Oxic Methanogenesis and Aquatic Methane Fluxes

800px-Canada_Island,_Lake_Lila

24th November 2014.

Recent research published in Nature Communications has shed new light on aquatic methane dynamics by demonstrating that methanogenesis occurs at the ecosystem-scale in oxic lake waters.

Despite being a recognised mechanism of methane production, the significance of methanogenesis in oxic waters has yet to be determined.  The researchers, from the Université du Québec à Montréal, used floating mesocosms that were closed at the bottom but open to the atmosphere, thus excluding non-pelagic methane sources.

During the 28-day experiment, dissolved methane concentrations increased in the mesocosms, and water/air fluxes ranged from 0.07 to 0.36 mmol m-2 day-1.  Furthermore, methane production was higher in mesocosms treated with nutrient enrichment, and lower in those treated with dissolved organic carbon (DOC).  The difference in methane concentrations and fluxes between treatments was linked to algal dynamics, specifically both gross primary production and net ecosystem production.  Analysis also suggested that the methane was produced through acetoclastic methanogenesis.  Considering the lake as a whole, mean fluxes from the control (i.e. not-treated) mesocosms accounted for approximately 20% of diffusive fluxes measured during summer, and were of a similar value to ebullitive fluxes.

The authors stress that this is not a missing measurement from aquatic carbon budgets, as this component is captured during standard flux measurements.  However, they point out that “incorporating the origin of the methane that outfluxes to the atmosphere from these lakes is critical to our understanding of the regulation of these fluxes and our capacity to predict their future change. In this regard, there are potentially large global implications for algal-driven oxic-water methanogenesis.”  Additionally, because methane dynamics differed in response to nutrient and DOC treatments, it is likely that aquatic methane emissions will be affected by environmental changes.

Reference:

Oxic water column methanogenesis as a major component of aquatic CH4 fluxes. 2014. Bogard, M.J., del Giorgio, P.A., Boutet, L., et al. Nature Communications, DOI: 10.1038/ncomms6350.

Photo: Canada Island, in Lake Lila, William C. Whitney Wilderness Area, Adirondack Park, Long Lake, NY, USA, by Mwanner. CC BY-SA 3.0.

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Methane, Methanogens and Permafrost

Permafrost_thaw_ponds_in_Hudson_Bay_Canada_near_Greenland

10th November 2014.

It has been known for some time that permafrost stores a significant proportion of global soil carbon, and that thawing permafrost acts as an atmospheric source of methane and carbon dioxide, thereby creating a positive feedback to climate change.  Earlier in 2014, an international team of researchers discovered a new methanogen in the class Methanomicrobia.  They named the new species Methanoflorens stordalenmirensis and proposed a new family for it: Methanoflorentaceae.

New research by the same team, at the same Swedish field site, has shed more light on the activities of M. stordalenmirensis.  The team used a natural gradient of permafrost thaw to determine how thaw-induced changes in vegetation and hydrology affect methane dynamics.  They found that average methane fluxes were zero at an intact permafrost site, but increased to 1.46 mg CH4 m−2 h−1 at a thawing Sphagnum site, and increased further to 8.75 mg CH4 m−2 h−1 at a fully thawed site dominated by Eriophorum.  There was also a significant difference in the isotopic signature of the produced methane: average δ13C of emitted methane was −79.6 ‰ at the Sphagnum site and −66.3 ‰ at the Eriophorum site.  A similar pattern was observed for porewater CH4 isotopes between the Sphagnum and Eriophorum site.

These changes in methane dynamics were accompanied by changes in the microbial communities of the sites.  For the intact site there was a low abundance of methanogens, whilst the communities at the Eriophorum site and the deeper (anaerobic) layers of the Sphagnum site contained 20-30 % methanogens.  The Sphagnum site was dominated by hydrogenotrophic methanogens, including species similar to M. stordalenmirensis, whilst the Eriophorum site featured a high abundance of acetoclastic methanogens in the genus Methanosaeta.

