Influences of fluctuating nutrient loadings on nitrate-reducing microorganisms in rivers

Rivers serve important functions for human society and are significantly impacted by anthropogenic nutrient inputs (e.g. organic and sulfur compounds). Reduced organic and sulfur compounds influence the nitrogen cycle as they are electron donors of microbial nitrate reduction. Water pollution caused by individual nutrients and the mechanisms have been studied, but how the variation in multiple nutrient loadings influences nitrate-reducing microorganisms is less understood. Two sets of microcosms were established and exposed to nitrate, along with either acetate or thiosulfate, at different times. Nutrient concentrations responded to the loading pollutant. The nutrient loading order was more important in shaping microbial community structure and microbial interactions through the exchange of growth-required substances. This indicated that upstream or historical nutrient inflows impacted current nitrate reduction by changing the seeding microbial community, highlighting the importance of river connectivity. Based on metatranscriptome analysis, although the order and type of nutrient loadings were equally important in regulating global transcriptomes, transcripts of enzymes for key metabolisms (nitrate reduction, sulfur oxidation, etc.) more actively responded to the nutrient type. The regulation of a small set of genes was sufficient to make the transition, while most transcripts were not degraded and regenerated. These insights are important for understanding the varying pollution status of rivers and for developing effective solutions, such as remediation.