2024 journal article
Inhibiting methanogenesis by targeting thermodynamics and enzymatic reactions in mixed cultures of rumen microbes <i>in vitro</i>
FRONTIERS IN MICROBIOLOGY, 15.
author keywords: methane mitigation; rumen microbiome; anaerobic fermentation; methanogenesis; thermodynamics; enzymatic reactions
open access via doi.org (publisher)
Source: Web Of Science
Added: September 9, 2024
Mitigation of enteric methane (CH 4 ) emissions from ruminant livestock represents an opportunity to improve the sustainability, productivity, and profitability of beef and dairy production. Ruminal methanogenesis can be mitigated via two primary strategies: (1) alternative electron acceptors and (2) enzymatic inhibition of methanogenic pathways. The former utilizes the thermodynamic favorability of certain reactions such as nitrate/nitrite reduction to ammonia (NH 3 ) while the latter targets specific enzymes using structural analogs of CH 4 and methanogenic cofactors such as bromochloromethane (BCM). In this study, we investigated the effects of four additives and their combinations on CH 4 production by rumen microbes in batch culture. Sodium nitrate (NaNO 3 ), sodium sulfate (Na 2 SO 4 ), and 3-nitro-1-propionate (3NPA) were included as thermodynamic inhibitors, whereas BCM was included as a enzymatic inhibitor. Individual additives were evaluated at three levels of inclusion in experiments 1 and 2. Highest level of each additive was used to determine the combined effect of NaNO 3 + Na 2 SO 4 (NS), NS + 3NPA (NSP), and NSP + BCM (NSPB) in experiments 3 and 4. Experimental diets were high, medium, and low forage diets (HF, MF, and LF, respectively) and consisted of alfalfa hay and a concentrate mix formulated to obtain the following forage to concentrate ratios: 70:30, 50:50, and 30:70, respectively. Diets with additives were placed in fermentation culture bottles and incubated in a water bath (39°C) for 6, 12, or 24h. Microbial DNA was extracted for 16S rRNA and ITS gene amplicon sequencing. In experiments 1 and 2, CH 4 concentrations in control cultures decreased in the order of LF, MF, and HF diets, whereas in experiments 3 and 4, CH 4 was highest in MF diet followed by HF and LF diets. Culture pH and NH 3 in the control decreased in the order of HF, MF, to LF as expected. NaNO 3 decreased ( p &lt; 0.001) CH 4 and butyrate and increased acetate and propionate ( p &lt; 0.03 and 0.003, respectively). Cultures receiving NaNO 3 had an enrichment of microorganisms capable of nitrate and nitrite reduction. 3NPA also decreased CH 4 at 6h with no further decrease at 24 h ( p &lt; 0.001). BCM significantly inhibited methanogenesis regardless of inclusion levels as well as in the presence of the thermodynamic inhibitors ( p &lt; 0.001) while enriching succinate producers and assimilators as well as propionate producers ( p adj &lt; 0.05). However, individual inclusion of BCM decreased total short chain fatty acid (SCFA) concentrations ( p &lt; 0.002). Inhibition of methanogenesis with BCM individually and in combination with the other additives increased gaseous H 2 concentrations ( p &lt; 0.001 individually and 0.028 in combination) while decreasing acetate to propionate ratio ( p &lt; 0.001). Only the cultures treated with BCM in combination with other additives significantly (p adj &lt; 0.05) decreased the abundance of Methanobrevibacter expressed as log fold change. Overall, the combination of thermodynamic and enzymatic inhibitors presented a promising effect on ruminal fermentation in-vitro , inhibiting methanogenesis while optimizing the other fermentation parameters such as pH, NH 3 , and SCFAs. Here, we provide a proof of concept that the combination of an electron acceptor and a methane analog may be exploited to improve microbial efficiency via methanogenesis inhibition.