Insights into the known ¹³C depletion of methane—contribution of the kinetic isotope effects on the serine hydroxymethyltransferase reaction

We determined the kinetic isotope effect on the serine hydroxymethyltransferase reaction (SHMT), which provides important C(1) metabolites that are essential for the biosynthesis of DNA bases, O-methyl groups of lignin and methane. An isotope effect on the SHMT reaction was suggested being responsible for the well-known isotopic depletion of methane. Using the cytosolic SHMT from pig liver, we measured the natural carbon isotope ratios of both atoms involved in the bond splitting by chemical degradation of the remaining serine before and after partial turnover. The kinetic isotope effect (13)(V(Max)/K(m)) was 0.994 0.006 and 0.995 0.007 on position C-3 and C-2, respectively. The results indicated that the SHMT reaction does not contribute to the (13)C depletion observed for methyl groups in natural products and methane. However, from the isotopic pattern of caffeine, isotope effects on the methionine synthetase reaction and on reactions forming Grignard compounds, the involved formation and fission of metal organic bonds are likely responsible for the observed general depletion of “activated” methyl groups. As metal organic bond formations in methyl transferases are also rate limiting in the formation of methane, they may likely be the origin of the known (13)C depletion in methane

Soil organic matter dynamics: a biological perspective derived from the use of compound-specific isotopes studies

Current attempts to explain the persistence of carbon in soils focuses on explanations such as the recalcitrant plant residues and the physical isolation of substrates from decomposers. A pool of organic matter that can persist for centuries to millennia is hypothesized because of the evidence provided by the persistence of pre-disturbance C in fallow or vegetation change experiments, and the radiocarbon age of soil carbon. However, new information, which became available through advances in the ability to measure the isotope signatures of specific compounds, favors a new picture of organic matter dynamics. Instead of persistence of plant-derived residues like lignin in the soil, the majority of mineral soil is in molecules derived from microbial synthesis. Carbon recycled multiple times through the microbial community can be old, decoupling the radiocarbon age of C atoms from the chemical or biological lability of the molecules they comprise. In consequence is soil microbiology, a major control on soil carbon dynamics, which highlights the potential vulnerability of soil organic matter to changing environmental conditions. Moreover, it emphasizes the need to devise new management options to restore, increase, and secure this valuable resource

Effect of peat quality on microbial greenhouse gas formation in an acidic fen

Peatlands play an important role in the global carbon cycle and represent both an important stock of soil carbon and a substantial natural source of relevant greenhouse gases like CO₂ and CH₄. While it is known that the quality of organic matter affects microbial degradation and mineralization processes in peatlands, the manner in which the quality of peat organic matter affects the formation of CO₂ and CH₄ remains unclear. In this study we developed a fast and simple peat quality index in order to estimate its potential greenhouse gas formation by linking the thermo-degradability of peat with potential anaerobic CO₂ and CH₄ formation rates. Peat samples were obtained at several depths (0–40 cm) at four sampling locations from an acidic fen (pH 4.7). CO₂ and CH₄ formation rates were highly spatially variable and depended on depth, sampling location, and the composition of pyrolysable organic matter. Peat samples active in CO₂ and CH₄ formation had a quality index above 1.35, and the fraction of thermally labile pyrolyzable organic matter (comparable to easily available carbon substrates for microbial activity) obtained by thermogravimetry was above 35%. Curie-point pyrolysis-gas chromatography/mass spectrometry mainly identified carbohydrates and lignin as pyrolysis products in these samples, indicating that undecomposed organic matter was found in this fraction. In contrast, lipids and unspecific pyrolysis products, which indicate recalcitrant and highly decomposed organic matter, correlated significantly with lower CO₂ formation and reduced methanogenesis. Our results suggest that undecomposed organic matter is a prerequisite for CH₄ and CO₂ development in acidic fens. Furthermore, the new peat quality index should aide the estimation of potential greenhouse gas formation resulting from peatland restoration and permafrost thawing and help yield more robust models of trace gas fluxes from peatlands for climate change research

