Effects of coenzyme M and methyl-coenzyme M on the efficacy of inhibitors of methanogenesis in rumen cultures

Background Understanding the mechanisms of action of compounds inhibiting methanogenesis can aid understanding the variation in their efficacy as feed additives for mitigating enteric methane (CH ₄ ) emissions from ruminants. 2-Bromoethanesulfonate (BES) and 3-nitrooxypropanol (3-NOP) inhibit methanogenesis by acting as structural analogs of methyl-coenzyme M (methyl-CoM), a methyl donor in the last reaction of methanogenesis. We hypothesized that a high concentration of methyl-CoM and its nonmethylated form, coenzyme M (CoM), would block the antimethanogenic effects of BES and 3-NOP in mixed rumen cultures and would not affect bromoform (BMF), which acts through a different mechanism. This hypothesis and the possible underlying mechanisms were examined in one ruminal serial culture and three batch culture experiments. Results Coenzyme M and methyl-CoM added at 1 mM blocked the inhibition of methanogenesis by BES but not by 3-NOP or BMF. 2-Bromoethanesulfonate strongly decreased the relative abundance of Methanobrevibacter ruminantium but did not affect other methanogens. Similar to CH ₄ production, the effect of BES on the relative abundance of M. ruminantium was reversed by CoM and methyl-CoM. Because M. ruminantium cannot synthesize CoM and must take up exogenous CoM, the sensitivity of methanogens to BES may depend on them lacking the genetic capacity to synthesize CoM, as BES and CoM compete for transmembrane transport. In contrast, 3-NOP diffuses across cell membranes and does not compete for transmembrane transport with CoM, which is consistent with its effects not being reversed by CoM or methyl-CoM. Bromoform does not act competitively with CoM or methyl-CoM so that the addition of CoM or methyl-CoM did not reverse the effects of BMF. Conclusions These results may explain previous observations of lack of persistence of BES in vivo and in continuous cultures. Long-term BES supplementation is thought to shift the archaeal community towards methanogens that do not require exogenous CoM and are tolerant to BES. The effects of 3-NOP and BMF were unaffected by the addition of CoM and methyl-CoM presumably because they can diffuse across cell membranes, and in the case of BMF, the inhibition of methyl-tetrahydromethanopterin: coenzyme M methyltransferase may not be relieved by CoM or methyl-CoM.