Plant mediated methane efflux from a boreal peatland complex

Purpose Aerenchymous plants are an important control for methane efflux from peatlands to the atmosphere, providing a bypass from the anoxic peat and avoiding oxidation in the oxic peat. We aimed to quantify the drivers of aerenchymous peatland species methane transport and the importance of this process for ecosystem-scale methane efflux. Methods We measured seasonal and interspecies variation in methane transport rate per gram of plant dry mass at a boreal fen and bog, which were upscaled to ecosystem-scale plant methane transport. Results Methane transport rate was better explained by plant species, leaf greenness and area than by environmental variables. Leaves appeared to transport methane even after senescence. Contrary to our expectations, both methane transport rate and the proportion of plant transport were lower in the fen (with greater sedge cover) than in the bog site. At the fen and bog, average methane transport rate was 0.7 and 1.8 mg g−1 d−1, and the proportion of seasonally variable plant transport was 7–41% and 6–90%, respectively. Species-specific differences in methane transport rate were observed at the ecosystem-scale: Scheuchzeria palustris, which accounted for 16% of the aerenchymous leaf area in the fen and displayed the greatest methane transport rate, was responsible for 45% of the ecosystem-scale plant transport. Conclusion Our study showed that plant species influence the magnitude of ecosystem-scale methane emissions through their properties of methane transport. The identification and quantification of these properties could be the pivotal next step in predicting plant methane transport in peatlands.

Methanotrophs: Discoveries, Environmental Relevance, and a Perspective on Current and Future Applications

Methane is the final product of the anaerobic decomposition of organic matter. The conversion of organic matter to methane (methanogenesis) as a mechanism for energy conservation is exclusively attributed to the archaeal domain. Methane is oxidized by methanotrophic microorganisms using oxygen or alternative terminal electron acceptors. Aerobic methanotrophic bacteria belong to the phyla Proteobacteria and Verrucomicrobia, while anaerobic methane oxidation is also mediated by more recently discovered anaerobic methanotrophs with representatives in both the bacteria and the archaea domains. The anaerobic oxidation of methane is coupled to the reduction of nitrate, nitrite, iron, manganese, sulfate, and organic electron acceptors (e.g., humic substances) as terminal electron acceptors. This review highlights the relevance of methanotrophy in natural and anthropogenically influenced ecosystems, emphasizing the environmental conditions, distribution, function, co-existence, interactions, and the availability of electron acceptors that likely play a key role in regulating their function. A systematic overview of key aspects of ecology, physiology, metabolism, and genomics is crucial to understand the contribution of methanotrophs in the mitigation of methane efflux to the atmosphere. We give significance to the processes under microaerophilic and anaerobic conditions for both aerobic and anaerobic methane oxidizers. In the context of anthropogenically influenced ecosystems, we emphasize the current and potential future applications of methanotrophs from two different angles, namely methane mitigation in wastewater treatment through the application of anaerobic methanotrophs, and the biotechnological applications of aerobic methanotrophs in resource recovery from methane waste streams. Finally, we identify knowledge gaps that may lead to opportunities to harness further the biotechnological benefits of methanotrophs in methane mitigation and for the production of valuable bioproducts enabling a bio-based and circular economy.

Methane efflux from an American bison herd

American bison (Bison bison L.) have recovered from the brink of extinction over the past century. Bison reintroduction creates multiple environmental benefits, but impacts on greenhouse gas emissions are poorly understood. Bison are thought to have produced some 2 Tg yr−1 of the estimated 9–15 Tg yr−1 of pre-industrial enteric methane emissions, but few measurements have been made due to their mobile grazing habits and safety issues associated with measuring non-domesticated animals. Here, we measure methane and carbon dioxide fluxes from a bison herd on an enclosed pasture during daytime periods in winter using eddy covariance. Methane emissions from the study area were negligible in the absence of bison (mean ± standard deviation = −0.0009 ± 0.008 µmol m−2 s−1) and were significantly greater than zero, 0.048 ± 0.082 µmol m−2 s−1, with a positively skewed distribution, when bison were present. We coupled bison location estimates from automated camera images with two independent flux footprint models to calculate a mean per-animal methane efflux of 58.5 µmol s−1 per bison, similar to eddy covariance measurements of methane efflux from a cattle feedlot during winter. When we sum the observations over time with conservative uncertainty estimates we arrive at 81 g CH4 per bison d−1 with 95 % confidence intervals between 54 and 109 g CH4 per bison d−1. Uncertainty was dominated by bison location estimates (46 % of the total uncertainty), then the flux footprint model (33 %) and the eddy covariance measurements (21 %), suggesting that making higher-resolution animal location estimates is a logical starting point for decreasing total uncertainty. Annual measurements are ultimately necessary to determine the full greenhouse gas burden of bison grazing systems. Our observations highlight the need to compare greenhouse gas emissions from different ruminant grazing systems and demonstrate the potential for using eddy covariance to measure methane efflux from non-domesticated animals.

