The stable isotopic signature of biologically produced molecular hydrogen (H₂)

Biologically produced molecular hydrogen (H2) is characterised by a very strong depletion in deuterium. Although the biological source to the atmosphere is small compared to photochemical or combustion sources, it makes an important contribution to the global isotope budget of H2. Large uncertainties exist in the quantification of the individual production and degradation processes that contribute to the atmospheric budget, and isotope measurements are a tool to distinguish the contributions from the different sources. Measurements of δ D from the various H2 sources are scarce and for biologically produced H2 only very few measurements exist. Here the first systematic study of the isotopic composition of biologically produced H2 is presented. In a first set of experiments, we investigated δ D of H2 produced in a biogas plant, covering different treatments of biogas production. In a second set of experiments, we investigated pure cultures of several H2 producing microorganisms such as bacteria or green algae. A Keeling plot analysis provides a robust overall source signature of δ D = −712‰ (±13‰) for the samples from the biogas reactor (at 38 °C, δ DH2O= +73.4‰), with a fractionation constant ϵH2-H2O of −689‰ (±20‰) between H2 and the water. The five experiments using pure culture samples from different microorganisms give a mean source signature of δ D = −728‰ (±28‰), and a fractionation constant ϵH2-H2O of −711‰ (±34‰) between H2 and the water. The results confirm the massive deuterium depletion of biologically produced H2 as was predicted by the calculation of the thermodynamic fractionation factors for hydrogen exchange between H2 and water vapour. Systematic errors in the isotope scale are difficult to assess in the absence of international standards for δ D of H2. As expected for a thermodynamic equilibrium, the fractionation factor is temperature dependent, but largely independent of the substrates used and the H2 production conditions. The equilibrium fractionation coefficient is positively correlated with temperature and we measured a rate of change of 2.3‰ / °C between 45 °C and 60 °C, which is in general agreement with the theoretical prediction of 1.4‰ / °C. Our best experimental estimate for ϵH2-H2O at a temperature of 20 °C is −731‰ (±20‰) for biologically produced H2. This value is close to the predicted value of −722‰, and we suggest using these values in future global H2 isotope budget calculations and models with adjusting to regional temperatures for calculating δ D values