Interaction between vitamin B12 and bromoform
Supplementation of ruminants with the bromoform-containing red alga Asparagopsis or pure bromoform preparations are potent strategies to mitigate enteric methane (CH4) emissions from ruminant livestock. It is known empirically that Asparagopsis and bromoform are less efficacious with high fiber diets, but the reasons explaining this interaction with diet composition are unknown. Bromoform and other CH4 halogenated analogs inhibit methanogenesis by reacting with cobamides that participate in the transfer of a methyl group from methyltetrahydromethanopterin: coenzyme M methyltransferase to coenzyme M in methanogenesis. Given that the rumen of ruminants consuming high forage diets has greater concentrations of vitamin B12 compared to diets that are high in concentrate, it is possible that Asparagopsis and bromoform preparations are less efficacious with high forage diets because there is more vitamin B12 to react with bromoform. We conducted one serial culture, and three batch cultures experiments, to examine the hypothesis that vitamin B12 can impair the efficacy at inhibiting methanogenesis of bromoform, other CH4 halogenated analogs, and Asparagopsis. Experiment 1 showed that vitamin B12 supplemented to rumen cultures in serial transfers 1 to 4 at 8.86 mg/L or spiked at 422 mg/L in serial transfer 5 can markedly reverse the inhibition of methanogenesis by bromoform supplemented at 0.50 uM. Bromoform increased the relative abundance of Methanobrevibacter spp. and vitamin B12 reversed this increase. In Experiments 2 and 3, vitamin B12 at a maximal concentration of 500 and 10 mg/L, respectively, did not impair the inhibition of methanogenesis by bromoform at 5.0 and 0.50 uM, in the same order. In Experiment 4, partial inhibition of methanogenesis by Asparagopsis was slightly reversed by vitamin B12 at 20 mg/L. Overall, the comparison of rumen concentrations of vitamin B12 and total cobamides reported in previous studies with the minimum concentrations of added vitamin B12 that we found could influence bromoform or Asparagopsis efficacy in our experiments, suggests that vitamin B12 and total cobamides are unlikely to influence the efficacy of Asparagopsis or bromoform preparations at inhibiting methanogenesis in vivo. However, results on the in vivo temporal dynamics of bromoform, dibromomethane, and bromomethane in the rumen of animals supplemented Asparagopsis and bromoform preparations are required to contrast with our results to fully evaluate this possibility.