The inhibitory effects of sunlight on nitrogen removal in riverine overlying water with suspended particles

Overlying water with suspended particles is a hot spot for nitrogen removal in river systems. Although light exposure affects nitrogen transformations and nitrogen removal in some environments, such effects have rarely been explored and quantified in riverine overlying water. Herein, we examined the difference between dark and light conditions in the community composition and abundance of nitrogen transformation microbes in simulated overlying water by high-throughput sequencing and qPCR. Moreover, N-15-labeling techniques were used to investigate variation in nitrogen removal rates (N-2 and N2O) as well as nitrification rates between dark and light conditions. We found apparent differences in the bacterial community between light and dark microcosms. The abundance of Cyanobacteria was greatly elevated in light microcosms, with the diazotroph nifH gene abundance being 7.4-fold higher in the light microcosm (P < 0.01). However, due to the vulnerability of some specifies to UV damage, the diazotroph species richness was reduced. The abundances of ammonia-oxidizing archaeal amoA, ammonia-oxidizing bacterial amoA, and denitrifying nirS genes were 80.1%, 46.3%, and 50.7% lower in the light microcosm, respectively, owing to the differential inhibition of sunlight exposure on these microbes. Both N-15-N-2 and N-15-N2O were significantly produced regardless of conditions with or without light. Due to the combined effects of reduced nitrification and denitrification, as well as potentially enhanced nitrogen fixation, the accumulated amounts of N-15-N-2 and N-15-N2O were 6.2% and 44.8% lower, respectively, in the light microcosm. This study quantifies the inhibitory effect of sunlight exposure on nitrogen removal in riverine overlying water and reveals the underlying mechanisms, providing insights into our understanding of nitrogen transformations in river systems.

Automated summary

This study investigated the differences in community composition and abundance of nitrogen transformation microbes under dark and light conditions in riverine overlying water. The results showed significant changes in the bacterial community under light exposure, with certain microbes being inhibited by sunlight. Additionally, the accumulated amounts of N-15-N-2 and N-15-N2O were reduced in the light microcosm, indicating that sunlight exposure decreases nitrogen removal in riverine overlying water.