The effect of Methylococcus capsulatus in mono- or co-culture with Methanobrevibacter smithii or with mixed rumen fluid on bacterial growth and methane gas production.

Providing an alternate electron sink to methane (CH4) is a crucial step in reducing enteric CH4 emissions. Comparing the effects of CH4-utilizing methanotroph bacteria (Methylococcus capsulatus; MC) and its combination with pure strains of rumen bacteria or mixed rumen fluid can offer biological insights into methanogenesis pathways and CH4 consumption. The primary objectives of this study were to investigate the impact of inoculating with M. capsulatus on the growth rates of rumen bacteria, including methanogens and mixed rumen fluid, as well as fermentation rates, ruminal gas production, CH4 emissions, and other environmental-impacting gases (N2O, H2S). Three experiments were carried out using in vitro ANKUM gas production systems (Exp. 1 and 2) and continuous recirculating flux chamber systems (Exp. 3). In Exp. 1, four strains of rumen bacteria-Streptococcus bovis [SB], Ruminococcus flavefaciens [RF], Methanobrevibacter smithii [MS], and M. capsulatus [MC]-were used to determine the effect of CH4-utilizing bacteria (e.g., MC) on specific growth rate, volatile fatty acid (VFA) production, and ruminal CH4 emissions in a combination with these bacterial strains. Results from Experiment 1 showed that MS produced the most CH4 among the strains. When cocultured with MC, no CH4 was detected, indicating that MC could utilize most of the CH4 produced in coculture with MS and other bacterial strains. There was little difference in total and cumulative gas production with varying MC doses (Exp. 2). However, in the presence of MC, CH4 production (percentage or g DM) decreased significantly (P < 0.01) as MC addition increased. Conversely, substrates containing both grain- and forage- based diets with MC increased N2O emissions per gram of DM (µg/g DM) or total N2O production (ppm), with treatment and basal diet interactions (P < 0. 01). In Exp. 3, using a continuous recirculating flux chamber system, CH4 flux significantly reduced (P < 0.001) over time in both basal diets with MC inoculum. However, fermentation rates varied between treatments and diets. These findings demonstrate that adding MC inoculum to in vitro rumen fermentation chambers significantly reduces CH4 emissions compared to controls.