Significant Fluctuation in the Global Sulfate Reservoir and Oceanic Redox State During the Late Devonian Mass Extinction

Sulfate plays an important role in the biosphere, hydrosphere, atmosphere, and lithosphere, and its concentration might have fluctuated greatly throughout the Earth’s history, in response to perturbations in the ocean-atmosphere system. Coupling high-resolution experimental results with an inverse modeling approach, we, here, show an unprecedented dynamic in the global sulfate reservoir during the Frasnian-Famennian (F-F) mass extinction, as one of the “Big five” Phanerozoic biotic crises. Notably, our results indicate that, in a relatively short time scale (~ 200 thousand years), seawater sulfate concentration would have dropped from several mM before the Upper Kellwasser Horizon (UKH) to a few hundreds of µM or perhaps lower at the dawn and during the UKH (more than 100 times lower than the modern level), and returned to around mM range after the event. The extremely low oceanic sulfate concentrations during the UKH would have enhanced the biological production of methane, leading to an increase in the methane efflux from the ocean and a warmer climate. Taken together, our findings indicate that the instability in the global sulfate reservoir may have played a major role in driving the Phanerozoic biological crises.

Assessing the potential for non-turbulent methane escape from the East Siberian Arctic Shelf

The East Siberian Arctic Shelf (ESAS) hosts large yet poorly quantified reservoirs of subsea permafrost and associated gas hydrates. It has been suggested that the global-warming induced thawing and dissociation of these reservoirs is currently releasing methane (CH4) to the shallow coastal ocean and ultimately the atmosphere. However, a major unknown in assessing the contribution of this CH4 flux to the global CH4 cycle and its climate feedbacks is the fate of CH4 as it migrates towards the sediment–water interface. In marine sediments, (an)aerobic oxidation reactions generally act as a very efficient methane sink. However, a number of environmental conditions can reduce the efficiency of this biofilter. Here, we used a reaction-transport model to assess the efficiency of the benthic methane filter and, thus, the potential for benthic methane escape across a wide range of environmental conditions that could be encountered on the East Siberian Arctic Shelf. Results show that, under steady-state conditions, anaerobic oxidation of methane (AOM) acts as an efficient biofilter. However, high CH4 escape is simulated for rapidly accumulating and/or active sediments and can be further enhanced by the presence of organic matter with intermediate reactivity and/or intense local transport processes, such as bioirrigation. In addition, in active settings, the sudden onset of CH4 flux triggered by, for instance, permafrost thaw or hydrate destabilization can also drive a high non-turbulent methane escape of up to 19 µmol CH4 cm−2 yr−1 during a transient, multi-decadal period. This “window of opportunity” arises due to delayed response of the resident microbial community to suddenly changing CH4 fluxes. A first-order estimate of non-turbulent, benthic methane efflux from the Laptev Sea is derived as well. We find that, under present-day conditions, non-turbulent methane efflux from Laptev Sea sediments does not exceed 1 Gg CH4 yr−1. As a consequence, we conclude that previously published estimates of ocean–atmosphere CH4 fluxes from the ESAS cannot be supported by non-turbulent, benthic methane escape.

Temporal water column dynamics control microbial methane oxidation above an active cold seep (Doggerbank, North Sea)

<p>Methane is a potent greenhouse gas with strongly increasing atmospheric concentrations since industrialisation. In the ocean, methane is most dominantly produced in sediments and is of microbial and/or thermogenic origin. Uprising methane may escape from the ocean floor to the overlying water column where it can be oxidized by methane oxidizing bacteria. The aerobic methane oxidation (MOx) is thus an important final barrier, which can mitigate methane release from the ocean to the atmosphere where it contributes to global warming. Nevertheless, there is rather little knowledge on the temporal dynamics of the microbial methane filter capacity in the water column. To gain a better understanding of the dynamics, we conducted two 48 hours’ time-series experiments during highly stratified conditions in summer and and mixed water column conditions in autumn above an active methane seep in the North Sea (Doggerbank, 41m water depth). At Doggerbank, dissolved CH<sub>4 </sub> δ<sup>13</sup>C-values (<-65 ‰) indicate a microbial CH<sub>4</sub> origin, and seismic data suggest a gas pocket at >50 m sediment depth. Our time series measurement showed that CH<sub>4</sub> concentrations were highly elevated with up to 2100 nM in bottom and 350 nM in surface waters under stratified conditions. The maxima showed a ~12h periodicity, indicating that the flux of CH<sub>4</sub> from the seep was linked to tidal dynamics with the lowest CH<sub>4</sub> concentrations at rising tide and enhanced flux at falling tide. In contrast, during mixed water column conditions we found lower maxima of only up to 450 nM. Yet, during mixed conditions we found that surface water methane concentrations were on average XX-fold higher compared to stratified conditions, suggesting a higher methane efflux to the atmosphere during this time period.  MOx activity showed a similar temporal behaviour suggesting that tidal dynamics are an important control on the efficiency of the microbial CH<sub>4</sub> filter in the water column. Under stratified conditions MOx rates were highest in bottom waters (<5.7 nM/day), however we also found high MOx rates in near-surface waters at times of elevated seep activity during stratified (<3.2 nM/day) and mixed water column conditions (<16.2 nM/day). Our results indicate that the efficiency of the microbial filter is affected by temporal dynamics and seasonality.</p>

