The role of Methanomassiliicoccales in trimethylamine metabolism in the rumen of dairy cows

A considerable amount of trimethylamine (TMA) is likely generated in the rumen; however, its metabolism is still unclear. This study aimed to investigate the role of Methanomassiliicoccales (Mmc) in TMA metabolism in the rumen of dairy cows. Three experiments, two rumen in vitro fermentation trials and one dairy cow in vivo trial, were conducted. Four groups were set in Experiment 1: control, nitroglycerin (NG, a methanogen inhibitor), TMA (7.2 mmol/L), and TMA + NG. The methanogenic activity was completely inhibited in the NG group, and no methane production was observed in the NG and TMA + NG groups. The TMA content hardly reduced in the TMA + NG group (6.9 mmol/L) following a 2 d-incubation; in contrast, it demonstrated a significant reduction by 47.2% in the TMA group. Methanogen 16S rRNA gene sequencing and real-time PCR showed that the relative abundance of Mmc increased in the TMA group (P = 0.005). The increase was mainly attributed to two species-level taxa, Group 9 sp. ISO4-G1 and Group 10 sp. Four groups were set in Experiment 2: control, NG, choline (choline chloride, 7.2 mmol/L), and choline + NG. Choline was completely degraded in 24 h, and the TMA content reached the peak point (7.3 mmol/L) in the fermentation culture. The TMA content remained relatively stable in the choline + NG group following the peak point. However, it started to decrease after 24 h in the choline group, corresponding to the rapid increase in methane production and the abundance of Mmc. Eight mid-lactating, rumen-fistulated Holstein cows were randomly assigned to the control (n = 4) or choline (n = 4) group in Experiment 3: In the choline group, cows were gradually supplemented with 100–250 g/(cow·d) of choline chloride over 4 weeks. Compared to the control group, TMA accumulated in the rumen fluid, and the abundance of Mmc 16S rRNA gene and choline-degrading bacterial cutC gene increased in the rumen content in the choline group (P < 0.050). The trimethylamine N-oxide content in the plasma and milk of the dairy cows was approximately 10 times higher in the choline group than that in the control at the end of the experiment. These findings revealed that Mmc played an important role in the elimination of TMA in the rumen. The accumulation of TMA in the rumen would lead to a large amount of TMA absorbed into the blood stream of the dairy cows.