Elucidating toxicity mechanisms of hexabromocyclododecane in marine microalga Chlorella salina: An integrated...

Hexabromocyclododecane (HBCD), a persistent brominated flame retardant, poses substantial ecological risks attributable to its bioaccumulation potential and toxicity. This study explored the toxic impacts of HBCD on the marine microalgae Chlorella salina using an integrated approach combining physiological, biochemical, and transcriptomic analyses. The microalgae was exposed to concentrations of 5, 50, and 100 μg·L⁻¹ of HBCD for 96 h. The results showed that HBCD significantly inhibited the growth of C. salina (p < 0.05), with a 21 % reduction in cell density at the highest concentration. Pigment analysis indicated that upon exposure to 100 μg·L⁻¹ HBCD, the levels of chlorophyll a, chlorophyll b, and carotenoids decreased by 17 %, 19 %, and 13 %, respectively (p < 0.05). Fourier transform infrared spectroscopy (FTIR) revealed concentration-dependent alterations in the composition, conformation, and functionality of key biomacromolecules. Specifically, lipid peroxidation was evidenced by decreased CH₂/lipid, CH₃/lipid, and olefinic=CH/lipid ratios, along with an increased carbonyl ester/lipid ratio. These findings were corroborated by elevated malondialdehyde (MDA) content and superoxide dismutase (SOD) activity. Alterations in the secondary structure of proteins were detected through decreased Amide I/Amide II and β-sheet/α-helix ratios. DNA damage involved a reversal of the B- to A-DNA transition and a shift from B- to Z-DNA conformational. Furthermore, transcriptomic analysis identified 4636 differentially expressed genes (DEGs) following exposure to 100 μg·L⁻¹ HBCD, which were predominantly enriched in pathways associated with fatty acid metabolism, energy metabolism, and cellular signaling. These findings provide mechanistic insights into the toxicity of HBCD in marine microalgae and highlight its potential ecological risks in marine environments.