Microbial cycling of mercury in contaminated pelagic and wetland sediments of San Pablo Bay, California

San Pablo Bay is an estuary, within northern San Francisco Bay, containing elevated sediment mercury (Hg) levels because of historic loading of hydraulic mining debris during the California gold-rush of the late 1800s. A preliminary investigation of benthic microbial Hg cycling was conducted in surface sediment (0–4 cm) collected from one salt-marsh and three open-water sites. A deeper profile (0–26 cm) was evaluated at one of the open-water locations. Radiolabeled model Hg-compounds were used to measure rates of both methylmercury (MeHg) production and degradation by bacteria. While all sites and depths had similar total-Hg concentrations (0.3–0.6 ppm), and geochemical signatures of mining debris (as εNd, range: –3.08 to –4.37), in-situ MeHg was highest in the marsh (5.4±3.5 ppb) and ≤0.7 ppb in all open-water sites. Microbial MeHg production (potential rate) in 0–4 surface sediments was also highest in the marsh (3.1 ng g^–1 wet sediment day^–1) and below detection (