Mercury distribution, methylation and volatilization in microcosms with and without the sea anemone Bunodosoma caissarum

•203Hg distribution was studied in microcosms with and without Bunodosoma caissarum.•B. caissarum presented a Hg bioconcentration factor of 70.•The presence of B. caissarum resulted in a higher volatilization of Hg.•More MeHg was found in the seawater of microcosms without B. caissarum.•Microbes associated with marine organisms play an understudied role in Hg cycle. Mercury (Hg) has a complex biogeochemical cycle in aquatic environments. Its most toxic form, methylmercury (MeHg), is produced by microorganisms. This study investigated how the sea anemone Bunodosoma caissarum affects Hg distribution, methylation and volatilization in laboratory model systems. 203Hg was added to microcosms and its distribution in seawater, specimens and air was periodically measured by gamma spectrometry. MeHg was measured by liquid scintillation. After the uptake period, specimens had a bioconcentration factor of 70 and in microcosms with and without B. caissarum, respectively 0.05% and 0.32% of the initial spike was found as MeHg. After depuration, MeHg in specimens ranged from 0.2% to 2.4% of total Hg. Microcosms with B. caissarum had higher Hg volatilization (58%) than controls (17%), possibly due to Hg2+ reduction mediated by microorganisms associated with its tissues and mucus secretions. Marine organisms and their associated microbiota may play a role in Hg and MeHg cycling.