Mercury Reduction, Uptake, and Species Transformation byFreshwater AlgaChlorella vulgarisunder Sunlit and Dark Conditions

As a major entry point of mercury (Hg) to aquaticfood webs, algae play an important role in taking up andtransforming Hg species in aquatic ecosystems. However, little isknown how and to what extent Hg reduction, uptake, and speciestransformations are mediated by algal cells and their exudates, algalorganic matter (AOM), under either sunlit or dark conditions. Here,usingChlorella vulgaris(CV) as one of the most prevalent freshwatermodel algal species, we show that solar irradiation could enhance thereduction of mercuric Hg(II) to elemental Hg(0) by both CV cellsand AOM. AOM reduced more Hg(II) than algal cells themselvesdue to cell surface adsorption and uptake of Hg(II) inside the cellsunder solar irradiation. Synchrotron radiation X-ray absorption near-edge spectroscopy (SR-XANES) analyses indicate that sunlightfacilitated the transformation of Hg to less bioavailable species, such as beta-HgS and Hg-phytochelatins, compared to Hg(Cysteine)2-like species formed in algal cells in the dark. Thesefindings highlight important functional roles and potential mechanisms of algae inHg reduction and immobilization under varying lighting conditions and how these processes may modulate Hg cycling andbioavailability in the aquatic environment.