5:15 PM - 7:15 PM
[MIS22-P10] Measurement of hydrongen isotope effect during the oxidation of atmospheric molecular hydrogen
Keywords:Soil sink, Hydrogen-oxidizing bacterium, Hydrogen isotope fractionation
Molecular hydrogen (H2) exists in the atmosphere at a concentration of about 530 ppb, and is thought to be maintained at a nearly constant level as a result of balance between the sources including oxidation of hydrocarbons in the atmosphere, generation from combustion, and microbial processes and the sinks of absorption by soil microorganisms and oxidative decomposition by atmospheric reaction. However, if H2 is increasingly used as an alternative energy source to fossil fuels, it is expected to leak into the atmosphere during production, transportation, storage, and consumption, increasing H2 concentrations and affecting atmospheric chemical reaction systems, which may indirectly contribute to global warming. In addition, the uncertainty in the estimated annual emission from each source or sink is particularly large for microbial processes, and there is no knowledge of how future climate change will affect microbial processes.
Hydrogen absorption by soil, which accounts for about 80% of H2 consumption processes, has been estimated based on observations and simulated experiments, but the fact that both hydrogen production and consumption occur in soil and that these processes are affected by various environmental factors is the cause of the large uncertainty. Therefore, it is useful to use stable isotope ratios that exhibit characteristic values depending on the source material, formation reaction, and decomposing reaction mechanism and progress. Previous studies have estimated isotope effects on soil uptake of hydrogen by correcting for the effects of formation, and have not been able to distinguish between isotope effects in physical diffusion and biological oxidation. Therefore, the objective of this study was to determine isotope effects on the hydrogen oxidation process by pure culture of soil-derived microorganisms.
Streptomyces avermitilis, a high-affinity hydrogen-oxidizing bacterium, was cultured on agar medium, and an experimental system was constructed to collect gas samples over time during incubation. Although S. avermitilis is able to oxidize H2 at atmospheric concentration level, they were incubated at 25°C in air containing 200 ppm H2 under atmospheric pressure in order to simplify the measurement of H2 concentration and isotope ratios in the samples. One mL of gas sample was collected over time, and the H2 was concentrated in a cryogenic concentration loop made of MolSieve capillary column, purified by GC (Carboxen-1010 PLOT column), and then introduced into an isotope ratio mass spectrometer to obtain H2 concentration (peak area) and isotope ratio (relative values to reference gas). During the preliminary 3–20 hour incubation experiment, a decrease in H2 concentration and an increase in isotope ratio were observed, and the isotope fractionation coefficient (α) was determined by fitting the Rayleigh equation. The obtained α value was smaller (corresponding to larger isotope effect) than that estimated in previous studies, and the results of two experiments differed significantly. The reasons for such differences will be discussed.
Hydrogen absorption by soil, which accounts for about 80% of H2 consumption processes, has been estimated based on observations and simulated experiments, but the fact that both hydrogen production and consumption occur in soil and that these processes are affected by various environmental factors is the cause of the large uncertainty. Therefore, it is useful to use stable isotope ratios that exhibit characteristic values depending on the source material, formation reaction, and decomposing reaction mechanism and progress. Previous studies have estimated isotope effects on soil uptake of hydrogen by correcting for the effects of formation, and have not been able to distinguish between isotope effects in physical diffusion and biological oxidation. Therefore, the objective of this study was to determine isotope effects on the hydrogen oxidation process by pure culture of soil-derived microorganisms.
Streptomyces avermitilis, a high-affinity hydrogen-oxidizing bacterium, was cultured on agar medium, and an experimental system was constructed to collect gas samples over time during incubation. Although S. avermitilis is able to oxidize H2 at atmospheric concentration level, they were incubated at 25°C in air containing 200 ppm H2 under atmospheric pressure in order to simplify the measurement of H2 concentration and isotope ratios in the samples. One mL of gas sample was collected over time, and the H2 was concentrated in a cryogenic concentration loop made of MolSieve capillary column, purified by GC (Carboxen-1010 PLOT column), and then introduced into an isotope ratio mass spectrometer to obtain H2 concentration (peak area) and isotope ratio (relative values to reference gas). During the preliminary 3–20 hour incubation experiment, a decrease in H2 concentration and an increase in isotope ratio were observed, and the isotope fractionation coefficient (α) was determined by fitting the Rayleigh equation. The obtained α value was smaller (corresponding to larger isotope effect) than that estimated in previous studies, and the results of two experiments differed significantly. The reasons for such differences will be discussed.