction sequence nitrate → Mn(1V)-oxides → Fe(II1)-oxides should occur in relation to pH and Eh (rH) and independently of the type of organism (nit+ or nit?) in question. This view is rejected by the results presented. With nit+ bacteria such as B. polymyxa, the enzyme nitrate reductase seems to act as one mechanism of iron- and manganese-reduction. However, with C. butyricum (nit?), another, so far unknown enzym system (ferrireductase?), should be made responsible for iron- and manganese-reduction. |
