Characterizing algal-bacterial symbiotic biofilms: Insights into coexistence of algae and anaerobic microorganisms

[Display omitted] •TN removal efficiency of APNABS exceeded 90% without aeration or external organics.•The algal-bacterial biofilm’s layered structure and composition were analyzed.•AnAOB and AOB occupied inner biofilm layer, and outer layer was dominated by algae.•Biofilm was divided into three zones based on the second derivative model of DO. This study constructed an integrated algae/partial nitrification/anammox biofilm system and operated it for 240 days. The total nitrogen removal efficiency exceeded 90 %. The structure, compositions, and function of this symbiotic biofilm, which played a pivotal role in the system, were analyzed in detail. Microscope photos and fluorescence in situ hybridization both showed that bacteria and algae were well integrated. The dissolved oxygen gradient further confirmed that different functional microorganisms grew at varying depths within biofilm. Algae formed an oxygen-producing zone (0–0.48 mm), followed by ammonia oxidizing bacteria (AOB) consuming oxygen to form an oxygen-consuming zone (0.48–0.86 mm), and anaerobic ammonia oxidizing bacteria (AnAOB) removed nitrogen in anaerobic zone (>0.86 mm). Chlorella, Nitrosomonas and Candidatus_Kuenenia were identified as the dominant algae, AOB and AnAOB, with relative abundances of 11.80 %, 19.77 % and 3.07 %, respectively. This layered biofilm benefitted providing a suitable environment for various microorganisms to survive within a complex biofilm.