Frank Keppler

Methane emissions from terrestrial plants under aerobic conditions

Methane is an important greenhouse gas and its atmospheric concentration has almost tripled since pre-industrial times. It plays a central role in atmospheric oxidation chemistry and affects stratospheric ozone and water vapour levels. Most of the methane from natural sources in Earths atmosphere is thought to originate from biological processes in anoxic environments. Here we demonstrate using stable carbon isotopes that methane is readily formed in situ in terrestrial plants under oxic conditions by a hitherto unrecognized process. Significant methane emissions from both intact plants and detached leaves were observed during incubation experiments in the laboratory and in the field. If our measurements are typical for short-lived biomass and scaled on a global basis, we estimate a methane source strength of 62–236 Tg yr-1 for living plants and 1–7 Tg yr-1 for plant litter (1 Tg = 1012 g). We suggest that this newly identified source may have important implications for the global methane budget and may call for a reconsideration of the role of natural methane sources in past climate change.

Halocarbons produced by natural oxidation processes during degradation of organic matter

Volatile halogenated organic compounds (VHOC) play an important role in atmospheric chemical processes—contributing, for example, to stratospheric ozone depletion. For anthropogenic VHOC whose sources are well known, the global atmospheric input can be estimated from industrial production data. Halogenated compounds of natural origin can also contribute significantly to the levels of VHOC in the atmosphere. The oceans have been implicated as one of the main natural sources, where organisms such as macroalgae and microalgae can release large quantities of VHOC to the atmosphere. Some terrestrial sources have also been identified, such as wood-rotting fungi, biomass burning and volcanic emissions. Here we report the identification of a different terrestrial source of naturally occurring VHOC. We find that, in soils and sediments, halide ions can be alkylated during the oxidation of organic matter by an electron acceptor such as Fe( III): sunlight or microbial mediation are not required for these reactions. When the available halide ion is chloride, the reaction products are CH 3Cl, C2H5Cl, C3H7Cl and C4H9Cl. (The corresponding alkyl bromides or alkyl iodides are produced when bromide or iodide are present.) Such abiotic processes could make a significant contribution to the budget of the important atmospheric compounds CH3Cl, CH3Br and CH3I.

Methoxyl groups of plant pectin as a precursor of atmospheric methane: evidence from deuterium labelling studies

* The observation that plants produce methane (CH4) under aerobic conditions has caused considerable controversy among the scientific community and the general public. It led to much discussion and debate not only about its contribution to the global CH4 budget but also about the authenticity of the observation itself. Previous results suggested that methoxyl groups of the abundant plant structural component pectin might play a key role in the in situ formation process of CH4. Here, this effect is investigated using an isotope labelling study. * Polysaccharides, pectin and polygalacturonic acid, with varying degrees of trideuterium-labelled methyl groups in the methoxyl moieties, were investigated for CH4 formation under UV irradiation and heating. * A strong deuterium signal in the emitted CH4 was observed from these labelled polysaccharides. * Results clearly demonstrate that ester methyl groups of pectin can serve as a precursor of CH4, supporting the idea of a novel chemical route of CH4 formation in plants under oxic environmental conditions.

Atmospheric constraints on global emissions of methane from plants

We investigate whether a recently proposed large source of CH_4 from vegetation can be reconciled with atmospheric measurements. Atmospheric transport model simulations with and without vegetation emissions are compared with background CH_4, δ^(13)C-CH_4 and satellite measurements. For present–day CH_4 we derive an upper limit to the newly discovered source of 125 Tg CH_4 yr^(−1). Analysis of preindustrial CH_4, however, points to 85 Tg CH_4 yr^(−1) as a more plausible limit. Model calculations with and without vegetation emissions show strikingly similar results at background surface monitoring sites, indicating that these measurements are rather insensitive to CH_4 from plants. Simulations with 125 Tg CH_4 yr^(−1) vegetation emissions can explain up to 50% of the previously reported unexpectedly high CH_4 column abundances over tropical forests observed by SCIAMACHY. Our results confirm the potential importance of vegetation emissions, and call for further research.

Abiotic Fe(III) induced mineralization of phenolic substances

The redox process between iron(III) (in dissolved form and as the mineral phase ferrihydrite) and phenolic substances has been examined. We investigated the relationship between the structure and reactivity for the dihydrobenzene reductants catechol, hydroquinone and resorcine, and for the 2-methoxyphenol guaiacol with iron(III), by determining the rate of the Fe(III) reduction as well as the production of CO2. This work demonstrates that catechol and guaiacol will be effectively oxidized to CO2 by reducing iron(III). Hydroquinone shows a reduction of iron(III), but no accompanying mineralization could be determined. In contrast, resorcine showed no reaction with Fe(II). The deciding factor on whether or not mineralization occurs were controlled by the position of the hydroxy groups. It is shown that phenolic substances with two hydroxy groups in the orthoposition or at least one hydroxy group and a methoxy group can be oxidized to CO2 while iron(III) is reduced.

