Simulated Sea Level Rise in Coastal Peat Soils Stimulates Mercury Methylation

Coastal wetlands are vulnerable to sea level rise with
unknown
consequences for mercury (Hg) cycling, particularly the potential
for exacerbating neurotoxic methylmercury (MeHg) production and bioaccumulation
in food webs. Here, the effect of sea level rise on MeHg formation
in the Florida Everglades was evaluated by incubating peat cores from
a freshwater wetland for 0–20 days in the laboratory at five
salinity conditions (0.16–6.0 parts-per-thousand; 0.20–454
mg L–1 sulfate (SO42–)) to simulate the onset of sea level rise within coastal margins.
Isotopically enriched inorganic mercury (201Hg(II)) was
used to track MeHg formation and peat-porewater partitioning. In all
five salinity treatments, porewaters became anoxic within 1 day and
became progressively enriched in dissolved organic matter (DOM) of
greater aromatic composition over the 20 days compared to ambient
conditions. In the four highest salinity treatments, SO42– concentrations decreased and sulfide concentrations
increased over time due to microbial dissimilatory SO42– reduction that was concurrent with 201Hg(II) methylation. Importantly, elevated salinity resulted in a
greater proportion of produced Me201Hg observed in porewaters
as opposed to bound to peat, interpreted to be due to the complexation
of MeHg with aromatic DOM released from peat. The findings highlight
the potential for enhanced production and mobilization of MeHg in
coastal wetlands of the Florida Everglades due to the onset of saltwater
intrusion.