The rumen harbors a highly diverse and complex mixture of microorganisms, including archaea (10⁸−10⁹/mL), bacteria (10¹⁰−10¹¹/mL), ciliate protozoa (10⁶/mL), and fungi (10⁶/mL), which facilitate the degradation of complex plant carbohydrates into small molecules [1] and ultimately provide metabolites that can be used by the ruminant animal ^[2][3][4][5][2,3,4,5]. Livestock are mainly fed with agricultural crops, which via microbial activity are converted to metabolic intermediates (i.e., volatile fatty acids (VFAs), such as acetate, butyrate and propionate, and hydrogen (H₂) and gaseous end products such as carbon dioxide (CO₂) and methane (CH₄) [6]. Increased microbial H₂ production and its subsequent accumulation, which can be promoted by a high-starch diet, have several detrimental effects on the rumen ecosystem and that can be attributed to a decrease in rumen pH triggered by starch fermentation. These effects include the deactivation of specific biomass-degrading enzymes from some of the most efficient fiber degraders of the rumen microbiome but also system-level responses, such as the reduction of feed conversion within the rumen ^[7][8][7,8]. Methanogens, a group of microbes belonging to the phylogenetic group of the archaea, combine molecular H₂ with CO₂ to produce CH₄ during methanogenesis, enabling the removal of H₂ from the system ^[9][10][9,10]. Although this removal of H₂ is important for maintaining a healthy rumen ecosystem, from the viewpoint of nutrient expenditure methanogenesis is a costly process, accounting for a gross energy intake loss of 2–12% in ruminants ^{[11][12][13][14]}[11,12,13,14]. Since the annual production of enteric CH₄ accounts for ~15% of total anthropogenic CH₄ emissions ^[11][15][11,15], with CH₄ having a global warming potential 23-fold higher than that of CO₂, there is also a real and severe environmental cost associated with the energy of the enteric CH₄ that is released into the atmosphere.

Strategies and factors for CH₄ abatement have been reviewed in the past ^{[1][9][12][16][17][18][19][20][21][22][23][24][25]}[1,9,12,16,17,18,19,20,21,22,23,24,25] and many of the strategies used to mitigate CH₄ from ruminants involve the use of antibiotics, ionophores [26], halogenated CH₄ analogues ^[27][28][29][27,28,29], heavy metals [30], lipid-rich materials such as coconut oil ^[31][32][33][31,32,33], probiotics [27], bacteriocin [34], and numerous chemicals ^[35][36][35,36]. Immunization against methanogens ^[37][38][37,38], elimination of ciliate protozoa (defaunation) both in in vivo and in vitro [39] and addition of acetogenic bacteria to rumen fluid ^[40][41][42][40,41,42] in in vitro experiments have also been tested. Use of toxic chemicals and antibiotics as inhibitors, although considered an option in the past, are no longer accepted due to rising concerns regarding their impact on the environment, the animal, and potentially on the consumer of the animal products [43]. Interventions using phage therapy, altering methanogenic diversity and chemogenomic approaches [6] are some of the more recent technologies, but the extent to which these processes remove and eliminate the produced H₂ still remains to be investigated. Therefore, a critical step for a successful CH₄ reduction strategy may be one that uses natural processes within the rumen. One such approach relies on establishing a non-methanogenic sink for H₂ produced during fermentation. This review will focus on these H₂ elimination pathways.

H₂ concentration plays a major role in the regulation of microbial fermentation in the rumen ^{[44][45][46][47]}[44,45,46,47]. The partial H₂ pressure is a key regulator of H₂ metabolism and the fate of ruminal H₂ disposal with dissolved H₂ gas and H₂ ion determining the redox potential of the rumen liquor. The efficient elimination of H₂ enhances fermentation by reducing its inhibitory effect on microbial growth and microbial degradation of plant material ^[48][49][48,49]. Destiny of H₂ liberation is associated with favorable thermodynamic changes and an inverse correlation between Gibbs free energy (ΔG⁰) and the minimum partial H₂-pressure that is required for a reaction to continue: a reaction is considered to be thermodynamically more competitive when its requirement of H₂ partial pressure is low [50]. Due to this central regulatory role in rumen fermentation, H₂ can be considered to be the currency of ruminal fermentation [51]. Removal of the major fraction of the rumen H₂ occurs via the methanogenic archaea to CH₄, during which four moles of H₂ are consumed and converted into one mole of CH₄, which is then released into the atmosphere though eructation. During hydrogenotrophic methanogenesis, methanogens use CO₂ as carbon source and terminal electron acceptor and H₂ as electron donor. Other non-methanogenic rumen microbes, using CO₂ and other electron acceptors such as sulfate, nitrate, and fumarate, compete with methanogens for H₂, but they play a less dominant role in the removal of H₂ from the rumen ecosystem ^[52][53][52,53]. Non-methanogenic bacteria that use H₂ as electron donor include acetogens that reduce CO₂ to form acetate by the Wood-Ljungdahl pathway [54], sulfate-reducing bacteria (SRB) that reduce sulfate to hydrogen sulfide [55], nitrate-reducing bacteria (NRB) that reduce nitrate (NO₃) to ammonia (NH₄) and fumarate-reducing bacteria that use H₂ to form succinate ^[56][57][56,57]. Succinate can subsequentially be decarboxylated to propionate, which is a valuable nutrient for the ruminant animal [58], either by the succinate producer itself or it can be transferred to succinate users as an intercellular electron carrier [45]. Figure 1 summarizes the microbial pathways for H₂ removal from the rumen.