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 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 the 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 the Hg biogeochemical modelling, mitigation strategies, and rational environmental risk assessment in the changing aquatic environment.