Simultaneous Trichloroacetate Dechlorination Metabolism and Nitrogen Fixation in Nitrogen-Limited Aquifers

Haloacetates, particularly trichloroacetate (TCAA), are pervasive contaminants, yet their biotransformation in anoxic, nutrient-limited environments remains poorly understood. Here, we elucidate a geographically widespread attenuation of haloacetates in nutrient-limited aquifers by field surveys across six Chinese provinces. Enrichment cultures metabolized TCAA as the sole carbon and energy source. Major metabolites (oxalate, acetate, CO₂) indicated complete hydrolytic dechlorination accompanied by fermentation and mineralization. Isotope labeling and chemical probe studies confirmed the simultaneous occurrence of TCAA dechlorination and nitrogen fixation under fixed nitrogen-limited conditions. Integrated metagenomic, metabolomic, and transcriptional analyses demonstrated nitrogen-regulated metabolic reconfiguration. Acetyl-CoA potentially associated with TCAA metabolism may enter the TCA cycle under nitrogen-limited conditions. Dehalogenase gene (had) homologues extensively co-occurred with homologues of nitrogenase genes (nifHDK) in oligotrophic environments. Potential cross-feeding interactions among unclassified Azospira sp., Ralstonia pickettii, and Azospira inquinata mediated carbon-nitrogen intermediate exchange. This study identifies a previously unrecognized process that enables simultaneous TCAA detoxification and nitrogen acquisition in oligotrophic aquifers, thereby proposing an energy-conserving and ecologically adaptive strategy for haloacetate bioremediation.