Catabolism of Dimethylsulfide and Methane Thiol by Methylotrophic Methanogens

Abstract

We demonstrated the ability of several methylotrophic methanogens to degrade dimethylsulfide and methane thiol to hydrogen sulfide, methane, and carbon dioxide. This is the first report of the growth of a pure culture of methanogen on methane thiol. Methanolobus siciliae HI350, a methylotrophic methanogen isolated from an oil well, was grown routinely on trimethylamine. When this culture was inoculated into a medium with 5 mM dimethyl sulfide, it began producing methane and hydrogen sulfide after a lag of several weeks. Methane production was slow, with an apparent microbial growth rate of 0.0033 h^-1 (about 3% as fast as growth on trimethylamine or methanol). The lag was shorter when 2 mM methane thiol was substrate. When a culture of M. siciliae HI350 growing on dimethylsulfide was subcultured on dimethylsulfide, the lag disappeared and growth rate was higher (0.087 h^-1). Dimethylsulfide-grown cultures also had no lag when transferred to media with methane thiol. Studies of cell-free extracts suggested that enzymes for the degradation of dimethylsulfide and methane thiol were inducible, whereas those for the degradation of methanol and trimethylamine were constitutive. Degradation of dimethylsulfide or methane thiol was complete, and stoichiometric quantities of methane and hydrogen sulfide were formed. Most surprisingly, this strain could be adapted to grow with high concentrations of dimethylsulfide or methane thiol, as high as 30 mM. Other methanogens which have been reported to catabolize dimethylsulfide are Methanolobus siciliae T4/M, Methanohalophilus zhilinaeae WeN5, Methanohalophilus oregonensis WALL, and strain GS-16. The M. siciliae strains and strain GS-16 were also able to use methane thiol as their catabolic substrate (other strains were not tested). We tested other methylotrophic methanogens for their ability to use dimethylsulfide at concentrations which did not inhibit their ability to degrade trimethylamine and found the following cultures unable to catabolize dimethylsulfide: Methanococcoides methylutens TMA-10, Methanolobus tindarius Tindari 3, Methanolobus vulcani PL-12/M, Methanohalophilus mahii SLP, Methanohalophilus halophilus Z-7982, Methanosarcina mazeii LYC, and Methanosarcina mazeii C 16.