Photosynthetic response of and sp. 1710 to zinc toxicity.

Zinc (Zn) is an essential trace element but can lead to water contamination and ecological deterioration when present in excessive amounts. Therefore, investigating the photosynthetic response of microalgae to Zn stress is of great significance. In this study, we assessed the photosynthetic responses of neutrophilic and acidophilic sp. 1710 to Zn exposure for 96 h. The specific growth rate (μ), chlorophyll-a (Chl-a) content, and chlorophyll fluorescence parameters were determined. The results demonstrated that sp. 1710 was much more tolerant to Zn than , with the half-maximal inhibitory concentration (IC50) values of 225.4 mg/L and 23.4  mg/L, respectively. The μ and Chl-a content of decreased in the presence of 15  mg/L Zn, whereas those of sp. 1710 were unaffected by as high as 100  mg/L Zn. Chlorophyll fluorescence parameters indicated that the regulation of energy dissipation, including non-photochemical quenching, played a crucial role in Zn stress resistance for both strains. However, in the case of , non-photochemical quenching was inhibited by 5  mg/L Zn in the first 48 h, whereas for sp. 1710, it remained unaffected under 100  mg/L Zn. sp. 1710 also exhibited a 20 times stronger capacity for regulating the electron transfer rate than under Zn stress. The light energy utilization efficiency (α) of sp. 1710 had the most highly non-linear correlation with μ, indicating the energy utilization and regulation process of sp. 1710 was well protected under Zn stress. Collectively, our findings demonstrate that the photosystem of sp. 1710 is much more resilient and tolerant than that of under Zn stress.