Evaluating Growth and Nitrogen and Phosphorus Removal of Four Microalgae in Different Nutrient Concentrations

The environmental problems brought about by factory-based aquaculture have become increasingly prominent. Reducing nitrogen and phosphorus concentrations in tailwater has become the key to tailwater management. In order to assess the potential of microalgae in removing nitrogen and phosphorus ions from aquaculture wastewater, four microalgae species, i.e., Chlorella sp., Dicrateria zhanjiangensis, Nitzschia closterium minutissima, and Platymonas subcordiformis, were used in this study, and their growth and nitrogen and phosphorus removal rates in four nutrient concentrations of simulated aquaculture wastewater were systematically evaluated. After 15 days of cultivation, the cell counts of all four types of microalgae increased. Three species, i.e., Chlorella sp., N. closterium minutissima, and P. subcordiformis, grew best in high PO₄^3- and low NH₄⁺ medium, whereas D. zhanjiangensis possessed best growth in low PO₄^3- and high NH₄⁺ medium. The removal rate of PO₄^3-, NH₄⁺, NO₃^-, and NO₂^- by four microalgae species exceeded 82.64%, 89.06%, 59.27%, and 42.15%, respectively, even though the four microalgae had different performance in the removal of nitrogen and phosphorus. All microalgae in the low-phosphorus groups removed PO₄^3- at significantly lower rates than those in the high-phosphorus groups, while high NH₄⁺ removal rates were observed in all four microalgae groups. Moreover, in phosphorus-limited conditions, four microalgae exhibit lower removal rates of NO₃^- when nitrogen content was high. The chlorophyll a contents of microalgae in four culture media strictly corresponded to their final cell densities. P. subcordiformis exhibited the highest intracellular polysaccharide accumulation in high PO₄^3- and low NH₄⁺ type medium, whereas D. zhanjiangensis demonstrated the strongest protein synthesis capacity in high PO₄^3- and low NH₄⁺ medium. The activities of acid phosphatase in all microalgae were higher under phosphorus-deficient conditions than phosphorus-sufficient conditions. Our results might provide useful references for microalgae selection in the treatment of different aquaculture wastewater conditions.