Quantitative evaluation of ruminal methane and carbon dioxide formation from formate through C-13 stable isotope analysis in a batch culture system

Methane produced from formate is one of the important methanogensis pathways in the rumen. However, quantitative information of CH₄ production from formate has been rarely reported. The aim of this study was to characterize the conversion rate (CR) of formic acid into CH₄ and CO₂ by rumen microorganisms. Ground lucerne hay was incubated with buffered ruminal fluid for 6, 12, 24 and 48 h. Before the incubation, ¹³C-labeled H¹³COOH was also supplied into the incubation bottle at a dose of 0, 1.5, 2.2 or 2.9 mg/g of DM substrate. There were no interactions (P>0.05) between dose and incubation time for all variables evaluated. When expressed as an absolute amount (ml in gas sample) or a relative CR (%), both ¹³CH₄ and ¹³CO₂ production quadratically increased (P<0.01) with the addition of H¹³COOH. The total ¹³C (¹³CH₄ and ¹³CO₂) CR was also quadratically increased (P<0.01) when H¹³COOH was added. Moreover, formate addition linearly decreased (P<0.031) the concentrations of NH₃-N, total and individual volatile fatty acids (acetate, propionate and butyrate), and quadratically decreased (P<0.014) the populations of protozoa, total methanogens, Methanosphaera stadtmanae, Methanobrevibacter ruminantium M1, Methanobrevibacter smithii and Methanosarcina barkeri. In summary, formate affects ruminal fermentation and methanogenesis, as well as the rumen microbiome, in particular microorganisms which are directly or indirectly involved in ruminal methanogenesis. This study provides quantitative verification for the rapid dissimilation of formate into CH₄ and CO₂ by rumen microorganisms.