The microbial oxidation of methanesulfonic acid in the marine environment
The biogeochemical transformations related to methanesulfonic acid (MSA) formation and degradation are discussed, with reference to the role of marine bacteria and the phylogeny and biochemistry of methylotrophic bacteria are briefly reviewed. The aims of the work presented were [i] to isolate novel MSA utilising bacteria from both seawater and freshwater samples, [ii] to characterise these isolates and [iii] to elucidate the mechanisms by which MSA is metabolised in these isolates. Isolation procedures for the enrichment of MSA-oxidizing bacterial from a wide range of seawater and freshwater sites, are described. Four methylotrophic bacterial strains, TR3, PSCB4 (marine isolates), FW2 and FW6 (freshwater isolates), capable of growth on MSA as a sole carbon source were isolated from the environment. MSA metabolism in strains TR3 and PSCB4 was initiated by an inducible NADB-dependant monooxygenase, which cleaved MSA into formaldehyde and sulfite. Formaldehyde was assimilated via the serine pathway. Cell suspensions of bacteria grown on MSA completely oxidized MSA to carbon dioxide and sulfite with a MSA: Oxygen stoichiometry of 1.0: 2.0. Oxygen electrode-substrate studies indicated the dissimilation of formaldehyde to formate and C02 for energy generation. Methanol was not an intermediate in MSA metabolism, although the strains could grow on methanol and other one-carbon compounds, as well as a variety of heterotrophic substrates. Initial studies of strains FW2 and FW6 indicated that they probably metabolised MSA in a similar way to the marine strains. Carbon dioxide was not fixed by ribulose bisphosphate carboxylase in strains TR3 and PSCB4. These novel facultative methylotrophs have the ability to mineralize MSA and may play an important role in the cycling of global sulfur, since MSA can be a major product from the oxidation of DMS, the principal biogeochemical organosulfur gas in the environment.