Microbiological hydrogen (H ) thresholds in anaerobic continuous-flow systems: Effects of system characteristics.

Hydrogen (H ) concentrations that were associated with microbiological respiratory processes (RPs) such as sulfate reduction and methanogenesis were quantified in continuous-flow systems (CFSs) (e.g., bioreactors, sediments). Gibbs free energy yield (ΔǴ ~ 0) of the relevant RP has been proposed to control the observed H concentrations, but most of the reported values do not align with the proposed energetic trends. Alternatively, we postulate that system characteristics of each experimental design influence all system components including H concentrations. To analyze this proposal, a Monod-based mathematical model was developed and used to design a gas-liquid bioreactor for hydrogenotrophic methanogenesis with Methanobacterium bryantii M.o.H. Gas-to-liquid H mass transfer, microbiological H consumption, biomass growth, methane formation, and Gibbs free energy yields were evaluated systematically. Combining model predictions and experimental results revealed that an initially large biomass concentration created transients during which biomass consumed [H ] rapidly to the thermodynamic H -threshold (≤1 nM) that triggerred the microorganisms to stop H oxidation. With no H oxidation, continuous gas-to-liquid H transfer increased [H ] to a level that signaled the methanogens to resume H oxidation. Thus, an oscillatory H -concentration profile developed between the thermodynamic H -threshold (≤1 nM) and a low [H ] (~10 nM) that relied on the rate of gas-to-liquid H -transfer. The transient [H ] values were too low to support biomass synthesis that could balance biomass losses through endogenous oxidation and advection; thus, biomass declined continuously and disappeared. A stable [H ] (1807 nM) emerged as a result of abiotic H -balance between gas-to-liquid H transfer and H removal via advection of liquid-phase.