Methane formation in aerobic environments

Methane (CH_4), the second principal anthropogenic greenhouse gas after CO_2, is the most abundant reduced organic compound in the atmosphere and plays a central role in atmospheric chemistry. Therefore a comprehensive understanding of its sources and sinks and the parameters that control emissions is prerequisite to simulate past, present and future atmospheric conditions. Until recently biological CH_4 formation has been associated exclusively with anoxic environments and methanogenic activity. However, there is growing and convincing evidence of alternative pathways in the aerobic biosphere including terrestrial plants, soils, marine algae and animals. Identifying and describing these sources is essential to complete our understanding of the biogeochemical cycles that control CH_4 in the atmospheric environment and its influence as a greenhouse gas.

Formation of chloroacetic acids from soil, humic acid and phenolic moieties

The mechanism of formation of chloroacetates, which are important toxic environmental substances, has been controversial. Whereas the anthropogenic production has been well established, a natural formation has also been suggested. In this study the natural formation of chloroacetic acids from soil, as well as from humic material which is present in soil and from phenolic model substances has been investigated. It is shown that chloroacetates are formed from humic material with a linear relationship between the amount of humic acid used and chloroacetates found. More dichloroacetate (DCA) than trichloroacetate (TCA) is produced. The addition of Fe(2+), Fe(3+) and H(2)O(2) leads to an increased yield. NaCl was added as a source of chloride. We further examined the relationship between the structure and reactivity of phenolic substances, which can be considered as monomeric units of humic acids. Ethoxyphenol with built-in ethyl groups forms large amounts of DCA and TCA. The experiments with phenoxyacetic acid yielded large amounts of monochloroacetate (MCA). With other phenolic substances a ring cleavage was observed. Our investigations indicate that chloroacetates are formed abiotically from humic material and soils in addition to their known biotic mode of formation.

Evidence for methane production by saprotrophic fungi

Methane in the biosphere is mainly produced by prokaryotic methanogenic archaea, biomass burning, coal and oil extraction, and to a lesser extent by eukaryotic plants. Here we demonstrate that saprotrophic fungi produce methane without the involvement of methanogenic archaea. Fluorescence in situ hybridization, confocal laser-scanning microscopy and quantitative real-time PCR confirm no contribution from microbial contamination or endosymbionts. Our results suggest a common methane formation pathway in fungal cells under aerobic conditions and thus identify fungi as another source of methane in the environment. Stable carbon isotope labelling experiments reveal methionine as a precursor of methane in fungi. These findings of an aerobic fungus-derived methane formation pathway open another avenue in methane research and will further assist with current efforts in the identification of the processes involved and their ecological implications.

Ultraviolet-radiation-induced methane emissions from meteorites and the Martian atmosphere

Almost a decade after methane was first reported in the atmosphere of Mars there is an intensive discussion about both the reliability of the observations—particularly the suggested seasonal and latitudinal variations—and the sources of methane on Mars. Given that the lifetime of methane in the Martian atmosphere is limited, a process on or below the planet’s surface would need to be continuously producing methane. A biological source would provide support for the potential existence of life on Mars, whereas a chemical origin would imply that there are unexpected geological processes. Methane release from carbonaceous meteorites associated with ablation during atmospheric entry is considered negligible. Here we show that methane is produced in much larger quantities from the Murchison meteorite (a type CM2 carbonaceous chondrite) when exposed to ultraviolet radiation under conditions similar to those expected at the Martian surface. Meteorites containing several per cent of intact organic matter reach the Martian surface at high rates, and our experiments suggest that a significant fraction of the organic matter accessible to ultraviolet radiation is converted to methane. Ultraviolet-radiation-induced methane formation from meteorites could explain a substantial fraction of the most recently estimated atmospheric methane mixing ratios. Stable hydrogen isotope analysis unambiguously confirms that the methane released from Murchison is of extraterrestrial origin. The stable carbon isotope composition, in contrast, is similar to that of terrestrial microbial origin; hence, measurements of this signature in future Mars missions may not enable an unambiguous identification of biogenic methane.

Enhanced formation of methane in plant cell cultures by inhibition of cytochrome c oxidase

The claim of methane (CH₄) formation in plants has caused much controversy and debate within the scientific community over the past 4 years. Here, using both stable isotope and concentration measurements, we demonstrate that CH₄ formation occurs in plant cell cultures that were grown in the dark under sterile conditions. Under non-stress conditions the plant cell cultures produced trace amounts [0.3-0.6 ng g⁻¹ dry weight (DW) h⁻¹] of CH₄ but these could be increased by one to two orders of magnitude (up to 12 ng g⁻¹ DW h⁻¹) when sodium azide, a compound known to disrupt electron transport flow at the cytochrome c oxidase (complex IV) in plant mitochondria, was added to the cell cultures. The addition of other electron transport chain (ETC) inhibitors did not result in significant CH₄ formation indicating that a site-specific disturbance of the ETC at complex IV causes CH₄ formation in plant cells. Our study is an important first step in providing more information on non-microbial CH₄ formation from living plants particularly under abiotic stress conditions that might affect the electron transport flow at the cytochrome c oxidase in plant mitochondria.