Nutrient removal performance and microbial community analysis in an integrated wastewater treatment system with macroalgae (U. meridionalis) and Bacillus

The integrated aquaculture wastewater treatment system has been widely applied and typically consists of multiple units, among which macroalgae are critical for nutrients removal. This study investigated the impact of the addition of Bacillus to the macroalgae unit on the nutrient removal performance of the system and its mechanisms by analyzing the bacterial community that was identified through high-throughput sequencing of 16S rRNA. The results showed that Bacillus significantly influenced the structural composition of the bacterial communities both in the water and on the algal surface within the macroalgae unit. The enrichment of bacteria promoted nitrogen cycling and algal growth, such as Algoriphagus and Marivita. Futhermore,a higher average variation degree was observed, suggesting forming a more stable bacterial community. The removal rates of nitrogenous nutrients, such as TAN, NO₃⁻-N, and NO₂⁻-N, improved, reaching 99.92%, 99.62%, and 99.70%, respectively. Additionally, the biomass of the algae (U. meridionalis) significantly improved, reaching 157%. The Bacillus also increased the contribution of stochastic processes (drift) in the assembly of the bacterial communities in both the water and on the algal surface. This study provides foundational support for optimizing integrated wastewater treatment systems.