Could the secret to lowering livestock's climate impact lie inside the gut of a cow? A recent study in Science has uncovered how rumen ciliates modulate methane (CH₄) emissions in ruminants. The study identified a previously unknown single-membrane hydrogen-producing organelle, termed the "hydrogenobody", that plays a key role in this mechanism.

The study was led collaboratively by Prof. MIAO Wei from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences (CAS), in partnership with researchers at Nanjing Agricultural University, Jianghan University, Jilin Agricultural University, and Northwestern Polytechnical University.

Methane is a potent greenhouse gas and a major contributor to global warming. The increase in global methane emissions is primarily driven by human activities, such as animal husbandry, rice cultivation, and the production and utilization of fossil fuels. Among these, methane emitted by ruminants like cattle and sheep accounts for over 30% of global anthropogenic methane emissions.

In ruminants, methane is mainly produced through anaerobic fermentation by rumen microorganisms, among which protozoan ciliates can constitute up to 50% of the total rumen microbial biomass. Rumen ciliates have long been recognized as important contributors to methane emissions in ruminants, but the mechanisms underlying their role remain poorly understood.

To address this question, the researchers constructed the world's largest genomic dataset for these organisms as part of the Protist 10,000 Genomes Project (P10K). This resource comprises 450 genomes spanning six families and 18 genera.

Based on morphological and functional characteristics, rumen ciliates can be broadly divided into two groups: Vestibuliferida, whose cells are entirely covered with cilia, and Entodiniomorphida, in which cilia are restricted to the oral region. The researchers also established anaerobic cultivation systems for three representative species, providing a foundation for future functional studies.

Rumen ciliate genome catalogue. (Image by IHB)

A key discovery of this study is a previously unknown single-membrane hydrogen-producing organelle, termed the "hydrogenobody".

Structurally distinct from the double-membraned hydrogenosome found in other protists, the hydrogenobody contains specific hydrogenases and oxygen reductases, enabling it to simultaneously produce hydrogen and scavenge oxygen.

By producing hydrogen, which is the primary substrate for methanogens, while maintaining an anaerobic environment, this organelle plays a central role in rumen methanogenesis and represents a promising target for methane mitigation strategies, according to the study.

Leveraging the newly established genomic catalog, the researchers analyzed nearly 2,000 rumen meta-omics datasets. The results revealed strong associations among rumen ciliates, methanogens, and methane emission levels.

Importantly, hydrogenobodies localize predominantly in cilia-rich regions. Their abundance is tens of times higher in Vestibuliferida than in Entodiniomorphida, which corresponds to a greater contribution to methane production. These findings point to Vestibuliferida ciliates as a key target group for interventions aimed at reducing ruminant methane emissions.

According to the researchers, this study challenges existing paradigms and provides novel insights into rumen ciliate–modulated methanogenesis, centered on a "hydrogenobody-mediated coupling of hydrogen production and oxygen scavenging", offering a theoretical basis for developing new strategies to mitigate methane emissions.

A novel hydrogen-producing organelle in rumen ciliates — "hydrogenobody". (Video by IHB)