Effects of three exogenous phytohormones on the growth, selenium absorption, and metabolism of the microalga Parachlorella kessleri under sodium selenite stress

Treatment of selenium (Se)-laden wastewater using microalgae has gained attention as it is environmentally friendly and cost effective; however, most microalgae are intolerant to Se. This study aimed to investigate the feasibility of alleviating Se stress in Parachlorella kessleri exposed to 8 mg/L Na2SeO3 using indoleacetic acid (IAA), gibberellin (GA3), and melatonin (MT) and to explore the effects of their concentrations (5–80 μM) on the growth adaptability and Se biotransformation efficiency of P. kessleri. The results showed that the alleviation of Se4+ stress depended on the type and concentration of phytohormone used; MT conferred the best protection against Se4+ stress, followed by GA3 and IAA. The 20-μM MT treatment produced the highest algal biomass and chlorophyll a content, i.e., the algal biomass and chlorophyll a content increased by 102.68 % and 232.68 %, respectively, compared with that in the negative control (Se4+ stress). This increase was attributed to the enhancement of MT-induced antioxidant capacity of P. kessleri, which increased the endogenous ascorbic acid and glutathione content and decreased the superoxide anion content, thereby reducing the damage caused by membrane peroxidation. Most importantly, MT effectively promoted the absorption of Se4+ by P. kessleri while enhancing Se volatilization metabolism, alleviating Se toxicity by reducing the excessive accumulation of Se in cells, and increasing the removal efficiency of Se in the medium up to 84.63 %. Although MT reduced Se accumulation in biomass, the organic Se proportion remained >97 % and predominantly in the form of selenomethionine. These results demonstrated that MT has immense potential to alleviate Se4+-induced stress in P. kessleri and can be used as an effective regulatory strategy to promote microalgae-based resourceful treatment of Se-laden wastewater and gain insights into the mechanisms underlying the alleviation of Se toxicity.