The enhanced performance of Chlorococcum sphacosum in rich carbon wastewater treatment under the influence of calcium ion

The performance of microalgae-based wastewater treatment process is probably affected by high concentrations of organic carbon in wastewater, and calcium ions may influence this process. Nevertheless, the phenomena and mechanism involved have not been well explored. Here the performance of Chlorococcum sphacosum in treating wastewater with high concentrations of organic carbon was studied by coupling calcium ion concentration regulation (adding CaCl2 and free Ca2+ chelator EGTA) via physicochemical and transcriptomic analysis. It was found that photosynthesis and nutrients removal were suppressed and lipid accumulation was enhanced as the microalga was used to treat wastewater with high concentrations of organic carbon (more than 1500 mg/L). Simultaneously, antioxidant enzymes (SOD, CAT and POD) activity and calcium signaling-related protein (CaM, CDPK, and Ca2+-ATPase) content rose implying that the microalga was in a state of stress and calcium was involved in stress response. It was found that EGTA treatment further weakened the performance of microalgal photosynthesis and nutrients removal. However, the supplementation of Ca2+ had the opposite effect. Transcriptomic analysis revealed that Ca2+ strengthened the heterotrophic decomposition process of carbon (up-regulating the gene expression related to glycolysis and tricarboxylic acid cycle) and inhibited lipid accumulation (down-regulating lipid synthesis) via Ca2+ signalings transduction. Briefly, Ca2+ alleviated the stress of high concentration of organic carbon and strengthened the performance treating the corresponding wastewater. [Display omitted] •High concentrations of organic carbon had adverse effect on microalgae.•High concentrations of organic carbon resulted in the stress of microalgae.•Ca2+ strengthened the microalgal performance of wastewater treatment.•Ca2+ alleviated the stress of high concentration of organic carbon on microalgae.