Betaine Modulates Rumen Archaeal Community and Functioning during Heat and Osmotic Stress Conditions In Vitro.

Rumen archaea play an important role in scavenging ruminal hydrogen (H₂) and thus facilitate rumen fermentation. They require optimum temperature and osmolality for their growth and metabolism; however, a number of external factors may put archaea under heat and osmotic stress. Betaine is an osmolyte, molecular chaperone, and antioxidant; therefore, it bears potential to combat against these stressors. In this in vitro study, three betaine levels, namely, 0 (control), 51 (low), and 286 (high) ppm, were used. Each of these was subjected to two temperatures (39.5 and 42°C) and two osmolality conditions (295 and 420 mOsmol kg^-1) with n = 6 per treatment. Sequencing analyses of the solid phase (which use solid materials containing primarily fibrous materials of low-density feed particles) and the liquid phase (rumen fermenter liquid) using 16S rRNA revealed that more than 99.8% of the ruminal archaea in fermenters belong to the phylum Euryarchaeota. At the genus level, Methanobrevibacter was the most prevalent in both phases, and Methanosaeta was only detected in the liquid phase. The genera Methanobrevibacter and Methanobacterium both showed a positive correlation with methane (CH₄) formation in the liquid and solid phases, respectively (P < 0.05). Heat stress increased the relative abundance of genus Methanimicrococcus at the expense of candidate archaeal genus Vadin CA11 (P < 0.05). In the solid phase, osmotic stress significantly reduced the Shannon and Simpson indices of diversity, and relative abundance was higher for Methanobrevibacter at the expense of Methanimicrococcus. In the liquid phase, osmotic stress increased not only the abundance-based coverage estimator (ACE) and singles parameters of diversity but also the relative abundances of Methanosphaera and Methanobacterium. The overall decrease in all gas parameters and estimated metabolic hydrogen ([2H]) utilization was observed during osmotic stress conditions (P < 0.05). Betaine enhanced the diversity of solid phase archaea as indicated by the increase in ACE and singles during heat stress, and only a high dose improved all diversity parameters in the liquid phase during osmotic stress (P < 0.05). Thus, betaine alleviates the effects of heat stress and osmotic stress on the archaea community.