Hydrogen online monitoring based on thermal conductivity for anaerobic microorganisms.

H 2 -producing microorganisms are a promising source of sustainable biohydrogen. However, most H 2 -producing microorganisms are anaerobes, which are difficult to cultivate and characterize. While several methods for measuring H 2 exist, common H 2 sensors often require oxygen, making them unsuitable for anaerobic processes. Other sensors can often not be operated at high gas humidity. Thus, we applied thermal conductivity (TC) sensors and developed a parallelized, online H 2 monitoring for time-efficient characterization of H 2 production by anaerobes. Since TC sensors are nonspecific for H 2 , the cross-sensitivity of the sensors was evaluated regarding temperature, gas humidity, and CO 2 concentrations. The systems' measurement range was validated with two anaerobes: a high H 2 -producer (Clostridium pasteurianum) and a low H 2 -producer (Phocaeicola vulgatus). Online monitoring of H 2 production in shake flask cultivations was demonstrated, and H 2 transfer rates were derived. Combined with online CO 2 and pressure measurements, molar gas balances of the cultivations were closed, and an anaerobic respiration quotient was calculated. Thus, insight into the effect of medium components and inhibitory cultivation conditions on H 2 production with the model anaerobes was gained. The presented online H 2 monitoring method can accelerate the characterization of anaerobes for biohydrogen production and reveal metabolic changes without expensive equipment and offline analysis.