To enhance the nutrient removal efficiency of Oocystis borgei for mariculture wastewater (MW), the effects and processes of three phytohormones on nitrogen and phosphorus removal from synthetic mariculture wastewater (SMW) by O. borgei under sequential batch operation were compared. The findings revealed that the supplementation with 10⁻⁶ M 3-indoleacetic acid (IAA), gibberellic acid (GA3), and zeatin (ZT) resulted in the most effective elimination, while there was no appreciable difference among them. The nitrogen and phosphorus indices of the effluent dramatically reduced (p < 0.01) upon the supplementation of phytohormones, and the removal effects were ranked as NO₃⁻-N > PO₄³⁻-P > NH₄⁺-N > NO₂⁻-N. The removal rates for NH₄⁺-N and PO₄³⁻-P were 0.72–0.74 mg·L⁻¹·d⁻¹ and 1.26–1.30 mg·L⁻¹·d⁻¹, respectively. According to physiological studies, phytohormones enhanced the levels of photosynthetic pigments and chlorophyll fluorescence parameters (Fv/Fm and φPSII), thereby improving photosynthetic activity. Additionally, they stimulated Nitrate Reductase (NR) and Glutamine Synthetase (GS) activities to promote nitrogen metabolism and increased Superoxide Dismutase (SOD), Catalase (CAT), and carotenoid contents to mitigate oxidative stress damage caused by abiotic stress. These activities contribute to the proliferation of O. borgei, which in turn resulted in an increase in the assimilation of nitrogen and phosphorus from SMW. In conclusion, phytohormone supplementation significantly increased nutrient removal from SMW by O. borgei in a sequential batch reactor, which has potential application in MW treatment.

Full-length transcriptome of Oocystis borgei under stress condition

Excessive ammonium nitrogen (NH₄ ⁺-N) in aquatic environments poses significant challenges for wastewater treatment and microalgae-based bioremediation. This study investigated the effects of phytohormones on enhancing the tolerance of Oocystis borgei to high NH₄ ⁺-N stress and explored the underlying mechanisms. Growth of O. borgei was significantly inhibited by NH₄ ⁺-N in a dose-dependent manner, with 500 mg·L⁻¹ NH₄ ⁺-N selected as the stress concentration. Supplementation with 10⁻⁶ M indole-3-acetic acid (IAA), gibberellic acid (GA3), or zeatin (ZT) significantly alleviated growth inhibition under high NH₄ ⁺-N stress, increasing cell density by 22–26% and specific growth rate by 217–282% compared to the HAN group. No significant differences were observed among the tested phytohormones. Phytohormone treatments mitigated photosynthetic inhibition by enhancing chlorophyll a (Chla) and carotenoid (Car) contents, as well as Fv/Fm and φPSII values. While rbcL gene expression remained suppressed, phytohormones reduced oxidative stress by decreasing malondialdehyde (MDA) content and modulating superoxide dismutase (SOD) and catalase (CAT) activities. Additionally, phytohormones promoted nitrogen metabolism by significantly increasing glutamine synthetase (GS) activity and gene expression. This study provides insights into the protective mechanisms of phytohormones against high NH₄ ⁺-N stress in O. borgei, offering potential strategies for enhancing algal-based treatment of high-ammonium wastewater.