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

Abstract

Biologically produced molecular hydrogen (H2) is characterized 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 molecular hydrogen (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 δD 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. We investigated δD of H2 produced in a biogas plant, covering different treatments of biogas production, and from several H2 producing microorganisms such as bacteria or green algae. A Keeling plot analysis provides a robust overall source sig nature of δD = –712‰ (±13‰) for the samples from the biogas reactor (at 38 °C, δDH2O = 73.4 ‰), with a fractionation constant εH2−H2O of –689‰ (±20 ‰). The pure culture samples from different microorganisms give a mean source signature of δD = –728‰ (± 39 ‰), and a fractionation constant εH2−H2O of –711‰ (± 45 ‰) between H2 and the water, respectively. The results confirm the massive deuterium depletion of biologically produced H2 as was predicted by calculation of the thermodynamic fractionation factors for hydrogen exchange between H2 and water vapor. As expected for a thermodynamic equilibrium, the fractionation factor is largely independent of the substrates used and the H2 production conditions. The predicted equilibrium fractionation coefficient is positively correlated with temperature and we measured a change 25 of 2.2‰/°C between 45 °C and 60 °C. This is in general agreement with the theoretical predictions.