Mechanisms of soil carbon storage in experimental grasslands

International audienceWe investigated the fate of root and litter derived carbon into soil organic matter and dissolved organic matter in soil profiles, in order to explain unexpected positive effects of plant diversity on carbon storage. A time series of soil and soil solution samples was investigated at the field site of The Jena Experiment. In addition to the main biodiversity experiment with C3 plants, a C4 species (Amaranthus retroflexus L.) naturally labeled with 13C was grown on an extra plot. Changes in organic carbon concentration in soil and soil solution were combined with stable isotope measurements to follow the fate of plant carbon into the soil and soil solution. A split plot design with plant litter removal versus double litter input simulated differences in biomass input. After 2 years, the no litter and double litter treatment, respectively, showed an increase of 381 g C m?2 and 263 g C m?2 to 20 cm depth, while 71 g C m?2 and 393 g C m?2 were lost between 20 and 30 cm depth. The isotopic label in the top 5 cm indicated that 11 and 15% of soil organic carbon were derived from plant material on the no litter and the double litter treatment, respectively. Without litter, this equals the total amount of carbon newly stored in soil, whereas with double litter this corresponds to twice the amount of stored carbon. Our results indicate that litter input resulted in lower carbon storage and larger carbon losses and consequently accelerated turnover of soil organic carbon. Isotopic evidence showed that inherited soil organic carbon was replaced by fresh plant carbon near the soil surface. Our results suggest that primarily carbon released from soil organic matter, not newly introduced plant organic matter, was transported in the soil solution and contributed to the observed carbon storage in deeper horizons

Stratosphere troposphere coupling: the influence of volcanic eruptions

Stratospheric sulfate aerosols produced by major volcanic eruptions modify the radiative and dynamical properties of the troposphere and stratosphere through their reflection of solar radiation and absorption of infrared radiation. At the Earth's surface, the primary consequence of a large eruption is cooling, however, it has long been known that major tropical eruptions tend to be followed by warmer than usual winters over the Northern Hemisphere (NH) continents. This volcanic "winter-warming" effect in the NH is understood to be the result of changes in atmospheric circulation patterns resulting from heating in the stratosphere, and is often described as positive anomalies of the Northern Annular Mode (NAM) that propagate downward from the stratosphere to the troposphere. In the southern hemisphere, climate models tend to also predict a positive Southern Annular Mode (SAM) response to volcanic eruptions, but this is generally inconsistent with post-eruption observations during the 20th century. We review present understanding of the influence of volcanic eruptions on the large scale modes of atmospheric variability in both the Northern and Southern Hemispheres. Using models of varying complexity, including an aerosol-climate model, an Earth system model, and CMIP5 simulations, we assess the ability of climate models to reproduce the observed post-eruption climatic and dynamical anomalies. We will also address the parametrization of volcanic eruptions in simulations of the past climate, and identify possibilities for improvemen

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 &delta; D from the various H₂ sources are scarce and for biologically produced H₂ only very few measurements exist. <br><br> Here the first systematic study of the isotopic composition of biologically produced H₂ is presented. In a first set of experiments, we investigated &delta; 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 &delta; D = &minus;712&permil; (±13&permil;) for the samples from the biogas reactor (at 38 °C, &delta; D_H2O= +73.4&permil;), with a fractionation constant &varepsilon;_H2-H2O of −689&permil; (±20&permil;) between H₂ and the water. The five experiments using pure culture samples from different microorganisms give a mean source signature of &delta; D = &minus;728&permil; (±28&permil;), and a fractionation constant &varepsilon;_H2-H2O of −711&permil; (±34&permil;) 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 &delta; D of H₂. <br><br> 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&permil; / °C between 45 °C and 60 °C, which is in general agreement with the theoretical prediction of 1.4&permil; / °C. Our best experimental estimate for &varepsilon;_H2-H2O at a temperature of 20 °C is −731&permil; (±20&permil;) for biologically produced H₂. This value is close to the predicted value of −722&permil;, and we suggest using these values in future global H₂ isotope budget calculations and models with adjusting to regional temperatures for calculating &delta; D values