Modelluntersuchungen zum Mechanismus der bakteriellen Eisenreduktion in Hydromorphen Böden

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.