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

In eutrophic coastal waters, aerobic methane-oxidizing bacteria (MOB) mitigate methane emissions by oxidizing benthic methane even in the stratified, anoxic water column. However, ongoing warming and eutrophication lead to extended stratification periods, enhancing anoxic and sulfidic conditions (euxinia), potentially affecting methane removal capacity. Here we compared overall water column methane removal between sites with irregular, seasonal and longer-term euxinia in the Stockholm Archipelago during summer 2022. The highest water-air methane emissions, bottom water-methane and sulfide accumulation, and the lowest methane oxidation potential were observed under longer-term euxinic bottom water conditions. While MOB relative abundance and potential activity indicated high functioning of the methane biofilter in the seasonally euxinic bottom water layer, the methane-filtering potential was much lower in the longer-term euxinic bottom water. Under persistent euxinic conditions, overall bacterial diversity and microbial network connectivity were lower, likely following a simultaneous shift in redox conditions and a shift toward anaerobic sulfur-cycling. This shift may force MOB to retreat from the euxinic bottom water into the narrow oxycline, reducing the capacity of the methane biofilter and resulting in higher methane emissions. These findings highlight the positive feedback loop that can further amplify oceanic methane emissions, particularly from eutrophic and shallow coastal waters prone to prolonged stratification under global warming.