Further analysis suggested that M. stordalenmirensis was the best single-variable predictor of isotopic patterns, although a multi-variable model also highlighted the importance of organic matter chemistry.  Altogether, these results show how ecosystem-scale methane dynamics are driven by changes in the composition of microbial communities.  Such research can be incorporated into future models of permafrost thaw and climate change methane feedbacks.

Lead author Carmody McCalley at the University of New Hampshire said: “By taking microbial ecology into account, we can accurately set up climate models to identify how much methane comes from thawing permafrost versus other sources such as fossil-fuel burning.”

References:

Discovery of a novel methanogen prevalent in thawing permafrost. 2014. Mondav, R., Woodcroft, B.J., Kim, E-H., et al. Nature Communications, 5, doi:10.1038/ncomms4212.

Methane dynamics regulated by microbial community response to permafrost thaw. 2014. McCalley, C.K., Woodcroft, B.J., Hodgkins, S.B., et al. Nature, 514, doi:10.1038/nature13798.

Photo: Permafrost thaw ponds, Hudson Bay, by Steve Jurvetson. CC BY 2.0

Temperature Dependence of Methane Fluxes Across Different Scales

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21st March 2014. A new paper published in Nature has shed light on the subject of the temperature dependence of methane fluxes measured across three different scales: cultures of methanogens, laboratory sediment incubations, and field measurements.  The meta-analysis considered wetlands (such as peat bogs), aquatic systems (such as lakes and rivers), and rice-paddy fields.

The analysis showed that the temperature dependence for the three different ecosystems was indistinguishable, with an average activation energy of 0.96 eV.  Furthermore, temperature dependence at the ecosystem scale was statistically identical to that of cultured methanogens (1.1 eV) and sediment incubations (0.93 eV).  The authors therefore suggest that the ecosystem-scale seasonal temperature dependence of methane fluxes is driven primarily by the methanogenic community.

Lead author, Dr Yvon-Durocher said: “This is important because biological methane fluxes are a major component of global methane emissions, but there is uncertainty about their magnitude and the factors that regulate them. This hinders our ability to predict the response of this key component of the carbon cycle to global warming. Our research provides scientists with an important clue about the mechanisms that may control the response of methane emissions from ecosystems to global warming.”

This similarity in temperature dependence across scales and ecosystems has important ramifications, as discussed by the paper.  For instance, the calculated temperature dependence of methane emissions is higher than that of carbon dioxide fluxes from both respiration (0.65 eV) and photosynthesis (0.3 eV).  The authors investigated the implications of this difference in activation energy in relation to a warming climate.  They found that the contribution of methane fluxes to total carbon emissions increased at elevated temperatures.  As methane is an extremely potent greenhouse gas, this finding suggests that positive feedbacks between climate change and the carbon cycle could be stimulated in future.  However, substantial variation existed in the temperature dependence of methane fluxes at the ecosystem scale, and the authors hypothesise that other factors such as water table depth, methanotrophy, substrate supply and microbial community structure could constrain changes on methane emissions in the long-term.

Dr Yvon-Durocher, from the Environment and Sustainability Institute (ESI) at the University of Exeter’s Penryn Campus in Cornwall, added: “The discovery that methane fluxes are much more responsive to temperature than the processes that produce and consume carbon dioxide highlights another mechanism by which the global carbon cycle may serve to accelerate rather than mitigate future climate change. However more research, using our results as a platform for refining Earth system models, is required to fully explore the consequences of our findings for future levels of climate change”.

Reference – Yvon-Durocher, G., Allen, A.P., Bastviken, D., Conrad, R., Gudasz, C., St-Pierre, A., Thanh-Duc, N., del Giorgio, P.A. 2014. Methane fluxes show consistent temperature dependence across microbial to ecosystem scales. Nature, doi:10.1038/nature13164

Photograph – a Welsh peatland on the island of Anglesey. Mike Peacock.