Aerobic-anaerobic interface oxygen

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... 

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 6.17 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 (D Angelo, E. M., and Reddy, K. R., unpublished results.)... 

FIGURE 6.18 Aerobic-anaerobic interfaces in a wetland soil dissolved oxygen concentration in a saturated... 

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. 

SRB, a diverse group of anaerobic bacteria isolated from a variety of environments, use sulfate in the absence of oxygen as the terminal electron acceptor in respiration. During biofilm formation, if the aerobic respiration rate within a biofilm is greater than the oxygen diffusion rate, the metal/biofilm interface can become anaerobic and provide a niche for sulfide production by SRB. The critical thickness of the biofilm required to produce anaerobie conditions depends on the availability of oxygen and the rate of respiration. The corrosion rate of iron and copper alloys in the presence of hydrogen sulfide is accelerated by the formation of iron sulfide minerals that stimulate the cathodic reaction. 

The flow in sanitary sewers may be controlled by gravity (gravity sewers) or pressure (pressure sewers). In a partially filled gravity sewer, transfer of oxygen across the air-water interface (reaeration) is possible, and aerobic heterotrophic processes may proceed. On the contrary, pressurized systems are full flowing and do not allow for reaeration. In these sewer types, anaerobic processes will, therefore, generally dominate. 

Exchange of volatile compounds across the air-water interface, e.g., oxygen (reaeration that affects aerobic or anaerobic conditions) and release of odorous substances Release of odorous substances to the urban atmosphere and change of reaeration due to a lower atmospheric oxygen concentration Extent of the processes... 

The biodegradation of crude oils in their reservoirs is well documented (M, 22). It occurs in the presence of meteoric water which supplies dissolved oxygen and nutrients including phosphate and fixed nitrogen. Microenvironments may exist in which aerobic and anaerobic activities occur in close proximity so that intermediates of aerobic metabolism may become substrates for anaerobic bacteria. In reservoirs, microbes are most active at the oil-water interface and at temperatures between about 20° and 60 to 75°C. 

Pore-water concentration profiles of redox-sensitive ions (nitrate, Mn, Fe, sulphate and sulphide) and nutrients (ammonium and phosphate) demonstrate the effects of degradation of OM. In freshwater sediments, the redox zones generally occur on a millimetre to centimetre scale due to the high input of reactive OM and the relatively low availability of external oxidators, especially nitrate and sulphate, compared to marine systems. A typical feature for organic-rich freshwater sediments deposited in aerobic surface waters, is the presence of anaerobic conditions close to the sediment-water interface (SWI). This is indicated by the absence of dissolved oxygen and the presence of reduced solutes (e.g. Mn, Fe and sulphides) in the pore water. Secondary redox reactions, like oxidation of reduced pore-water and solid-phase constituents, and other postdepositional processes, like precipitation-dissolution... 

The thickness of the aerobic layer varies from <1 mm to 3 cm. In relation to anaerobic soil volume, the aerobic soil volume at the soil-floodwater interface is small. However, this thin aerobic interface in the proximity of anaerobic soil is key to many unique biogeochemical processes functioning in wetlands. The differentiation of a wetland soil or sediment into two distinct zones as a result of limited oxygen penetration into the soil was first described by Pearsall and Mortimer (1939) and Mortimer (1941). 

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. 

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