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.

Emission of methane, carbon monoxide, carbon dioxide and short-chain hydrocarbons from vegetation foliage under ultraviolet irradiation.

The original report that plants emit methane (CH4 ) under aerobic conditions caused much debate and controversy. Critics questioned experimental techniques, possible mechanisms for CH4 production and the nature of estimating global emissions. Several studies have now confirmed that aerobic CH4 emissions can be detected from plant foliage but the extent of the phenomenon in plants and the precise mechanisms and precursors involved remain uncertain. In this study, we investigated the role of environmentally realistic levels of ultraviolet (UV) radiation in causing the emission of CH4 and other gases from foliage obtained from a wide variety of plant types. We related our measured emissions to the foliar content of methyl esters and lignin and to the epidermal UV absorbance of the species investigated. Our data demonstrate that the terrestrial vegetation foliage sampled did emit CH4 , with a range in emissions of 0.6-31.8 ng CH4 g(-1) leaf DW h(-1) , which compares favourably with the original reports of experimental work. In addition to CH4 emissions, our data show that carbon monoxide, ethene and propane are also emitted under UV stress but we detected no significant emissions of carbon dioxide or ethane.

Global methane emission estimates from ultraviolet irradiation of terrestrial plant foliage.

SUMMARY: *Several studies have reported in situ methane (CH(4)) emissions from vegetation foliage, but there remains considerable debate about its significance as a global source. Here, we report a study that evaluates the role of ultraviolet (UV) radiation-driven CH(4) emissions from foliar pectin as a global CH(4) source. *We combine a relationship for spectrally weighted CH(4) production from pectin with a global UV irradiation climatology model, satellite-derived leaf area index (LAI) and air temperature data to estimate the potential global CH(4) emissions from vegetation foliage. *Our results suggest that global foliar CH(4) emissions from UV-irradiated pectin could account for 0.2-1.0 Tg yr(-1), of which 60% is from tropical latitudes, corresponding to < 0.2% of total CH(4) sources. *Our estimate is one to two orders of magnitude lower than previous estimates of global foliar CH(4) emissions. Recent studies have reported that pectin is not the only molecular source of UV-driven CH(4) emissions and that other environmental stresses may also generate CH(4). Consequently, further evaluation of such mechanisms of CH(4) generation is needed to confirm the contribution of foliage to the global CH(4) budget.

The role of ultraviolet radiation, photosensitizers, reactive oxygen species and ester groups in mechanisms of methane formation from pectin.

Ultraviolet (UV) radiation has recently been demonstrated to drive an aerobic production of methane (CH(4)) from plant tissues and pectins, as do agents that generate reactive oxygen species (ROS) in vivo independently of UV. As the major building-blocks of pectin do not absorb solar UV found at the earth's surface (i.e. >280 nm), we explored the hypothesis that UV radiation affects pectin indirectly via generation of ROS which themselves release CH(4) from pectin. Decreasing the UV absorbance of commercial pectin by ethanol washing diminished UV-dependent CH(4) production, and this was restored by the addition of the UV photosensitizer tryptophan. Certain ROS scavengers [mannitol, a hydroxyl radical ((*)OH) scavenger; 1,4-diazabicyclo[2.2.2] octane; and iodide] strongly inhibited UV-induced CH(4) production from dry pectin. Furthermore, pectin solutions emitted CH(4) in darkness upon the addition of (*)OH, but not superoxide or H(2)O(2). Model carbohydrates reacted similarly if they possessed -CH(3) groups [e.g. methyl esters or (more weakly) acetyl esters but not rhamnose]. We conclude that UV evokes CH(4) production from pectic methyl groups by interacting with UV photosensitizers to generate (*)OH. We suggest that diverse processes generating (*)OH could contribute to CH(4) emissions independently of UV irradiation, and that environmental stresses and constitutive physiological processes generating ROS require careful evaluation in studies of CH(4) formation from foliage.

Ultraviolet radiation drives methane emissions from terrestrial plant pectins.

Recent studies demonstrating an in situ formation of methane (CH(4)) within foliage and separate observations that soil-derived CH(4) can be released from the stems of trees have continued the debate about the role of vegetation in CH(4) emissions to the atmosphere. Here, a study of the role of ultraviolet (UV) radiation in the formation of CH(4) and other trace gases from plant pectins in vitro and from leaves of tobacco (Nicotiana tabacum) in planta is reported. Plant pectins were investigated for CH(4 )production under UV irradiation before and after de-methylesterification and with and without the singlet oxygen scavenger 1,4-diazabicyclo[2.2.2]octane (DABCO). Leaves of tobacco were also investigated under UV irradiation and following leaf infiltration with the singlet oxygen generator rose bengal or the bacterial pathogen Pseudomonas syringae. Results demonstrated production of CH(4), ethane and ethylene from pectins and from tobacco leaves following all treatments, that methyl-ester groups of pectin are a source of CH(4), and that reactive oxygen species (ROS) arising from environmental stresses have a potential role in mechanisms of CH(4) formation. Rates of CH(4 )production were lower than those previously reported for intact plants in sunlight but the results clearly show that foliage can emit CH(4) under aerobic conditions.

Reactive oxygen species in aerobic methane formation from vegetation.

The first report of aerobic methane emissions from vegetation by an unknown mechanism suggested that this potential new source may make a significant contribution to global methane emissions. We recently investigated possible mechanisms and reported experiments in which UV-irradiation caused methane emissions from pectin, a major plant cell wall polysaccharide. Our findings also suggest that UV-generated reactive oxygen species (ROS) release methane from pectin. This has implications for all other, UV-independent processes which may generate ROS in or close to the plant cell wall and suggests a need to evaluate additional systems for ROS-generated methane emissions in leaves.