The influence of microplastics on mercury biogeochemistry in San Francisco Bay and Lake Erie

Microplastics are an emerging contaminant that threaten the health of aquatic ecosystems. These small (<5 mm) fragments of deteriorated plastic concentrate contaminants and are consumed by zooplankton, fish, marine mammals, and sea birds. Microplastics also host microbial biofilms that may alter the biogeochemical cycling of metals such as mercury. We investigated the influence of microplastics on mercury cycling in San Francisco Bay and Lake Erie. Plastic abundance ranged from 1,000 to 2,000 items km^-2 in San Francisco Bay and 8,000 to 65,000 items km^-2 in Lake Erie. Concentrations of total mercury adsorbed to plastic fragments were less than 15 ng/g in Lake Erie with ~1% as methylmercury. In San Francisco Bay, concentrations of total mercury adsorbed to microplastics were 25-100 ng/g and methylmercury concentrations were within the range of Bay plankton. Partition coefficients for plastic in each system were 1–3 orders of magnitude lower than natural particles. Metagenomic sequencing was used to characterize microbial biofilms attached to plastic debris and mercury cycling genes from the mer operon (i.e. merA) were identified. Incubation experiments with enriched mercury isotopes in seawater revealed lower production rates of methylmercury with the addition of microplastics. Microfibers were found in fish guts from both San Francisco Bay and Lake Erie, however, there was no correlation between fiber ingestion and fish mercury concentrations. Microplastics adsorb primarily inorganic mercury from aquatic environments and may aid in the detoxification (i.e. reduction & demethylation) of methylmercury.