Mercury\nReduction, Uptake, and Species Transformation\nby Freshwater Alga Chlorella vulgaris under Sunlit and Dark Conditions

As\na major entry point of mercury (Hg) to aquatic food webs, algae\nplay an important role in taking up and transforming Hg species in\naquatic ecosystems. However, little is known how and to what extent\nHg reduction, uptake, and species transformations are mediated by\nalgal cells and their exudates, algal organic matter (AOM), under\neither sunlit or dark conditions. Here, using Chlorella\nvulgaris (CV) as one of the most prevalent freshwater\nmodel algal species, we show that solar irradiation could enhance\nthe reduction of mercuric Hg­(II) to elemental Hg(0) by both CV cells\nand AOM. AOM reduced more Hg­(II) than algal cells themselves due to\ncell surface adsorption and uptake of Hg­(II) inside the cells under\nsolar irradiation. Synchrotron radiation X-ray absorption near-edge\nspectroscopy (SR-XANES) analyses indicate that sunlight facilitated\nthe transformation of Hg to less bioavailable species, such as β-HgS\nand Hg-phytochelatins, compared to Hg­(Cysteine)2-like species\nformed in algal cells in the dark. These findings highlight important\nfunctional roles and potential mechanisms of algae in Hg reduction\nand immobilization under varying lighting conditions and how these\nprocesses may modulate Hg cycling and bioavailability in the aquatic\nenvironment.