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aWith permission from Atlas (1984).

surface acts as an anode in an electrochemical reaction and is transformed to Fe2+. An equivalent number of H+ ions are produced at the cathode site. The anaerobic, sulfate-reducing bacteria such as Desulfobacter and Desulfovibrio produce S2-, which reacts with Fe2+ to produce FeS. At the same time, hydroxyls from water react with H+ ions. The overall reaction is

The conditions required for the reaction include anaerobic sites at redox potentials less than -400 mV, a pH greater than 5.5, low free-O2 content, and the presence of SO4". Under these conditions, an Fe pipe of 3-mm wall thickness can be corroded through in 5 to 7 years. This is one of the most costly microbial reactions in nature. It means that buried Fe pipes must be continually replaced or else protected by wrapping with asphalt or plastic. Additional protection is obtained by maintaining a small electrical current along the pipe (cathodic protection) to prevent the formation of an electrode half-cell.

Other corrosion processes are accomplished by miscellaneous aerobic and facultative bacteria (Table 15.14). Metal pipes are not the only ones to be affected by microbially induced corrosion. Concrete is a moderately porous mixture of alkaline precipitates and mineral aggregate. Strong acids react with the alkaline materials, causing structural degradation. Concrete sewer pipes are subject to corrosion by a mutualism between autotrophs and heterotrophs and succession of acidophiles (Little et al., 2000; Fig. 15.12).

Sewage

FIGURE 15.12 Conceptual diagram of the mutualistic relationship between sulfur-reducing bacteria and thiobacilli that causes corrosion in concrete sewer pipes (with permission from Little et al., 2000).

Sewage

FIGURE 15.12 Conceptual diagram of the mutualistic relationship between sulfur-reducing bacteria and thiobacilli that causes corrosion in concrete sewer pipes (with permission from Little et al., 2000).

Anaerobic conditions in the sewage support sulfur-reducing bacteria that convert sulfate and organic S to H2S, which volatilizes to the sewer atmosphere. The H2S redissolves in condensate upon reaching the sewer crown, where a second community of aerobic microorganisms including thiobacilli oxidizes it to corrosive H2SO4. The organism involved in the latter step was called Thiobacillus concre-tivorous when isolated, but was found to be Acidithiobacillus thiooxidans.

conclusion: microorganisms as unifiers of elemental cycles in soil

Soil microorganisms not only have a profound influence on the fluxes and cycling of individual element cycles, but also are strong integrators of the various element cycles. They integrate C, N, S, P, and various metal element cycles through biomass production in two related ways: (1) the stoichiometry of biomass production and (2) the selection of alternative electron acceptors under different redox conditions.

Stoichiometric relationships between substrates and decomposers may provide a framework to understanding how the internal cycling of nutrient elements in soil is controlled. It would follow that the release of inorganic forms of N, S, and P would occur following the oxidation of C to CO2 in ratios similar to those present in the organic component from which the C was derived. The frequent reporting of average nutrient concentrations and stoichiometric ratios of organisms and soil (e.g., Table 15.3) suggests that these values are reasonably constant and the approach sound. However, the stabilization of organic materials containing N, S, and P in soil is strongly influenced by the soil matrix itself through various physical and chemical protection mechanisms. The protection afforded by the inorganic matrix is a distinguishing feature of the soil environment compared to aquatic systems. Consequently, while biomass stoichiometry may be a dominant control on nutrient cycling in aquatic systems (e.g., Redfield, 1958), its potential control is shared with overriding protection mechanisms in terrestrial systems.

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