Sulfide Toxicity as Key Control on Anaerobic Oxidation of Methane in Eutrophic Coastal Sediments

Coastal zones account for 75% of marine methane emissions,
despite
covering only 15% of the ocean surface area. In these ecosystems,
the tight balance between methane production and oxidation in sediments
prevents most methane from escaping into seawater. However, anthropogenic
activities could disrupt this balance, leading to an increased methane
escape from coastal sediments. To quantify and unravel potential mechanisms
underlying this disruption, we used a suite of biogeochemical and
microbiological analyses to investigate the impact of anthropogenically
induced redox shifts on methane cycling in sediments from three sites
with contrasting bottom water redox conditions (oxic-hypoxic-euxinic)
in the eutrophic Stockholm Archipelago. Our results indicate that
the methane production potential increased under hypoxia and euxinia,
while anaerobic oxidation of methane was disrupted under euxinia.
Experimental, genomic, and biogeochemical data suggest that the virtual
disappearance of methane-oxidizing archaea at the euxinic site occurred
due to sulfide toxicity. This could explain a near 7-fold increase
in the extent of escape of benthic methane at the euxinic site relative
to the hypoxic one. In conclusion, these insights reveal how the development
of euxinia could disrupt the coastal methane biofilter, potentially
leading to increased methane emissions from coastal zones.