💨 Coastal wetlands are key ecosystems that play a significant role in the global carbon and nitrogen cycle. They have traditionally been considered a small source of methane (CH₄). Methane is a potent greenhouse gas that has a significant global warming potential. However, recent studies have shown that CH₄ emissions from coastal wetlands are highly variable and can often exceed carbon sequestration in terms of CO₂ equivalent.

Methane dynamics in these ecosystems are primarily driven by two opposing processes: the production of CH₄ by methanogens and its subsequent oxidation, facilitated by methanotrophs. Methane oxidation serves as a critical control mechanism that reduces up to 90 % of methane produced in soils under anoxic conditions before it is dissipated into the atmosphere. In oxygen-deficient environments, typical of coastal zones, anaerobic methane oxidation (AMO) pathways predominate.

One of the main types of AMO in coastal wetlands is sulfate-dependent anaerobic methane oxidation (S-DAMO). This is because sulfate (SO₄²⁻) is the main redox-active compound in seawater and is introduced into these systems by tidal inputs. S-DAMO is carried out by anaerobic methanotrophic archaea (ANME), primarily the ANME-1 and ANME-2c groups.

The new study, which uses a five-year field experiment in a brackish coastal wetland, examined how global climate change, specifically warming and elevated carbon dioxide (eCO₂), affect sulfate dynamics and S-DAMO activity. The experiment, conducted at the Smithsonian's Global Change Research Wetland, included four climate regimes: ambient conditions, eCO₂ alone, warming alone (+5.1 °C above ambient), and a combination of warming and eCO₂ (+5.1 °C + 350 ppm CO₂).

The results showed that sulfate dynamics responded differently to individual climatic influences. Warming has reduced sulfate concentrationsThis decrease is likely due to an accelerated rate of sulfate reduction at higher temperatures, which is not fully compensated by oxidation processes. The decrease in sulfate concentration due to warming subsequently reduced S-DAMO speedsConsistent with this finding, Warmer areas showed increased CH₄ emissionsThe study suggests that reduced S-DAMO activity contributed to higher methane emissions during warming.

On the contrary, increased CO₂ concentration (eCO₂) led to an increase in sulfate concentration. This effect is thought to be linked to the observed increase in plant root productivity under eCO₂. Greater root growth improves the transport of oxygen from the atmosphere to the soil. The increased oxygen availability likely promoted the oxidation of sulfides (a product of sulfate reduction) back to sulfate, thereby regenerating this key electron acceptor for S-DAMO. Higher sulfate concentrations led to stimulation of S-DAMO ratesThese findings suggest that sulfate availability limits S-DAMO rates in relatively low salinity environments such as this study site. Consistent with the increased S-DAMO activity, eCO₂ areas showed reduced CH₄ emissions.

The combined effects of warming and eCO₂ led to S-DAMO rates that were higher than those under ambient conditions or warming alone. It seems that the effect of eCO₂ on increasing sulfate availability and thus S-DAMO was able to compensate to some extent for the negative effect of warming. A positive correlation was also observed between S-DAMO rates and sulfate concentrations, as well as between S-DAMO rates and the abundance of the ANME-1 and ANME-2c genes, the microorganisms responsible for S-DAMO.

A study at this site found that nitrite/nitrate-dependent anaerobic methane oxidation (N-DAMO) played only a minor role, likely due to low nitrogen inputs compared to some coastal areas in Asia where N-DAMO is more significant. This highlights that the main mechanism of AMO may vary depending on local conditions, particularly the availability of different electron acceptors.

These findings underscore the potential of climate change to alter soil AMO activity through changes in sulfate dynamics. The authors suggest that it is it is necessary to include these processes in prediction models for a more accurate representation of methane dynamics in coastal wetlands and better predictions of future CH₄ emissions in a changing climate. The extent to which S-DAMO mitigates methane emissions may vary depending on site conditions, but even the low percentage of methane removed (7-12 % at this site) is significant given the potency of methane as a greenhouse gas. Spring

Glossary of key terms

• AMO (Anaerobic Methane Oxidation): A microbial process in which methane is oxidized (decomposed) in the absence of oxygen, often with the help of other electron acceptors.
• ANME (Anaerobic Methanotrophic) archaea: A group of archaea responsible for anaerobic methane oxidation.
• S-DAMO (Sulfate-Dependent Anaerobic Methane Oxidation): A form of AMO where sulfate (SO4 2-) serves as an electron acceptor.
• N-DAMO (Nitrate/Nitrite-Dependent Anaerobic Methane Oxidation): A form of AMO where nitrate (NO3 -) or nitrite (NO2 -) serves as the electron acceptor.
• Redox Potential: A measure of the tendency of a chemical environment to accept or donate electrons; a low redox potential indicates reducing (oxygen-free) conditions.
• Sulfate Reduction: A microbial process in which sulfate is reduced to sulfide (H2S) under anaerobic conditions.
• Sulfate Regeneration: The process by which sulfide is oxidized back to sulfate, often in the presence of oxygen.
• eCO2 (Elevated CO2): Increased concentration of atmospheric carbon dioxide above ambient levels.
• Gdw: Grams of dried matter (soil), used to normalize microbial activity or abundance.
• 13C stable isotope technique: A technique using the heavier isotope of carbon (13C) to monitor and measure the rate of biogeochemical processes, such as methane oxidation.
• qPCR (quantitative Polymerase Chain Reaction): A laboratory technique used to quantify the amount of specific DNA, for example to determine the abundance of particular microorganisms.
• Methanogens: Microorganisms that produce methane as a byproduct of their metabolism.
• Methanotrophs: Microorganisms that metabolize methane as a source of energy and carbon.