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This study evaluates a mixed-culture microbial fuel cell (MFC) as a rapid, real-time biosensor for detecting water toxicity caused by heavy metals and benzene, toluene, ethylbenzene, and xylene (BTEX) compounds. A dual-chamber MFC (200 mL) equipped with carbon fiber electrodes was operated with an electroactive biofilm on the anode, and toxicity responses were quantified using inhibition ratios (IR) derived from voltage suppression following contaminant exposure. The biosensor effectively detected copper (Cu²⁺) and mercury (Hg²⁺) over a concentration range of 2-20 mg L^-1, exhibiting strong linear relationships (R² = 0.9928 and R² = 0.9811, respectively). Mixed-culture biofilms showed a 1.64-fold increase in sensitivity compared to pure cultures and demonstrated stable baseline signals. The biosensor also responded linearly to benzene and xylene (R² = 0.9876 and R² = 0.9811) with rapid response times of 1.0-3.5 min. Microbial community analysis indicated increased richness and evenness following MFC operation, supporting biofilm stability during short-term exposure. The developed biosensor is intended for early-warning and process-health monitoring in industrial and wastewater systems, where rapid detection of toxicity spikes is critical. These findings demonstrate the feasibility of MFC-based biosensors as low-cost, in situ tools for integrative toxicity surveillance.