Microbial diversity supports nitrification: insights from a full-scale anoxic/oxic wastewater treatment process.

Nitrification is a crucial process for removing nitrogen in wastewater treatment systems. While previous research has primarily focused on nitrifying microorganisms, the impact of the broader diversity of non-nitrifying microbes on nitrification remains insufficiently explored. This study examined the relationship between microbial diversity and nitrification performance, taking into account the associations with nitrifiers in a full-scale anoxic/oxic activated sludge process. The microbial community was divided into a nitrifier subcommunity, comprising taxa directly responsible for nitrification, and nitrifier-associated subcommunities, comprising taxa that potentially interact with nitrifiers. Notably, the nitrifier-associated subcommunity consistently exhibited greater explanatory power for variations in nitrification rates than both the total microbial community and the nitrifier subcommunity, regardless of environmental and feeding conditions. Furthermore, the explanatory power of the nitrifier-associated subcommunity increased with stricter association cutoffs, with a core set of 13 taxa identified at an association cutoff of 0.6, accounting for up to 33% of the observed variations in the nitrification rate. Random forest ranked the dynamics of the core nitrifier-associated subcommunity among the top 3 most important variables out of 30 environmental and microbial factors, collectively explaining 82% of the variation in the nitrification rate. These findings underscore the importance of broader microbial diversity-beyond the diversity of nitrifiers alone-in supporting nitrification performance, highlighting the contribution of non-nitrifying microbes through their associations with nitrifiers. This study offers novel insights into the relationship between biodiversity and nitrification, providing valuable implications for managing microbial communities to optimize nitrogen removal in wastewater treatment ecosystems.IMPORTANCENitrification is essential for removing nitrogen from wastewater. While most research has concentrated on the nitrifiers directly responsible for this process, the role of the broader microbial community in influencing nitrification has been less explored. This study reveals a previously unrecognized relationship: the diversity of microbial taxa associated with nitrifiers explains more variation in nitrification rates than the diversity of either the total microbial community or the nitrifiers alone in a real-world wastewater treatment plant. By demonstrating the influential role of the diversity of the broader microbial community in shaping nitrification performance, this work introduces a new perspective on the ecological drivers of nitrification dynamics beyond nitrifier functional diversity. This insight provides a foundation for better microbial community management to achieve stable and efficient nitrogen removal. Our results deepen current understanding of the ecological complexity underpinning wastewater treatment processes, stressing the importance of microbial diversity in wastewater treatment system functionality.