Wetlands play a dual role in the global greenhouse gas (GHG) cycle, acting as important carbon sinks, while also producing up to 33% of global methane (CH4) (Jackson, et al 2020). Methanogenesis is not typically associated with hypersaline wetlands due to competition from sulfate-reducing microbes, capable of more energy efficient metabolism while competing for similar substrates within the same niche. However, metabolism of compatible solutes (synthesised as a salinity stress adaptation by halophilic microbes) may introduce an interesting evolutionary edge in this cycle for hypersaline methanogens (Welsh 2000).
Methane production may be further enhanced by wind-induced sediment resuspension. In anoxic sediment, CH4 production is typically controlled by methanotrophy in the oxic water column. However, sediment resuspension may create anoxic micro-niches near the water-atmosphere interface, potentially enhancing atmospheric CH4 flux.
Our study site, The Coorong, is a degraded shallow coastal lagoon at the estuary of the Murray-Darling Basin, which experiences a warm-temperate to arid climate. It functions as a reverse estuary and is subject to hypersalinity, particularly during drought conditions, reaching up to 5 times marine salinity (Moseley, et al 2017). Resuspension events are a regular feature of the system, driven by high wind energy. A 4-7-fold increase in CH4 concentration was measured during a 2021 resuspension event, which may represent a novel pathway of CH4 release to the atmosphere, bypassing coupled methanotrophy in the oxic water column.
The dynamics between GHG emissions and microbial regulation in wetland ecosystems persists as a knowledge gap in global climate modelling. To address this, we integrate microbial ecology, biogeochemistry and physical limnology to elucidate methane flux pathways in hypersaline environments, working towards improving robustness of global GHG budgets.