Application of indole-3-acetic acid in microalgae cultivation to improve the feasibility of simultaneously purifying wastewater, fixing CO2 and producing fatty acids under Hg stress

Abstract

Alleviating the toxicity of mercury (Hg), improving the yield of Chlorella vulgaris ( C. vulgaris ) biomass and fatty acids, purifying wastewater, and promoting carbon dioxide (CO 2 ) fixation are important goals of research in the cultivation of microalgae in wastewater-injected flue gas . Accordingly, indole-3-acetic acid (IAA) was selected as an additive to help achieve these goals. The results showed that IAA could reduce cell damage and malondialdehyde (MDA) content, which may be attributed to the improved activities of detoxification components in reactive oxygen species (superoxide dismutase (SOD), catalase (CAT), etc.) and the improved expression of important photosynthesis genes ( psbA , rbcL , etc.), ultimately leading to enhanced chlorophyll synthesis rates and biomass under 0–30 μg m −3 Hg concentrations. Moreover, wastewater was found to have a high bioremediation efficiency because C. vulgaris grew well in all treatments, and the rates of nitrogen and phosphorus removal were higher than 97.64%. Additionally, the CO 2 fixation rate (170.98–220.92 mg CO 2 ·L −1 ·d −1 ) of C. vulgaris was enhanced with the addition of IAA under Hg stress. More importantly, it was found that 2 mg L −1 IAA is beneficial for improving lipid accumulation (34040.42 and 72564.81 μg g −1 ) under 10–30 μg m −3 Hg concentrations, and the lipid content increased by 384.90% and 65.19% at 10 and 30 μg m −3 Hg respectively. A modified Droop kinetic function that was related to both the phosphorus quota and nitrogen quota was implemented and described C. vulgaris growth well ( R 2 ≥0.994). • Flue gas with Hg has the potential to cultivate microalgae for biodiesel production. • IAA attenuated the inhibitory effect of Hg on C. vulgaris. • IAA enhanced the CO 2 -fixation ability of C. vulgaris under Hg stress. • IAA induced fatty acid biosynthesis, especially for MUFAs, in C. vulgaris cells.