Introduction
The complex microbial community that inhabits the rumen allows ruminants to digest and transform fibrous carbohydrates unavailable to humans into useful products such as meat, milk, wool and traction. Critical to the symbiosis between the rumen microbiota and the host animal is the anaerobic condition of the rumen, which prevents the complete oxidation of carbohydrates to carbon dioxide (CO2) and water. Instead, carbohydrates are incompletely oxidized to volatile fatty acids (VFA) and gases, with the host animal absorbing and utilizing the VFA as sources and precursors of energy, fat, glucose, and non-essential amino acids (Armstrong and Blaxter, 1957).
Rumen fermentation not only provides the ruminant with VFA. Part of the negative Gibbs energy change (ΔG) associated with fermentation is used by rumen microbes to generate ATP that can be utilized for microbial growth, active transport of substrates, and motility. Microbial growth produces microbial protein, which is the principal (Wallace et al., 1997) and most economical source of amino acids for ruminants. Rumen microorganisms can also synthesize water-soluble vitamins, which thus do not need to be included in most ruminants diets (Weiss, 2017).
A product of rumen fermentation is methane (CH4), which is a potent greenhouse gas when released to the atmosphere, and also a loss of energy for ruminants (Eckard et al., 2010; Martin et al., 2010). Through the formation of CH4 and the profile of VFA produced, rumen fermentation has important consequences for animal productivity and the environment. Understanding how rumen fermentation is controlled can help designing strategies to manipulate it in desired directions. Central to rumen metabolism are the dynamics of metabolic hydrogen ([H]) production and utilization. The idea of understanding rumen energy metabolism as [H] flows through different biochemical pathways is not new (Czerkawski, 1986; Hegarty and Gerdes, 1999). The objectives of this paper are to review and critically examine [H] flows as the unifying principle to understand rumen fermentation. In particular, this paper will discuss (i) The control of the VFA profile and CH4 production by dihydrogen (H2), (ii) The principles underlying the competition for H2, (iii) The potential inhibitory effects of H2 and other intercellular electron (e-) carriers on the rates of fermentation and digestion in the rumen, and (iv) The relationship between the flows of [H] and microbial growth. All of these aspects have implications to animal productivity and the environment mediated by ruminal and post-absorptive metabolism.