Mechanisms underlying the interactions and adaptability of nitrogen removal microorganisms in freshwater sediments

Microorganisms in eutrophic water play a vital role in nitrogen (N) removal, which contributes significantly to the nutrient cycling and sustainability of eutrophic ecosystems. However, the mechanisms underlying the interactions and adaptation strategies of the N removal microorganisms in eutrophic ecosystems remain unclear. We thus analyzed field sediments collected from a eutrophic freshwater ecosystem, enriched the N removal microorganisms, examined their function and adaptability through amplicon, metagenome and metatranscriptome sequencing. We found that the N removal activities could be affected through potential competition and inhibition among microbial metabolic pathways. High-diversity microbial communities generally increased the abundance and expression of N removal functional genes. Further enrichment experiments showed that the enrichment of N removal microorganisms led to a development of simplified but more stable microbial communities, characterized by similar evolutionary patterns among N removal microorganisms, tighter interactions, and increased adaptability. Notably, the sustained provision of NH₄⁺ and NO₂^- during the enrichment could potentially strengthen the interconnections among denitrification, anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) processes. Moreover, the identification of shared metabolic traits among denitrification, anammox and DNRA implies important cooperative associations and adaptability of N removal microorganisms. Our findings highlight the microbial interactions affect the adaptive strategies of key microbial taxa involved in N removal.