The interplay of hematite and photic biofilm triggers acceleration of biotic nitrate removal.

The soil-water interface is replete with photic biofilm and iron minerals; however, the potential of how iron minerals promote biotic nitrate removal is still unknown. This study investigates the physiological and ecological responses of photic biofilm to hematite (Fe2 O3 ), in order to explore a practically feasible approach for in-situ nitrate removal. The nitrate removal rate of photic biofilm was significantly higher in the presence of Fe2 O3 (92.5%) compared to the control (82.8%). Results show that the presence of Fe2 O3 changed the community composition of photic biofilm, facilitates the thriving of Magnetospirillum and Pseudomonas, and promotes the growth of photic biofilm represented by the extracellular polymeric substance (EPS) and the content of chlorophyll. The presence of Fe2 O3 also induces oxidative stress (•O2 - ) in photic biofilm, which was demonstrated by electron spin resonance spectrometry. However, the photic biofilm could improve the EPS productivity to prevent the entrance of Fe2 O3 to cells in the biofilm matrix and mitigate oxidative stress. The Fe2 O3 then promoted the relative abundance of Magnetospirillum and Pseudomonas and the activity of nitrate reductase, which accelerates nitrate reduction by photic biofilm. This study provides an insight into the interaction between iron minerals and photic biofilm and demonstrates the possibility of combining biotic and abiotic methods to improve the in-situ nitrate removal rate.