Ruminants harbour a variety of microorganisms — bacteria, archaea, fungi, viruses and protozoa — in their rumen, which drive substantial methane emissions.
Particularly, ciliate protozoa account for approximately a quarter of the total microbial biomass in the rumen ecosystem and have a key role in rumen methanogenesis, but the mechanisms underlying this process are currently unknown.
Next, the authors screened all rumen ciliate genomes for hydrogenase genes.
Hydrogenases catalyse the production of hydrogen, which is subsequently used as a substrate for methanogenesis, through an interspecies hydrogen transfer process from ciliates to methanogens.
Through heterologous expression in Escherichia coli the authors confirmed hydrogen production by the hydrogenases.
Methane is a potent greenhouse gas contributing to global warming. Ruminants harbour a variety of microorganisms — bacteria, archaea, fungi, viruses and protozoa — in their rumen, which drive substantial methane emissions. Particularly, ciliate protozoa account for approximately a quarter of the total microbial biomass in the rumen ecosystem and have a key role in rumen methanogenesis, but the mechanisms underlying this process are currently unknown. In a recent study, Xie et al. reveal the mechanistic basis of ciliate-mediated methanogenesis in ruminants.
Next, the authors screened all rumen ciliate genomes for hydrogenase genes. Hydrogenases catalyse the production of hydrogen, which is subsequently used as a substrate for methanogenesis, through an interspecies hydrogen transfer process from ciliates to methanogens. The authors found a dominant hydrogenase type present in all ciliates except Isotricha species, with a structure comprising an N-terminal signal peptide, a central hydrogenase domain and a C-terminal transmembrane helix. Through heterologous expression in Escherichia coli the authors confirmed hydrogen production by the hydrogenases. Because these hydrogenases lacked canonical mitochondrial or hydrogenosomal targeting sequences, which indicates a cellular localization distinct from hydrogenosomes or cytoplasmic hydrogenases, the authors used immunofluorescence and transmission and scanning electron microscopy, to analyse hydrogenase localization in Dasytricha. They identified vesicles located beneath the cell membrane and within the ectoplasm, around the basal bodies of cilia, corresponding to hydrogen-producing subcellular organelles (referred to as hydrogenobodies). These vesicles had a distinct morphology that differentiates them from hydrogenosomes found in other protozoa and fungi.
