Distinct cyanobacteria/ eukaryotic algae types and varying initial algae-to-bacteria ratios drive the spatial partitioning and reduction of antibiotic resistance genes in algal-bacterial consortia.

This study investigated synergistic pollutant removal and antibiotic resistance gene (ARG) reduction by consortia of Bacillus licheniformis with Chlorella or Synechocystis. Using the initial algal-to-bacterial cell ratio as the primary variable, we evaluated growth, microbial community, pollutant removal, and ARG distribution across the water phase, algal cells, and extracellular polymeric substance (EPS) layers. B. licheniformis enhanced total algal biomass and absolute photosynthetic pigment production, while the ratio significantly influenced specific pigment, protein, and polysaccharide contents per unit biomass, indicating distinct resource allocation strategies. Both consortia efficiently removed nutrients, performing best at a low ratio of 1:3, which also differentially shaped microbial communities and their functions within soluble (S-EPS), loosely bound (LB-EPS), and tightly bound (TB-EPS) EPS. ARGs showed pronounced spatial partitioning: in the aqueous phase, monofunctional resistance genes were substantially reduced, whereas multidrug resistance and regulatory genes were enriched. Across compartments, Chlorella accumulated ARGs in the middle EPS layer (LB-EPS > S-EPS > TB-EPS), whereas Synechocystis enriched the inner layer (TB-EPS > LB-EPS > S-EPS) with higher intracellular abundance, with Chlorella showing greater stability in ARG regulation. These findings identify spatial partitioning as a key determinant of ARG fate in algal-bacterial consortia, guiding safer wastewater treatment design.