Effect of 3-nitrooxypropanol (3-NOP) on rumen bacteria via changes in dissolved hydrogen (H2) concentrations. A Effect of 3-NOP on dissolved H2 in lactating dairy cows; the bacterial genera appearing in the oval shapes are rapid (orange) and slow (blue) fermenters that were increased in 3-NOP treated cows at week 8 and week 12, respectively (see panel B). B Fold change (log 10) between control and 3-NOP-treated cows at weeks 4, 8, and 12 in the relative abundance of selected bacterial genera in DNA-based 16S rRNA analysis. The selection of bacterial genera was based on significant differences (glmer test) between treatment groups (P < 0.05). C Fold change (log 10) between control and 3-NOP treated cows at weeks 4, 8, and 12 in hydrogenase content in metagenomes (metaG) and metatranscriptomes (metaT). Hydrogenase content is shown based on hydrogenase subgroup. These are divided into fermentative hydrogenases (H2-producing; group A1, A2, B [FeFe]-hydrogenases), bifurcating hydrogenases (bidirectional; group A3, A4 [FeFe]-hydrogenases), respiratory hydrogenases (H2-uptake; group 1a, 1b, 1c, 1d, 1e, 1f, 1g, 1h, 1i, 1j [NiFe]-hydrogenases), respiratory hydrogenases (H2-evolving; group 4b, 4d [NiFe]-hydrogenases), alternative and sensory hydrogenases (H2-uptake; 2a, 2b [NiFe]-hydrogenases), cofactor-coupled bidirectional hydrogenases (3b, 3d, [NiFe]-hydrogenases), methanogenic hydrogenases (H2-uptake; group 1k, 3a, 3c, 4h, 4i [NiFe]-hydrogenases, [Fe]-hydrogenases), energy-converting hydrogenases (bidirectional; group 4a, 4c, 4e, 4f, 4g [NiFe]-hydrogenases), and sensory hydrogenases (group C [FeFe]-hydrogenases). Positive and negative log 10-fold change is the increased and decreased relative abundance, respectively, in 3-NOP compared with controls cows.