Interactions between microbial processes and geochemical transformations under anaerobic conditions: a review

Anaerobiosis in soil can have undesirable environmental effects on the soil, the deep vadose zone, the aquifer and the atmosphere. These effects include the mobilisation of metals and the emission of greenhouse gases resulting from microbial processes interacting closely with geochemical transformations. The main pathways of anaerobic microbial catabolism are respiration, fermentation, acetogenesis and methanogenesis. On the one hand, geochemical properties of the solution affect microbial activities through substrate availability, thermodynamic regulations and inhibition. On the other hand, microbial activities affect the pH of the soil solution, its reduction/oxidation level, the complexation of metals with volatile fatty acids, and solid particles by modulating adsorption/desorption and dissolution/precipitation reactions. However, some of the solid phases (e. g. calcite, metal oxides and oxyhydroxides) can partially buffer the evolution of the solution. Abiotic transformations can also partly regenerate terminal electron acceptors used by micro-organisms. Only a few models combine microbial and geochemical transformations. Most of them were initially proposed to describe anaerobic biodigestors. In this specific case, geochemical transformations are restricted to acid/base reactions, and reactions between solids and the solution are ignored. Various environmental problems could benefit from improved biogeochemical models, which have to distinguish explicitly between microbial and abiotic transformations because of their kinetic behaviour and their different causal factors.