Rumen microbial degradation of bromoform from red seaweed (Asparagopsis taxiformis) and the impact on rumen fermentation and methanogenic archaea

BACKGROUND: The red macroalgae Asparagopsis is an effective methanogenesis inhibitor due to the presence of halogenated methane (CH(4)) analogues, primarily bromoform (CHBr(3)). This study aimed to investigate the degradation process of CHBr(3) from A. taxiformis in the rumen and whether this process is diet-dependent. An in vitro batch culture system was used according to a 2 × 2 factorial design, assessing two A. taxiformis inclusion rates [0 (CTL) and 2% DM diet (AT)] and two diets [high-concentrate (HC) and high-forage diet (HF)]. Incubations lasted for 72 h and samples of headspace and fermentation liquid were taken at 0, 0.5, 1, 3, 6, 8, 12, 16, 24, 48 and 72 h to assess the pattern of degradation of CHBr(3) into dibromomethane (CH(2)Br(2)) and fermentation parameters. Additionally, an in vitro experiment with pure cultures of seven methanogens strains (Methanobrevibacter smithii, Methanobrevibacter ruminantium, Methanosphaera stadtmanae, Methanosarcina barkeri, Methanobrevibacter millerae, Methanothermobacter wolfei and Methanobacterium mobile) was conducted to test the effects of increasing concentrations of CHBr(3) (0.4, 2, 10 and 50 µmol/L). RESULTS: The addition of AT significantly decreased CH(4) production (P = 0.002) and the acetate:propionate ratio (P = 0.003) during a 72-h incubation. The concentrations of CHBr(3) showed a rapid decrease with nearly 90% degraded within the first 3 h of incubation. On the contrary, CH(2)Br(2) concentration quickly increased during the first 6 h and then gradually decreased towards the end of the incubation. Neither CHBr(3) degradation nor CH(2)Br(2) synthesis were affected by the type of diet used as substrate, suggesting that the fermentation rate is not a driving factor involved in CHBr(3) degradation. The in vitro culture of methanogens showed a dose-response effect of CHBr(3) by inhibiting the growth of M. smithii, M. ruminantium, M. stadtmanae, M. barkeri, M. millerae, M. wolfei, and M. mobile. CONCLUSIONS: The present work demonstrated that CHBr(3) from A. taxiformis is quickly degraded to CH(2)Br(2) in the rumen and that the fermentation rate promoted by different diets is not a driving factor involved in CHBr(3) degradation.