Effect of light wavelengths on algal-bacterial symbiotic particles (ABSP): Nitrogen removal, physicochemical properties, community structure

The accumulation of nitrogenous waste, such as ammonia, and its biological nitrification products, namely nitrite and nitrate, poses one of the primary challenges in the water treatment process for Recirculating Aquaculture Systems (RAS). This study proposed the incorporation of light and algal-bacterial symbiotic particles (ABSP)into RAS as a viable solution to address these challenges. The findings indicated that ABSP achieved the RAS water quality standards (ammonia nitrogen <0.5 mg/L, nitrite nitrogen <0.1 mg/L) by the 14th day of operation, while red light achieved and maintained these standards from as early as the 9th day. Further analysis of chemical stoichiometry and functional genes revealed that microalgae (32.7%, 15.6%, 0%) and bacteria (47.7%, 55.8%, 48.4%) were responsible for the assimilation and removal of most organic and nutrient substances under red light, white light, and dark conditions, respectively. Additionally, it was observed that the presence of algae and bacteria in ABSP was significantly influenced by the wavelength of light. Under red light, an optimal distribution of functional species for both microalgae (Characium, Microcoleus, and Uronema) and bacteria (Nakamurella and Micropruina) was achieved, which is crucial for the self-sustaining synergistic reaction and stability of ABSP. This study may offer a promising engineering alternative with enormous potential to achieve energy-efficient and environmentally sustainable RAS water treatment.