Manganese biotic reduction

Based on critical reviews, Lovley (1991, 2004) concluded that there are potential mechanisms for the abiotic reduction of Fe(III) and Mn(IV), but the significance of this process is minimal as compared to biotic reduction catalyzed by microbial activities. Typically, the end products of Fe(II) and Mn(II) are measured as indicators of the biotic and abiotic reduction of Fe(III) and Mn(IV) in anaerobic environments. The reduction of Fe(III) and Mn(IV) as a function of Eh is shown in Figures 10.10 and 10.11. Sodium acetate extractable iron and manganese in anaerobic soils represents Fe(II) and Mn(II), end products of reduction. As expected, extractable Mn(II) and Fe(II) concentrations are low nnder oxidized conditions and increase with a decrease in the Eh of soil. The accumulation of Mn(II) occurs at higher Eh values than the accumulation of Ee(II), suggesting Mn(IV) reduction precedes Fe(III) reduction. Because the reduction of Ee(III) and Mn(IV) occurs... 

Similar to the possibility of concurrent reduction of sulfate and ferric iron by a culture of a single bacteria (Coleman et al. 1993 see section 7.4.3.4) other iron reducing bacteria were found to additionally maintain dissimilation with more than one electron acceptors under suboxic conditions (Lovley and Phillips 1988) or even under oxic conditions (Myers and Nealson 1988a). In the presence of Fe(III) and Mn(IV) strain MR-1 was found to reduce both but additional manganese reduction occurred due to the immediate abiotic reaction with released Fe (Myers and Nealson 1988b). The interactions of biotic and abiotic reactions are shown in Fig. 7.17. 

Wetlands exhibit distinct redox gradients between the soil and overlying water column and in the root zone (Chapter 4), resulting in aerobic interfaces. For example, the aerobic layer at the soil-floodwater interface is created by a slow diffusion of oxygen and the rapid consumption at the interface. The thin aerobic layer at the soil-floodwater interface and around roots functions as an effective zone for aerobic oxidation of Fe(ll) and Mn(II). Below this aerobic layer there exists the zone of anaerobic oxidation of Fe(ll) and Mn(ll) and reduction of Fe(III) and Mn(IV). The juxtaposition of aerobic and anaerobic zones creates conditions of intense cycling of iron and manganese mediated by both biotic and abiotic reactions. 

Unique characteristics of ferromanganese nodules and associated oxidation-reduction reactions have been used by soil scientists as morphological indicators to help identify hydric soils (see Chapter 3). These characteristics are termed by soil scientists as redoximorphic features however, various terms such as redox concentrations, redox depletions, and reduced matrix are synonymously used for the oxidation-reduction of iron and manganese and their respective concentrations. We prefer not to define these characteristics as redoximorphic features because oxidation-reduction reactions not only involve iron and manganese but also a range of elements that support biotic communities in the biosphere. 

Explain the difference between biotic and abiotic iron and manganese reduction. Which is the dominant process ... 

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