Physiological and transcriptomic responses of the microalga Isochrysis galbana during exposure to Hg(II) stress

Heavy metal contamination, particularly mercury (Hg), represents a substantial threat to aquatic ecosystems and primary producers. In this study, we systematically examined the impacts of varying concentrations of Hg(II) on Isochrysis galbana in terms of growth, chlorophyll a content, soluble protein levels, and ultrastructure. Results demonstrated that when Hg(II) concentrations exceeded 0.2 mg/L, the growth of I. galbana was significantly inhibited. At Hg(II) concentrations below 0.2 mg/L, the activities of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) were enhanced. Further analyses revealed that Hg(II) was detoxified through binding to functional groups on the cell surface and to macromolecular compounds within the cell via hydrogen and ionic bonds. X-ray photoelectron spectroscopy measurements indicated the possible existence of accumulated mercury in the forms of Hg₃(PO₄)₂ or HgO. Notably, morphological analysis disclosed chromatin agglutination, cell fragmentation, and other typical apoptotic features in I. galbana cells following exposure to Hg(II). Transcriptome analysis further showed that Hg(II) significantly disrupted the expression of genes involved in photosynthesis, tricarboxylic acid (TCA) cycle, and DNA replication pathways, which consequently affected the growth and metabolism of I. galbana, ultimately leading to growth inhibition. Collectively, these findings offer novel insights into the biochemical and physiological response mechanisms by which Hg impacts aquatic primary producers.