Linking composition and function: a two-year metatranscriptomic study of microbial adaptation in an industrial wastewater treatment plant.

Industrial saline wastewater presents a major challenge due to high salinity. While most previous studies focus on domestic activated sludge processes using 16S rRNA sequencing, this study provides metatranscriptomic insight into an industrial saline wastewater treatment plant in the Netherlands operating a facultative activated sludge process. The process, characterized by alternating aeration and non-aeration phases, generates dynamic redox conditions that drive microbial succession and adaptive metabolic responses over time, in contrast to conventional activated sludge processes that typically operate under more stable aeration regimes and comparatively steady redox conditions. Over two years and four months, 32 metatranscriptomic profiles were generated alongside comprehensive physicochemical and operational measurements. The activated sludge harbored a diverse, metabolically active community contributing to nitrogen, carbon, and phosphorus removal, as annotated via SEED Subsystems. Functional gene richness saturated more rapidly than taxonomic richness, reflecting substantial functional redundancy, while complementarity effects under lower-diversity conditions suggested cooperative interactions that enhance ecosystem functionality. Notably, Thermodesulfobacteriota, including sulfur-reducing Desulfobulbia, emerged as a core and environmentally filtered group under saline conditions. This study demonstrates that longitudinal metatranscriptomic profiling can capture temporally resolved functional responses in industrial microbial communities, offering insight into microbial resilience, adaptation, and the ecological principles underpinning engineered bioremediation systems.