The normal maximum H2 yield in mesophilic biohydrogen (bioH 2) fermentation is ∼2 mol of H2/(mol of glucose). Thermodynamics could be the most fundamental control for bioH2 formation, since proton reduction is strongly energy consuming (+79.4 kJ/(mol of H2)). However, most of the electron equivalents in glucose do not accumulate in H2 but in a range of organic acids and alcohols. Thus, evaluating the hypothesis of thermodynamic control requires the full stoichiometry of the fermentation. We carried out batch bioH2 reactions with a range of pH values that yielded H2 yields from 0 to ∼2 mol of H2/(mol of glucose). We constructed complete electron equivalent (e- equiv) balances for high or low H2 yield by measuring all e- sinks. The highest H2 yield occurred with pH ∼4 and was coincident with major butyrate accumulation; ethanol or lactate correlated to reduced H2 yields at pH 7 and 10, respectively. Although the Gibb's free energies for all overall reactions were similar (-10.6 to -11.2 kJ/(e- equiv)), thermodynamics controlled the H 2-producing reaction coupled to ferredoxin; this reaction was favorable at acidic pH but thermodynamically blocked at pH 10. Also, butyrate formation was the most thermodynamically favorable reaction that produced ATP after glycolysis.
ASJC Scopus subject areas
- Environmental Chemistry