Aerobic-Anaerobic Interfaces

There has been a long controversy in the literature concerning the lowest O2 concentrations where denitrification can occur (Robertson and Kuenen, 1984). Some believe it can occur at aerobic-anaerobic interfaces (Christensen and Tiedje, 1988 Bonin et al., 1989), while others believe that low O2 conditions inhibit synthesis of the essential enzymes for denitrification (Payne, 1976 Kapralek et al., 1982). It has also been speculated for some time that anoxic microsites may allow for the occurrence of denitrification in aerobic environments (Jannasch, 1960). Laboratory studies indicate that there are differential effects of O2 on the different steps of denitrification, whereby the NO3- reduction step is less sensitive than N( >7 or N2O reduction (Bonin and Raymond, 1990). 

The factors that influence the flux of energy through aerobic-anaerobic interface ecosystems have been the focus of this review chapter. Here we consider the net effect of these influences on carbon metabolism in marine and freshwater ecosystems. Thamdrup (2000) recently compiled studies that reported the relative contributions of O2 reduction, Fe(III) reduction, and SO4 reduction to carbon metabolism in marine ecosystems (n = 16). On average, the dominant pathway was SO4 reduction (62 17%, X SD). Aerobic respiration and Fe(III) respiration contributed equally to carbon metabolism (18 10% and 17 15%, respectively). Compared to previous compilations, —50% of the amount... 

Roden E. E., Sobolev D., Glazer B., and Luther G. W. (in press) New insights into the biogeochemical cycling of iron in circumneutral sedimentary environments potential for a rapid microscale bacterial Fe redox cycle at the aerobic-anaerobic interface. In Iron in the Natural Environment Biogeochemistry, Microbial Diversity, and Bioremediation (eds. J. D. Coates and C. Zhang). Kluwer, (in press). 

Molecular oxygen regulates both aerobic and anaerobic microbial metabolisms (Bodelier, 2003). Because of intense competition for O2, aerobic rhizosphere bacteria may be adapted to low O2 concentrations and periods of anoxia. Competition for O2 among microbial species, and between biotic and abiotic processes, has received relatively little attention (Laanbroek, 1990) but is likely to influence rhizosphere oxidation rates. Aerobic processes that occur in the wetland rhizosphere are those found in other aerobic-anaerobic interface environments and... 

Pimenov, N.V., Rusanov, I.I., Yusupov, S.K., Fridrich, j., Lein, A., Wehrli, B. and Ivanov, M.V. (2000) Microbial processes at the aerobic-anaerobic interface in the deep-water zone of the Black Sea. Microbiology 69, 435 48. 

The accumulation of reduced compounds in the anaerobic or reduced soil layer results in the establishment of concentration gradients across the aerobic-anaerobic interface. The concentration of reduced compounds is usually higher in the anaerobic layer, which results in upward diffusion into the aerobic soil or floodwater where they are oxidized. Similarly, some of the dissolved oxidized compounds diffuse downward, that is, from the floodwater or aerobic soil layer into the underlying anaerobic soil layer, where they will be reduced. For example, the steep gradients in ammonium concentrations in the soil profile are due to diffusion into aerobic soil layer or floodwater, and subsequent oxidation to nitrate (Figure 3.11). The nitrate formed diffuses downward into the anaerobic soil layer and is consumed as electron acceptor by microorganisms (Figure 3.12). Similar oxidation... 

Oxygen supply in wetlands is restricted to the water column and to a thin layer of surface soil. Oxygen is also transported by wetland macrophytes to their root zone, resulting in the creation of aerobic conditions on root snrfaces (see Chapters 3 and 6 for a detailed discussion on aerobic-anaerobic interfaces in wetlands). 

FIGURE 6.16 Schematic showing aerobic-anaerobic interfaces in a wetland soil. 

FIGURE 6.17 Aerobic-anaerobic interfaces in a wetland soil dissolved oxygen concentration in soil and water column. (D Angelo, E. M., and Reddy, K. R., unpublished results.)... 

Once the reductants present in the oxidized root zone are depleted, subsequent oxidation of these compounds will depend on the diffusive resupply across the aerobic-anaerobic interface. The ability of wetland plants to transport oxygen has attracted biologists and engineers to include this process into designing constructed wetlands for wastewater treatment. 

Existence of aerobic-anaerobic interface at the soil-water interface and in plant rhizo-sphere, which provides condition for removal of a wide range of pharmaceuticals, some of these degrade best under aerobic condition and others under anaerobic condition... 

Nitrification-denitrification reactions are the major pathway for N loss from wetland ecosystems to the atmosphere. The process involves a series of sequential microbial processes that include mineralization of organic nitrogen to ammonium, oxidation of ammonium to nitrate, and denitrification of nitrate to nitrous oxide or dinitrogen gas. Nitrification-denitrification in wetlands occurs primarily in two zones the aerobic-anaerobic interface at the surface of the flooded soil or sediment, and the oxidized rhizosphere of wetland plants (Figure 16.6). 

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