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 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. In a first set of experiments, we investigated δD 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 δD = -712‰ (±13 ‰) for the samples from the biogas reactor (at 38 δC, δDH2O = +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 δD = -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 δD 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‰/ δC between 45 δC and 60 δC, which is in general agreement with the theoretical prediction of 1.4‰/ δC. Our best experimental estimate for εH₂-H₂O at a temperature of 20 δC 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 δD values. [ABSTRACT FROM AUTHOR]