Functional Redundancy Enables Stable Treatment Performance Across Taxonomically Diverse Dairy Wastewater Microbiomes.

Biological treatment of dairy wastewater depends on complex microbial communities that vary widely across reactors, influent sources and operating conditions. Whether stable treatment performance arises from conserved taxa or from redundancy in functional potential remains unclear. In this study, we conducted a meta-analysis and reprocessing of 239 samples from nine independent studies spanning six countries and multiple reactor configurations. Taxonomic profiles showed extreme heterogeneity, with no ASV detected in at least 50% of samples and most genera exhibiting strong study-specific patterns. In contrast, the predicted functional repertoire was highly conserved. PICRUSt2 analysis identified 196 MetaCyc pathways present in at least 90% of samples, and these pathways accounted for approximately 90% of total predicted functional abundance. Generalised linear latent variable models showed that temperature and pH created consistent but modest ecological gradients, while study- and reactor-level factors dominated community structure. Despite substantial taxonomic turnover, functional signatures associated with fermentation, biosynthesis, nitrogen cycling, and degradation of complex substrates remained stable across systems. Treatment-stage transitions also followed predictable metabolic trajectories. These results show that dairy wastewater treatment systems are taxonomically flexible but functionally robust, and that extensive redundancy supports consistent metabolic capacity across diverse operational and environmental contexts.