ABSTRACT: Widespread organic pollutants such as BTEX (benzene, toluene, ethylbenzene, and xylene) are traditionally considered to enhance soil carbon loss through mineralization and ecotoxicity. Challenging this paradigm, we reveal that BTEX can stimulate microbial carbon chain elongation (CE)—a previously overlooked carbon fixation pathway—thereby reshaping soil carbon dynamics. Through phased amplicon sequencing, metagenomics, and metaproteomics, we demonstrate that BTEX exerts bidirectional regulation on CE at both taxonomic and molecular levels. Specifically, BTEX selectively enriches Clostridium_sensu_stricto_12 and Rummelibacillus, while suppressing Acinetobacter, a key CE contributor in natural soils. BTEX also inhibits Petrimonas, a syntrophic degrader of medium-chain fatty acids (MCFAs), promoting MCFAs accumulation. Moreover, BTEX-degrading bacteria establish cooperative interactions with CE bacteria, facilitating the sequestration of carbon as MCFAs rather than complete mineralization to CO₂, with Bacillus bridging both metabolic roles. At the molecular level, BTEX enhances CE by accelerating substrate uptake and acetyl-CoA flux into the reverse β-oxidation (RBO) pathway. Multi-omics analysis revealed that BTEX downregulates fatty acid biosynthesis (FAB), another pathway of CE, through fabR, acrR, and fadR while maintaining NADH availability to relieve Rex-mediated inhibition of the key RBO enzyme gene bcd. However, excessive BTEX disrupts metabolic homeostasis and suppresses CE activity. Collectively, our findings redefine the ecological implications of aromatic hydrocarbon contamination by uncovering its capacity to modulate anaerobic carbon fixation and retention in soil microbial communities. This work highlights a previously unrecognized link between pollutant degradation and biogenic carbon sequestration, with broader implications for understanding soil biogeochemical resilience under anthropogenic pressure.
Wanqian Guo 
