Role of phytoplankton in aquatic mercury speciation and transformations

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 Hg-II) into elemental Hg (Hg-0), 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.

Automated summary

Phytoplankton can directly and indirectly influence the biogeochemical cycling and transformations of mercury. However, the current understanding of how phytoplankton affects these processes is limited. Recent research has shown that different microalgal species and cyanobacteria contribute to the transformations and reduction of mercury, and the biomolecules released by phytoplankton also participate in abiotic reduction. However, further studies are needed to explore the underlying mechanisms and the importance of these interactions in the aquatic environment.