Influence of bacterial load on the development and performance of a photoheterotrophic biofilm community for wastewater polishing

• Microalga co-cultured with the selected bacteria is promising for WW bioremediation. • The three microbial consortia mutually augmented their growth in a biofilm. • C. vulgaris co-cultured with the three bacteria improved biomass productivity. • The presence of R. fascians increased the dual-species biofilm thickness and EPS production. • Nutrients removal by photoheterotrophic biofilms was comparable to axenic C. vulgaris. A novel photoheterotrophic dual-species biofilm was evaluated for wastewater (WW) treatment and microbial growth under variable cell loads. For that, Chlorella vulgaris was tested under axenic conditions and in combination with three bacteria, Rhodococcus fascians , Acinetobacter calcoaceticus, and Leucobacter sp., assessing the influence of each bacterial strain in biomass productivity, growth rate, thickness, roughness, and production extracellular polymeric substances (EPS) of each co-culture. The cultivation of C. vulgaris with bacteria improved biomass productivity and growth rate for almost all cases (except when using the lowest concentration of Leucobacter sp.) particularly the biofilms containing R. fascians , with biomass productivity of 7.46 ± 1.1 g/L.d when the initial optical density (OD) proportions were 0.1:0.025 for microalga and bacterium, respectively, and a growth rate of 0.97 ± 0.2 d¯ 1 in 72 h for the initial OD 0.05:0.05. Moreover, the presence of bacteria affected the dual-species biofilm thickness and EPS production, especially those with R. fascians . Nutrient removal by the dual-species biofilms was comparable to the axenic C. vulgaris biofilms, except with Leucobacter sp., while the single-species bacterium cultivation led to nutrient accumulation. This study identified symbiotic behaviours between C. vulgaris and the selected bacteria in WW polishing, where microbial load and proportion influenced the growth dynamics and WW treatment efficiency.