In situ assessment of the initial phase of wastewater biofilm formation: Effect of the presence of algae in an aerobic bacterial biofilm system

•The initial formation process of algal-bacterial biofilm was more stable.•The algal-bacterial flocs tend to form a thicker, denser, and rougher biofilm.•Algal inoculation increased hydrophobicity and attachment strength of flocs EPSs.•The community diversity increased and functional bacteria enriched after adding algae. The initial start-up attachment stage that dominates biofilm formation is an unstable process and is time-consuming. In the present study, Chlorella sp. was introduced into a general aerobic biofilm system to explore whether the addition of algae improved the initial attachment phase of biofilm. Compared with those of the bacterial biofilms, the initial algal-bacterial biofilms were more stable and had a thicker, denser, and rougher surface. Further investigation suggested that the concentration of extracellular polymeric substances (EPSs) in the algal-bacterial biofilm was 31.33 % greater than that in the bacterial biofilm. Additionally, the algal-bacterial flocs had greater free energies of absolute cohesion (ΔGcoh) and adhesion energy (∆Gadh) than did the bacterial flocs. These phenomena contribute to the speediness and stabilization of initial algal-bacterial start-up biofilms. Specifically, algae inoculation increased microbial community diversity and promoted the growth of bacterial members related to biofilm development. In conclusion, both physicochemical interactions and biological processes strongly influence microbial attachment during the initial biofilm formation process and further promote strengthening. [Display omitted]