Inhibitory effects of sulfur compounds on methane oxidation by a methane-oxidizing consortium

Kinetic and enzymatic inhibition experiments were performed to investigate the effects of methanethiol (MT) and hydrogen sulfide (H₂S) on methane oxidation by a methane-oxidizing consortium. In the coexistence of MT and H₂S, the oxidation of methane was delayed until MT and H₂S were completely degraded. MT and H₂S could be degraded, both with and without methane. The kinetic analysis revealed that the methane-oxidizing consortium showed a maximum methane oxidation rate (V_max) of 3.7 mmol g-dry cell weight (DCW)^-1 h^-1 and a saturation constant (K_m) of 184.1 μM. MT and H₂S show competitive inhibition on methane oxidation, with inhibition values (K_i) of 1504.8 and 359.8 μM, respectively. MT was primary removed by particulate methane monooxygenases (pMMO) of the consortium, while H₂S was degraded by the other microorganisms or enzymes in the consortium. DNA and mRNA transcript levels of the pmoA gene expressions were decreased to ~10⁶ and 10³ pmoA gene copy number g-DCW^-1 after MT and H₂S degradation, respectively; however, both the amount of the DNA and mRNA transcript recovered their initial levels of ~10⁷ and 10⁵ pmoA gene copy number g-DCW^-1 after methane oxidation, respectively. The gene expression results indicate that the pmoA gene could be rapidly reproducible after methane oxidation. This study provides comprehensive information of kinetic interactions between methane and sulfur compounds.