Phylogenetic and Ecological Characteristics of Uncultured Succinivibrionaceae Detected in the Rumen of Low Methane‐Producing Cows

Some uncultured Succinivibrionaceae have been suggested to be associated with lower methane production in the rumen of cows, but this remains underexplored. This study aimed to elucidate the phylogenetic and ecological characteristics of uncultured Succinivibrionaceae . Phylogenetic differences between uncultured Succinivibrionaceae and other Succinivibrionaceae species were determined. The uncultured Succinivibrionaceae ‐specific primer set was newly designed. Real‐time PCR was performed on 96 rumen fluid samples obtained from 69 Holstein cows and 12 calves to quantify the abundances of uncultured Succinivibrionaceae , Succinivibrio dextrinosolvens , Ruminobacter amylophilus , and archaea, and their abundances were compared. The 16S rRNA gene sequences of uncultured Succinivibrionaceae clustered separately from those of known Succinivibrionaceae species and were identified as a distinct bacterial cluster (US cluster). The US was highly abundant in calves and lactating cows but was scarce in dry cows. The short‐chain fatty acid ratio [(C2 + C4)/C3] negatively and positively correlated with the US and other known Succinivibrionaceae species, respectively. The archaea‐to‐bacteria ratio negatively correlated with the US but positively correlated with known Succinivibrionaceae species. These phylogenetic and ecological differences between the US and known Succinivibrionaceae species suggest that the US has a different niche and functional role in the rumen from those of known Succinivibrionaceae species.