Metabolic shifts of microbial communities exposed to high concentrations of tire microplastics: Effects on nitrogen cycling and pollutant degradation

Tire microplastics (TMPs) are abundant in urban runoff and introduce physical and chemical stressors to wastewater biocenoses. In this study, aerobic granular sludge was exposed to high TMP concentrations, intentionally selected to elucidate the mechanisms underlying microbial and functional responses, and microbial activity was quantified by respirometry alongside RNA-sequencing-based metatranscriptomic profiling. Exposure to TMPs did not impair overall wastewater treatment performance. Nitrification efficiency increased from 82.6 ± 3.2% in the control reactor to 96.7 ± 0.5% under TMP exposure, whereas total extracellular polymer substances (EPS) content decreased by more than 40%. With TMPs in the influent, the total oxygen uptake rate (OUR) increased up to 5.5-fold, while nitrifier-specific OUR rose at 50 mg TMPs/L but declined at 100–500 mg TMPs/L despite enrichment of nitrification genes, suggesting a TMP-associated decoupling between transcription and respiratory activity. TMPs exposure increased the abundance of Chloroflexota from 0.5% to 1.6% and enriched genera associated with xenobiotic degradation, EPS production and nitrogen transformations such as e.g. Pseudomonas (1.14–2.33%), Micavibrio (0.75–2.83%), Accumulibacter (0.02–1.38%), Zoogloea (0.09–1.13%) or Candidatus Nitrotoga (0-0.45%). Metatranscriptomic analysis indicated that bacterial communities underwent significant metabolic reprogramming to counteract environmental stress, optimize energy production, and switch to alternative carbon and electron flux strategies. TMPs favoured classical and complete nitrification (comammox) in granules. Bacteria downregulated denitrification gene transcription while concomitantly upregulating genes involved in xenobiotic degradation functions, reflecting an adaptive mechanism that prioritizes the removal of toxic compounds to ensure survival. The results indicate that exposure to TMPs acts as a selective pressure, reducing community diversity while favoring the proliferation of stress-tolerant microorganisms. The findings of this study provide insight into how TMPs may influence key nitrogen cycle processes in engineered wastewater bioreactors.