Zinc Concentration Dictates the Response of the Diatom Thalassiosira weissflogii to Ocean Acidification: Alleviation at Toxic Levels versus Exacerbation at Sub-Toxic levels

Ocean acidification (OA) and zinc pollution co-occur in marine ecosystems, yet phytoplankton responses show conflicting patterns. We examined OA (1200 µatm CO2) and zinc (sub-toxic SZn: 1.20 mg L-1; toxic TZn: 2.40 mgL-1) effects on Thalassiosira weissflogii. OA inhibited growth under SZn but enhanced it under TZn. Under SZn, OA elevated oxidative stress markers-with superoxide dismutase (SOD) activity increasing by 48.12%, catalase (CAT) activity by 114.22%, and malondialdehyde (MDA) content by 76.27% compared to the SZn treatment-and increased cellular Zn content by 49.56% relative to SZn alone. Conversely, under TZn, OA reduced SOD by 61.31%, CAT by 45.45%, MDA by 25.29%, and cellular Zn by 35.96% compared to the TZn treatment and stimulated glycan/protein production. Gene expression revealed OA suppressed photosynthesis, TCA cycle, and oxidative phosphorylation genes under SZn. Under TZn, OA alleviated photosynthetic inhibition and upregulated gluconeogenesis, glycolysis, and oxidative phosphorylation genes. These findings indicate that OA exacerbates the detrimental effects of sub-toxic Zn on T. weissflogii, while mitigating the toxicity of Zn at lethal concentrations. This study provides a potential mechanistic explanation for contradictory reports on microalgal responses to heavy metal stress under OA and significantly advances our understanding of marine organismal adaptation to multiple stressors, offering critical insights into the ecological risks posed by zinc under future OA scenarios.