Conserved sulfate removal pathway in microalgae: A cross-species investigation for high-sulfate wastewater treatment

The discharge of high-sulfate wastewater from industries such as metallurgy, papermaking, and food processing poses a growing threat to environmental safety. While previous studies have focused on the physiological responses of microalgae to sulfate stress, the specific pathways through which microalgae remove sulfate remain poorly elucidated. This study investigated the response characteristics and removal mechanisms of three microalgal species (Chlorella vulgaris, Chlorella pyrenoidosa and an indigenous strain (Chlorella sp.) isolated from municipal wastewater) under high sulfate conditions. All three species maintained robust growth and efficiently removed ammonium and phosphate, with removal rates exceeding 70%. Notably, among the three species, C. pyrenoidosa and the indigenous strain demonstrated high tolerance to sulfate stress without significant oxidative stress, while only C. vulgaris exhibited a marked increase in ROS levels because of sulfate-induced physiological stress. Sulfate removal was facilitated through the synergistic action of three principal mechanisms including bioadsorption, bioaccumulation, and biotransformation/assimilation. Among these, biotransformation/assimilation was the dominant pathway, contributing over 70% of the total sulfate removal in all species, underscoring a conserved, non-species-specific mechanism. Further analysis via three-dimensional fluorescence spectroscopy (3D-EEM), Fourier transform infrared spectroscopy (FTIR) and compositional characterization of extracellular polymeric substances (EPS) revealed that protein-dominated functional groups such as N-H and C=O served as the primary binding sites, suggesting that bioadsorption likely proceeded through coordination complexation and electrostatic interactions. These findings provide new insights into the microbial treatment of sulfate-rich wastewater and highlight the potential of microalgae-based technologies for sustainable wastewater remediation.