Why do sulfate-reducing bacteria outcompete methanogenic bacteria for substrates?

The apparent Ks values for H2 of several phylogenetically distant strains of both methanogenic bacteria and sulfate-reducing bacteria were measured. The sulfate reducers had Ks values of about 2 μM whereas the Ks values of the methanogens were 6-20 μM. This indicates that probably all sulfate-reducing bacteria have a higher substrate affinity for H2 than the methanogenic bacteria. Difference in substrate affinity can thus account for the inhibition of methanogenesis from H2 and CO2 in sulfate-rich ecosystems (mainly saltwater marshes), where the H2 concentration is well below 5 μM. Possible explanations for this general phenomenon are discussed.