Microbial metabolites and the reductive dissolution of manganese oxides: Oxalate and pyruvate

Microorganisms may participate in the reductive dissolution of Mn oxides in sediments either directly, by using Mn oxides as a terminal electron acceptor, or indirectly, through the production and excretion of reductants and other molecules that perturb the chemical conditions in sediments. If the latter process is predominant, it should be possible to calculate rates of Mn mobilization from sediments from a knowledge of the kinetics of the reductive dissolution reaction, once the structures and concentrations of reductants found in sediments are known. Laboratory experiments show that synthetic Mn (III, IV) oxides are reduced and dissolved by the microbial metabolites oxalate and pyruvate at appreciable rates. This provides additional evidence that indirect involvement of microbes in the mobilization of Mn in sediments can occur. Rates of reductive dissolution increase as the pH is decreased; the effect of pH on dissolution rate is more pronounced for oxalate than for pyruvate. Ways in which chemical conditions in freshwater and marine sediments affect rates of Mn mobilization are discussed.