Sediment microbial ecology and methane dynamics during resuspension events in a hypersaline coastal lagoon

Abstract

Wetlands play a dual role in the global greenhouse gas (GHG) cycle, acting as important carbon sinks, while also influencing GHG production, particularly methane (CH4). 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, salinity stress adaptations employed by resident methanogenic Archaea may introduce an interesting evolutionary edge in this cycle. 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 shallow coastal lagoon in south-eastern Australia, 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. Resuspension events are a regular feature of the system, driven by high wind energy. A 4-7-fold increase in CH4 concentration was recently measured during a 2021 resuspension event, which may represent a novel pathway of CH4release to the atmosphere, bypassing coupled methanotrophy in the oxic water column. The dynamics between GHG emissions and microbial regulation persists as a knowledge gap in global climate modelling. To address this, we take a multi-disciplinary approach, integrating microbial ecology, biogeochemistry and hydrodynamic modelling to elucidate methane flux pathways in hypersaline environments, improving robustness of global GHG budgets.