Intracellular speciation and transformation of inorganic mercury in marine phytoplankton

•The ability of marine phytoplankton to convert inorganic mercury was examined.•All tested algae could convert Hg(II) into gaseous Hg (DGM), phytochelatin-binding Hg (PC), and metacinnabar (β-HgS).•Of the added Hg(II), less than 5% was reduced to DGM per day in all species.•Of the intracellular Hg, <20% and 20–90% were chelated by PCs and transformed into β-HgS, respectively.•Intracellular biotransformation was more important than bioaccumulation in Hg(II) detoxification.

Metal speciation is closely related to toxicity in aquatic organisms, but quantitative study of mercury transformation has rarely been reported. In this study, the ability of three marine phytoplankton species, including a green alga Chlorella autotrophica, a flagellate Isochrysis galbana and a diatom Thalassiosira weissflogii, to convert inorganic mercury were examined. We found that all algae tested were able to transform Hg(II) into dissolved gaseous mercury (DGM), phytochelatin (PC) complexes and metacinnabar (β-HgS). The most tolerant species, T. weissflogii, generally produced the highest level of PCs and β-HgS. Attributed to the highest DGM production ability, C. autotrophica accumulated the least Hg, but was the most sensitive due to low PC induction and β-HgS formation. Of the added Hg(II), less than 5% was reduced to DGM per day in all species. Of the intracellular Hg, <20% and 20–90% were chelated by PCs and transformed into β-HgS, respectively. These results suggest that intracellular biotransformation might be more important than bioavailability regulation in Hg(II) detoxification in marine phytoplankton.