Role of phosphate in microalgal-bacterial symbiosis system treating wastewater containing heavy metals

Abstract

Phosphorus is one of the important factors to successfully establish the microalgal-bacterial symbiosis (MABS) system. The migration and transformation of phosphorus can occur in various ways, and the effects of phosphate on the MABS system facing environmental impacts like heavy metal stress are often ignored. This study investigated the roles of phosphate on the response of the MABS system to zinc ion (Zn²⁺). The results showed that the pollutant removal effect in the MABS system was significantly reduced, and microbial growth and activity were inhibited with the presence of Zn²⁺. When phosphate and Zn²⁺ coexisted, the inhibition effects of pollutants removal and microbial growth rate were mitigated compared to that of only with the presence of Zn²⁺, with the increasing rates of 28.3% for total nitrogen removal, 48.9% for chemical oxygen demand removal, 78.3% for chlorophyll-a concentration, and 13.3% for volatile suspended solids concentration. When phosphate was subsequently supplemented in the MABS system after adding Zn²⁺, both pollutants removal efficiency and microbial growth and activity were not recovered. Thus, the inhibition effect of Zn²⁺ on the MABS system was irreversible. Further analysis showed that Zn²⁺ preferentially combined with phosphate could form chemical precipitate, which reduced the fixation of MABS system for Zn²⁺ through extracellular adsorption and intracellular uptake. Under Zn²⁺ stress, the succession of microbial communities occurred, and Parachlorella was more tolerant to Zn²⁺. This study revealed the comprehensive response mechanism of the co-effects of phosphate and Zn²⁺ on the MABS system, and provided some insights for the MABS system treating wastewater containing heavy metals, as well as migration and transformation of heavy metals in aquatic ecosystems.

Keywords: Growth characteristics; Heavy metal; Microalgal-bacterial symbiosis system; Phosphate; Pollutants removal.