Fig. 1
General scheme of methanogenic organic compound degradation and the “H2 conflict.” a Scheme for the degradation of organic macromolecules and the major intermediates, including AAs (blue or purple [see below]), sugars (red), FAs (green), and H2 (orange). b Gibbs free energy change for the degradation of representative AAs with low (isoleucine; blue) and high (glutamine; purple) calculated H2 tolerance, sugar (glucose), and fatty acid (FA) (butyrate) and H2-oxidizing CO2-reducing methanogenesis with varying H2 partial pressures. The vertical dotted lines indicate each pathway’s threshold H2 concentration at which ∆G becomes 0 kJ/mol. ∆G values are calculated as the ∆Greaction + (mol ATP generated/mol reaction)*∆GATPsynthesis (see details below). The H2 partial pressure range at which each metabolism is thermodynamically favorable is shown at the top (horizontal bars with corresponding colors). Hydrogen partial pressures that overlap with those for H2-oxidizing CO2-reducing methanogenesis are indicated (solid colors) and would be permissive for that reaction. Metabolisms with [H2]max less than 100 Pa and greater than 100 Pa are respectively defined as H2-sensitive and H2-tolerant. The following conditions were used for calculations—10 μM butyrate, 300 μM acetate, 0.1 μM amino acids and sugars, 1 mM NH4+, 50 mM HCO3-, 50 kPa CH4, pH of 7, and 37 °C. ∆GATPsynthesis is assumed to be 60 kJ/mol. For butyrate, isoleucine, glutamine, glucose, and H2/CO2 methanogenesis, ATP yields of 0.33, 1, 1.33, 4.67, and 0.2 were assumed. The ATP yields are calculated as follows: ATPgenerated – ATPconsumed – x*(NADHgenerated –NADHconsumed) + x*(FdH2generated – FdH2consumed) – 2x*(ETFH2generated – ETFH2consumed) – 2x*(quinolgenerated – quinolconsumed), where x is the ATP synthase ATP:H+ ratio (assumed to be 1:3 for organotrophy and 1:5 for methanogenesis in this figure). Abbreviations: NADH—reduced nicotinamide adenine dinucleotide; FdH2—reduced ferredoxin; ETF—reduced electron transfer flavoprotein