Role of phytoplankton in aquatic mercury speciation and transformations

Environmental context Understanding mercury transformations in the aquatic environment is of utmost importance for the improvement of mercury biogeochemical modelling and sound environmental risk assessment. In such a context, we discuss critically the advancement in the knowledge on the role of the phytoplankton (algae and cyanobacteria) in mercury cycling and transformations in the aquatic environment. Important research advances revealed that different microalgal species and cyanobacteria contribute: to biotic reduction of inorganic mercury to elemental mercury; to demethylation of methylmercury and transformation of inorganic mercury into metacinnabar; and to production of different biomolecules which can contribute to abiotic mercury reduction. Abstract Phytoplankton may directly influence biogeochemical cycling and transformations of mercury (Hg) through biotic transformations of the accumulated metal via methylation/demethylation and reduction/oxidation, and indirectly, through the excretion of low and high molecular weight ligands, likely triggering or influencing different abiotic transformation pathways as well as the transformations carried out by bacteria. However, unlike the extensive work already done on the role of bacteria in Hg transformations, the current knowledge about the influence of phytoplankton (algae and cyanobacteria) on such processes is still limited. Critical evaluation of the existing advances in the research topic revealed that different microalgal species and cyanobacteria contribute to the biotic reduction of inorganic mercury (iHg or HgII) into elemental Hg (Hg0), monomethylmercury (MeHg) demethylation and transformation of iHg into metacinnabar. The low and high molecular weight biomolecules released by phytoplankton can complex Hg species and contribute to abiotic mercury reduction. Despite these advances, the underlying mechanisms and their importance in the aquatic environment are to be explored and detailed. The development of novel molecular, stable isotope-based and multi-omics approaches would provide further impetus for the understanding of the key interactions between Hg species and phytoplankton. Such understanding will be of utmost importance for the improvement of Hg biogeochemical modelling, mitigation strategies and rational environmental risk assessment in the changing aquatic environment.