Methane efflux from an American bison herd

American bison (Bison bison L.) have recovered from the brink of extinction over the past century. Bison reintroduction creates multiple environmental benefits, but their impacts on greenhouse gas emissions are poorly understood. Bison are thought to have produced some 2 Tg year−1 of the estimated 9–15 Tg year−1 of pre-industrial enteric methane emissions, but few contemporary measurements have been made due to their mobile grazing habits and safety issues associated with direct measurements. Here, we measure methane and carbon dioxide fluxes from a bison herd on an enclosed pasture during daytime periods in winter using eddy covariance. Methane emissions from the study area were negligible in the absence of bison (mean ± standard deviation = 0.0024 ± 0.042 μmol m−2 s−1) and were significantly greater than zero, 0.048 ± 0.082 μmol m−2 s−1 with a positively skewed distribution, when bison were present. We coupled an eddy covariance flux footprint analysis with bison location estimates from automated camera images to calculate a mean (median) methane flux of 38 μmol s−1 (22 μmol s−1) per animal, or 52 ± 14 g CH4 day−1 (31 g CH4 day−1), less than half of measured emission rates for range cattle. Emission estimates are subject to spatial uncertainty in bison location measurements and the flux footprint, but from our measurements there is no evidence that bison methane emissions exceed those from cattle. We caution however that our measurements were made during winter and that evening measurements of bison distributions were not possible using our approach. Annual measurements are ultimately necessary to determine the greenhouse gas burden of bison grazing systems. Eddy covariance is a promising technique for measuring ruminant methane emissions in conventional and alternate grazing systems and can be used to compare them going forward.

Assessing the potential for non-turbulent methane escape from the East Siberian Arctic Shelf

East Siberian Arctic Shelf (ESAS) hosts large, yet poorly quantified reservoirs of subsea permafrost and associated gas hydrates. It has been suggested the global-warming induced thawing and dissociation of these reservoirs is currently releasing methane to the shallow shelf ocean and ultimately the atmosphere. However, the exact contribution of permafrost thaw and methane gas hydrate destabilization to benthic methane efflux from the warming shelf and ultimately methane-climate feedbacks remains controversial. A major unknown is the fate of permafrost and/or gas hydrate-derived methane as it migrates towards the sediment-water interface. In marine sediments, (an)aerobic oxidation reactions generally act as extremely efficient biofilters that often consume close to 100 % of the upward migrating methane. However, it has been shown that a number of environmental conditions can reduce the efficiency of this biofilter, thus allowing methane to escape to the overlying ocean. Here, we used a reaction-transport model to assess the efficiency of the benthic methane filter and, thus, the potential for permafrost and/or gas hydrate derived methane to escape shelf sediments under a wide range of environmental conditions encountered on East Siberian Arctic Shelf. Results of an extensive sensitivity analysis show that, under steady state conditions, anaerobic oxidation of methane (AOM) acts as an efficient biofilter that prevents the escape of dissolved methane from shelf sediments for a wide range of environmental conditions. Yet, high CH4 escape comparable to fluxes reported from mud-volcanoes is simulated for rapidly accumulating (sedimentation rate > 0.7 cm yr−1) and/or active (active fluid flow > 6 cm yr−1) sediments and can be further enhanced by mid-range organic matter reactivity and/or intense local transport processes, such as bioirrigation. In active settings, high non-turbulent methane escape of up to 19 μmolCH4 cm−2 yr−1 can also occur during a transient, multi-decadal period following the sudden onset of CH4 flux triggered by, for instance, permafrost thaw or hydrate destabilization. This window of opportunity arises due to the time needed by the microbial community to build up an efficient AOM biofilter. In contrast, seasonal variations in environmental conditions (e.g. bottom water SO42−, CH4 flux) exert a negligible effect on CH4 efflux through the sediment-water interface. Our results indicate that present and future methane efflux from ESAS sediments is mainly supported by methane gas and non-turbulent CH4 efflux from rapidly accumulating and/or active sediments (e.g. coastal settings, portions close to river mouths or submarine slumps). In particular active sites on the ESAS may release methane in response to the onset or increase of permafrost thawing or CH4 gas hydrate destabilization rates. Model results also reveal that AOM generally acts as an efficient biofilter for upward migrating CH4 under environmental conditions that are representative for the present-day ESAS with potentially important, yet unquantified implications for the Arctic ocean's alkalinity budget and, thus, CO2 fluxes. The results of the model sensitivity study are used as a quantitative framework to derive first-order estimates of non-turbulent, benthic methane efflux from the Laptev Sea. We find that, under present day conditions, AOM is an efficient biofilter and non-turbulent methane efflux from Laptev Sea sediments does not exceed 1 GgCH4 yr−1. As a consequence, we state that previously published estimates of fluxes from ESAS water into atmosphere cannot be supported by non-turbulent methane escape from the sediments, but require the build-up and preferential escape of benthic methane gas from the sediments to the atmosphere that matches or even exceeds such estimated fluxes.