Mercury Reduction, Uptake, and Species Transformation by Freshwater Alga Chlorella vulgaris under Sunlit and Dark Conditions

Abstract

As a major entry point of mercury (Hg) to aquatic food webs, algae play an important role in taking up and transforming Hg species in aquatic ecosystems. However, little is known how and to what extent Hg reduction, uptake, and species transformations are mediated by algal cells and their exudates, algal organic matter (AOM), under either sunlit or dark conditions. Here, using Chlorella vulgaris (CV) as one of the most prevalent freshwater model algal species, we show that solar irradiation could enhance the reduction of mercuric Hg­(II) to elemental Hg(0) by both CV cells and AOM. AOM reduced more Hg­(II) than algal cells themselves due to cell surface adsorption and uptake of Hg­(II) inside the cells under solar irradiation. Synchrotron radiation X-ray absorption near-edge spectroscopy (SR-XANES) analyses indicate that sunlight facilitated the transformation of Hg to less bioavailable species, such as β-HgS and Hg-phytochelatins, compared to Hg­(Cysteine)2-like species formed in algal cells in the dark. These findings highlight important functional roles and potential mechanisms of algae in Hg reduction and immobilization under varying lighting conditions and how these processes may modulate Hg cycling and bioavailability in the aquatic environment.

Transformation of Mercuric Ions to Mercury Nanoparticles in Diatom Chaetoceros curvisetus

“…In consideration of both the distribution and transport tendency of Hg intracellularly, it was suspected that HgNPs formation in the diatom Chaetoceros curvisetus occurred inside the cells, most likely on the inner membrane. Owing to the chemical composition of HgNPs (elements Hg and S) (Figure ), the potential sulfur sources of HgNPs formed intracellularly were cysteines from assimilatory sulfate reduction and other metabolic pathways. ,, …”

“…Until now, particulate HgS (β-HgS) has been detected in algae, while HgNPs have not been identified. − Several studies have indicated that algae are capable of transforming dissolved divalent Hg (Hg­(II)) into various species, such as Hg­(Cysteine) 2 , phytochelatin-binding mercury (Hg­(PCs)), and insoluble mineral mercury (β-HgS, 20–90%), under both light and dark conditions. , Facilitated by sunlight, Hg­(II) and Hg­(Cysteine) 2 can further convert into more stable forms, Hg­(PCs) and β-HgS . These high proportions of β-HgS formation mediated by algae suggest the crucial process of Hg transformation in algae, especially the formation of particulate HgS.…”

Gaseous Mercury Exchange from Water–Air Interface in Differently Impacted Freshwater Environments

“…Among them, Rhodobacteraceaea and Gammaproteobacteria contain known Hg reducers. A recent study by Liang et al demonstrated that Hg reduction mediated by phytoplankton and algal cell exudates can occur either under sunlit or dark conditions [123].…”

Role of phytoplankton in aquatic mercury speciation and transformations

“…Production of DGM was observed for 3 other diatoms species (Wu and Wang 2014), suggesting an important role of intracellular thiols. The reduction of iHg was, however, observed in cultures of Chlorella vulgaris, but the organic matter released or obtained after cell degradation was shown to reduce more iHg than living algal cells themselves (Liang et al 2022). Nevertheless, the fundamental mechanisms 13 involved in the biological reduction process remain poorly understood.…”

“…Isochrysis galbana, and marine diatom Thalassiosira weissflogii could transform iHg into metacinnabar (β-HgS) (Wu and Wang 2014). Sunlight was also shown to facilitate the transformation of Hg to less bioavailable species, such as β-HgS and Hg-PCs (Liang et al 2022).…”