The stable isotopic signature of biologically produced molecular hydrogen (H<sub>2</sub>)

Abstract

Abstract. Biologically produced molecular hydrogen (H 2 )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 H 2 . 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 H 2 sources are scarce and for biologically produced H 2 only very few measurements exist.Here the first systematic study of the isotopic composition of biologically produced H 2 is presented. In a first set of experiments, we investigated δD of H 2 produced in a biogas plant, covering different treatments of biogas production. In a second set of experiments, we investigated pure cultures of several H 2 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 • C, δD H 2 O = +73.4 ‰), with a fractionation constant ε H 2 -H 2 O of −689 ‰ (±20 ‰) between H 2 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 ε H 2 -H 2 O of −711 ‰ (±34 ‰) between H 2 and the water. The results confirm the massive deuterium depletion of biologically produced H 2 as was predicted by the calculation of the thermodynamic fractionation factors for hydrogen exchange between H 2 and water vapour. Systematic errors in the isotope scale are difficult to assess in the absence of international standards for δD of H 2 .As expected for a thermodynamic equilibrium, the fractionation factor is temperature dependent, but largely independent of the substrates used and the H 2 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 ε H 2 -H 2 O at a temperature of 20 • C is −731 ‰ (±20 ‰) for biologically produced H 2 . This value is close to the predicted value of −722 ‰, and we suggest using these values in future global H 2 isotope budget calculations and models with adjusting to regional temperatures for calculating δD values.

Isotopic signatures of production and uptake of H<sub>2</sub> by soil

“…Kawagucci et al (2010) proposed that microbiological H 2 consumption and production could destroy the thermal isotopic equilibrium between H 2 and H 2 O in low-temperature hydrothermal fluids. Luo et al (1991) and Walter et al (2012) found fractionation factors of 0.448, 0.401 and 0.363 for H 2 generated from water by different N 2 -fixing bacteria in the laboratory.…”

“…By assuming α in = 0.945 (overall fractionation factor as determined in our deposition experiments), δD soil = −530 ‰ (averaged δD soil of Cabauw net-emission experiments) and δD 0 = −611 ‰ (averaged of δD(H 2 ) derived from laboratory experiments in Luo et al (1991) and Walter et al, 2012), we would obtain f in = 0.97. That is, 97 % of H 2 produced by N 2 fixation would be removed within soil before entering atmosphere.…”

“…and 0.401 ± 0.002 for Anabaena sp., respectively. Walter et al (2012) reported α H 2 /H 2 O = 0.363 ± 0.019 for the N 2 -fixing rhizobacterium Azospirillum brasiliensis. It has been proposed that microbiological H 2 consumption and production could modify the thermal isotopic equilibrium between H 2 and H 2 O in lowtemperature hydrothermal fluids (Kawagucci et al, 2010).…”

“…It should be noted that the estimation of f in is very uncertain due to the lack of information about α in and δD 0 . By using the lower limit of α in (0.911) in our experiment and the upper limit of δD 0 (−569 ‰) in Luo et al (1991) and Walter et al (2012), we calculate a lower limit of f in to be 0.62. The upper limit of f in is 1.00 when α in approaches 1.…”

“…In order to compare our δD soil with the fractionation factors between H 2 and H 2 O found by Luo et al (1991) and Walter et al (2012), we converted their fractionation factors to δD(H 2 ) by assuming the δD(H 2 O) to be the same as that of global rainwater (−37.8 ‰, Hoffmann et al, 1998). This results in δD(H 2 ) values of −651 to −569 ‰ for their N 2 -fixing bacteria.…”