Effects and mechanisms of phytohormones in enhancing the resistance of Oocystis borgei to high ammonium nitrogen stress

Abstract

Abstract 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.