Long-Term Euxinia Restricts Microbial Methane Removal in Eutrophic Coastal Basins

Area of Science:

• Marine microbial ecology
• Biogeochemical cycles
• Climate change impacts

Background:

• Aerobic methane-oxidizing bacteria (MOB) in coastal waters mitigate methane emissions.
• Increased warming and eutrophication lead to prolonged stratification, enhancing anoxic and sulfidic conditions (euxinia).
• Euxinia may negatively impact the methane removal capacity of MOB.

Purpose of the Study:

• Compare water column methane removal across sites with varying degrees of euxinia.
• Investigate the effects of prolonged euxinia on methane-oxidizing bacteria and overall methane emissions.
• Understand the feedback loop between stratification, euxinia, and methane emissions in coastal ecosystems.

Main Methods:

• Field study in the Stockholm Archipelago during summer 2022.
• Comparison of methane emissions, methane and sulfide concentrations, and methane oxidation potential across sites.
• Assessment of MOB abundance, activity, bacterial diversity, and microbial network connectivity.

Main Results:

• Longer-term euxinic conditions correlated with higher methane emissions and sulfide accumulation.
• Methane oxidation potential was significantly lower in persistently euxinic bottom waters.
• While MOB showed activity in seasonally euxinic layers, their capacity was reduced in longer-term euxinic zones.
• Persistent euxinia led to lower bacterial diversity and network connectivity, favoring sulfur-cycling bacteria.

Conclusions:

• Prolonged euxinia in coastal waters reduces the methane biofilter capacity by impacting MOB.
• This reduction can lead to a positive feedback loop, amplifying methane emissions.
• Shallow, stratified coastal waters under global warming are particularly vulnerable to increased methane release.