Copper and zinc interact significantly in their joint toxicity to Chlamydomonas reinhardtii: Insights from physiological and transcriptomic investigations

Copper (Cu) and zinc (Zn) often discharge simultaneously from industrial and agricultural sectors and cause stress to aquatic biota. Although microalgae have been extensively investigated for their responses to Cu or Zn exposure, how they cope with the mixtures of two metals, especially at transcriptomic level, remains largely unknown. In this study, Chlamydomonas reinhardtii was exposed to environmentally relevant concentrations of two metals. It was found that Zn promoted the entry of Cu into the algal cells. With the increase of combined toxicity, extracellular polymeric substances (EPS) and cell wall functional groups immobilized significant amounts of Cu and Zn. Furthermore, C. reinhardtii adjusted resistance strategies internally, including starch consumption and synthesis of chlorophyll and lipids. Upon high level of Cu and Zn coexistence, synergistic effects were observed in lipid peroxidation and catalase (CAT) activity. Under 1.05 mg/L Cu + 0.87 mg/L Zn, 256 differentially expressed genes (DEGs) were mainly involved in oxidative phosphorylation, ribosome, nitrogen metabolism; while 4294 DEGs induced by 4.21 mg/L Cu + 3.48 mg/L Zn were mainly related to photosynthesis, citric acid cycle, etc. Together, this study revealed a more comprehensive understanding of mechanisms of Cu/Zn detoxification in C. reinhardtii, emphasizing critical roles of photosynthetic carbon sequestration and energy metabolism in the metal resistance. [Display omitted] •Cu and Zn joint toxicity was studied at physiological and transcriptomic levels.•Both metals were mainly adsorbed extracellularly in the high level of coexistence.•Multiple genes were upregulated in energy metabolism and photosynthesis.•Carbon flux switched from starch to lipid synthesis under the combined stress.•CAT played an important role in the detoxification of Cu and Zn mixture.