Environmental controls on microbial methane oxidation in the coastal ocean

Abstract

Methane is a potent greenhouse gas that significantly contributes to global warming, yet it often receives less attention than carbon dioxide. While anthropogenic sources of methane are well-documented, natural sources, especially marine environments, remain less understood, introducing uncertainties into the global methane budget. Coastal waters on the inner continental shelf, despite covering a small fraction of the global ocean, are major contributors to marine methane emissions due to their high productivity and the presence of cold seeps. These shallow ecosystems often exhibit elevated methane concentrations, with their close seafloor-atmosphere proximity limiting the effectiveness of methane-oxidizing bacteria (methanotrophs) to fully mitigate emissions. Dynamic factors such as ocean currents, tidal fluctuations, seasonal variations, and land runoff further influence microbial activity and the efficiency of the "microbial methane filter." This thesis investigates the relationship between coastal waters and atmospheric methane levels, focusing on how environmental factors affect microbial methane oxidation. To address limitations of prior studies, this research incorporates high-frequency measurements across diverse coastal regions, including the Doggerbank seep area, the Dutch Wadden Sea, and Arctic seep areas north of Svalbard. Complementary laboratory experiments simulated future climate scenarios to evaluate methanotroph resilience to environmental changes in temperature, salinity, and methane concentration. In the Doggerbank seep area, tides and seasonal stratification strongly influenced methane release. Falling tides triggered methane emissions and increased methanotroph activity, while fully mixed autumn conditions reduced microbial efficiency, leading to higher atmospheric methane release. In the Wadden Sea, warmer seasons showed elevated methane concentrations and oxidation rates but also increased atmospheric emissions. Even in colder seasons, methane supersaturation persisted, with wind and tidal currents transporting methane to the North Sea. These findings emphasize the interplay of seasonal, tidal, and environmental dynamics in shaping methane fluxes. In Arctic regions, studies revealed that bottom waters in seep systems north of Svalbard contained higher methane concentrations and supported diverse methanotrophic communities. Hydrological connectivity, driven by the West Spitsbergen Current, influenced community similarities between sites. While methanotrophs rapidly responded to elevated methane levels, their capacity to fully oxidize methane was constrained by water currents, underscoring the role of physical transport in methane dispersal. Laboratory experiments highlighted the adaptability of methanotrophs to varying methane levels and environmental conditions, with shifts in methane availability significantly impacting microbial community composition. Functional redundancy within these communities suggested their capacity to adapt to high-methane scenarios in a changing ocean. This research advances understanding of methane dynamics in coastal waters, demonstrating how environmental factors govern the microbial methane filter's efficiency. Insights from this work are crucial for predicting methane emissions under future climate conditions and contribute to broader efforts to mitigate greenhouse gas emissions.