Nitrate and Inhibition of Ruminal Methanogenesis: Microbial Ecology, Obstacles, and Opportunities for Lowering Methane Emissions from Ruminant Livestock

Abstract

Ruminal methane production is among the main targets for greenhouse gas (GHG) mitigation for the animal agriculture industry. Many compounds have been evaluated for their efficacy to suppress enteric methane production by ruminal microorganisms. Of these, nitrate as an alternative hydrogen sink has been among the most promising, but it suffers from variability in efficacy for reasons that are not understood. The accumulation of nitrite, which is poisonous when absorbed into the animal’s circulation, is also variable and poorly understood. This review identifies large gaps in our knowledge of rumen microbial ecology that handicap the further development and safety of nitrate as a dietary additive. Three main bacterial species have been associated historically with ruminal nitrate reduction, namely Wolinella succinogenes, Veillonella parvula, and Selenomonas ruminantium, but others almost certainly exist in the largely uncultivated ruminal microbiota. Indications are strong that ciliate protozoa can reduce nitrate, but the significance of their role relative to bacteria is not known. The metabolic fate of the reduced nitrate has not been studied in detail. It is important to be sure that nitrate metabolism and efforts to enhance rates of nitrite reduction do not lead to the evolution of the much more potent GHG, nitrous oxide. The relative importance of direct inhibition of archaeal methanogenic enzymes by nitrite or the efficiency of capture of hydrogen by nitrate reduction in lowering methane production is also not known, nor are nitrite effects on other members of the microbiota. How effective would combining mitigation methods be, based on our understanding of the effects of nitrate and nitrite on the microbiome? Answering these fundamental microbiological questions is essential in assessing the potential of dietary nitrate to limit methane emissions from ruminant livestock.

Application of meta-omics techniques to understand greenhouse gas emissions originating from ruminal metabolism

“…Around 90% of the methane produced by ruminants is derived from the rumen [5], where methanogenic archaea convert the H 2 and CO 2 produced by the protozoa, bacteria and fungi to methane [6]. Worldwide research efforts have investigated various mitigation strategies, particularly feed additives that might inhibit H 2 production, provide an alternative H sink or inhibit the archaea themselves [7–10]. Other strategies include chemogenomics and immunization [11–13].…”

Current available strategies to mitigate greenhouse gas emissions in livestock systems: an animal welfare perspective

“…However, nitrate must be supplemented with caution as it can be toxic above certain doses leading to methaemoglobinaemia and carcinogenesis (Sinderal and Milkowski, 2012). The reviews by Bruning-Fann and Kaneene (1993) and more recently by Lee and Beauchemin (2014) and Yang et al (2016) discuss in detail nitrate's role in metabolism, animal production, enteric CH 4 emissions and toxicity, and how it may be safely used in practice.…”

Bakteri Denitrifikasi Inaktif Sebagai Suplemen Untuk Mengurangi Gas Metana dari Cairan Rumen Sapi

“…Dugaan lainnya adalah amonia yang dihasilkan langsung diasimilasi menjadi protein baru, yang dimanifestasikan dalam bentuk peningkatan biomassa protozoa (Gambar 3); yang memerlukan pemeriksaan lebih lanjut. Selain itu, nitrit dapat dimetabolisme langsung menjadi produk akhir selain amonia, seperti gas-gas nitrogen dan nitrous-oxide, yang juga merupakan penyusun gas rumah kaca [14].…”

“…Dalam proses denitrifikasi, amonia dihasilkan dari reduksi nitrit. Reduksi nitrit berlangsung lebih lambat dibandingkan dengan reduksi nitrat menjadi nitrit, yang dapat menyebabkan akumulasi nitrit, sehingga konsentrasi amonia yang rendah menggambarkan proses denitrifikasi yang rendah (14).…”

“…Sementara itu, produksi metana pada perlakuan tanpa penambahan bakteri denitrifikasi pada jam ke-48 malah meningkat sebesar 267% dibandingkan pada jam ke-24. Hal ini sesuai dengan hasil penelitian lain yang menunjukkan bahwa denitrifikasi menghambat metanogenesis, yaitu jumlah gas metana yang dihasilkan berbanding terbalik dengan jumlah senyawa-senyawa NO x -yang ditambahkan karena mereka merupakan akseptor elektron yang lebih disukai [14].…”