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.