Mechanisms of enhanced synergistic pollution reduction and carbon fixation induced by microalgal-bacterial interactions within different biofilm structures.

Microalgae can achieve simultaneous pollutant removal and carbon fixation in wastewater treatment. However, unimicrobial algal systems face challenges of limited performance and poor biofilm adhesion. This study introduced bacteria as mediators and examined the mechanisms involved in different spatial structures of microalgal-bacterial biofilms. Results showed that, compared to the unimicrobial microalgal biofilm (UMB), the hybrid microalgal-bacterial biofilm (HMBB) and stratified microalgal-bacterial biofilm (SMBB) enhanced CO2 fixation from 20.04 % to 31.50 % and 35.30 %, respectively, with biomass increasing from 62.1 mg/g to 77.6 mg/g and 93.0 mg/g. The SMBB system exhibited the strongest enhancement, particularly in microalgal photosynthetic activity and total EPS. Protein (PN) and polysaccharide (PS) concentrations reached 57.28 mg/L and 26.45 mg/L, which were 43.27 % and 17.45 % higher than those in HMBB, respectively. The increased PN improved hydrophobicity, thereby enhancing biofilm formation and biomass accumulation in the microalgal-bacterial systems, with a bacterial-to-microalgal biomass ratio of 1:1.19, compared to 1:1.01 in HMBB. The microalgal-bacterial interaction comprised: (i) microalgae facilitating the enrichment of pollutant-degrading bacteria (e.g., Acinetobacter and Pseudomonas); and bacterial modulation of algal metabolism through (ii) upregulation of key genes associated with photosynthetic carbon fixation (e.g., GOT1, 12.57-fold), (iii) stimulation of hydrophobic amino acid synthesis (e.g., ilvE, 12.56-fold), and (iv) activation of pathways related to nitrogen-phosphorus metabolism and the TCA cycle. In general, bacterial inoculation contributes three main advantages for microalgal: enhanced biofilm adhesion, increased carbon sequestration, and improved pollutant removal efficiency, and the stratified microalgal-bacterial biofilm (SMBB) provides the most pronounced improvement.