Effect of light intensity on nitrogen removal, enzymatic activity and metabolic pathway of algal-bacterial symbiosis in rotating biological contactor treating mariculture wastewater

[Display omitted] •The optimal light intensity for bacterial-algal symbiosis was 80 μE/(m2·s).•The change of light intensity obviously varied the bacterial and algal community.•High enzymatic activity and photosynthesis promoted the nitrogen conversion.•Oxidative damage at 100 μE/(m2·s) light intensity affected nitrogen metabolism.•Key functional pathways had different responses to the change in light intensity. An algal-bacterial symbiosis (ABS) system was developed on a rotating biological contactor treating mariculture wastewater, and its nitrogen removal, enzymatic activity and metabolic pathways were investigated under different light intensities. The nitrogen removal efficiency increased when light intensities ranged from 20 to 80 μE/(m2·s) but declined under 100 μE/(m2·s). Higher enzymatic activities under 80 μE/(m2·s) facilitated nitrogen conversion, light utilization, ATP supply and photosynthesis. Reactive oxygen species accumulation activated antioxidant pathways under 20 and 100 μE/(m2·s). Functional bacteria including Sedimentitalea, Thauera and Dechloromonas as well as Chlorella sorokinian, Dunaliella, Pleurosira laevis and Microcystis were enriched under 80 μE/(m2·s). Abundant photosynthesis-related genes (petC, Lca3/4 and atpH/A) supported energy supply and electron transport. Conversely, lower proportions of IDH3, gltB, and acnA/B under 20 and 100 μE/(m2·s) hindered tricarboxylic acid cycle, reducing NADPH and energy production. These results enhance the understanding on the effect of light intensity on ABS system treating mariculture wastewater.