Visual biosensing of bioaccessible mercury in aquatic environments using chromoprotein-based whole-cell sensors

Mercury contamination poses severe ecological and human health risks through bioaccumulation in aquatic food webs. Conventional instrumental methods are sensitive but unsuitable for rapid on-site screening due to their high cost and complex pretreatment requirements. Here, we developed a dual-mode whole-cell biosensor by integrating a mercury-responsive MerR regulatory circuit with chromoprotein reporters. Three candidate chromoproteins were screened for heterologous expression and thermal stability in Escherichia coli, and amilCP was selected as the optimal reporter. The biosensor supports both naked-eye semi-quantification via red-green-blue (RGB) analysis of cell pellets and quantitative spectrophotometric readout following heat lysis, eliminating organic solvent extraction. For inorganic mercury, the sensor achieved detection limits of 78.125 nM by imaging and 46.17 nM by spectrophotometry, with a dynamic range extending to 2500 nM. For methylmercury, intracellular and extracellular demethylation strategies yielded complementary profiles, with the integrated transport-cleavage configuration achieving a detection limit of 0.49 nM in LB medium. Matrix-spiking validation in surface water achieved RGB imaging LODs of 39.06 nM for Hg(II) and 3.91 nM for MeHg, demonstrating robust matrix tolerance and supporting the platform's potential as a cost-effective tool for mercury screening in aquatic environments.