We present a new concept for whole-cell biosensors that couples the response to Hg ²⁺ with bioluminescence and bacterial aggregation. This allows us to use the bacterial aggregation to preconcentrate the bioluminescent bacteria at the substrate surface and increase the sensitivity of Hg ²⁺ detection. This whole-cell biosensor combines a Hg ²⁺ -sensitive bioluminescence reporter and light-responsive bacterial cell-cell adhesions. We demonstrate that the blue luminescence in response to Hg ²⁺ is able to photoactivate bacterial aggregation, which provides a second readout for Hg ²⁺ detection. In return, the Hg ²⁺ -triggered bacterial aggregation leads to faster sedimentation and more efficient formation of biofilms. At low Hg ²⁺ concentrations, the enrichment of the bacteria in biofilms leads to an up to 10-fold increase in the signal. The activation of photoswitchable proteins with biological light is a new concept in optogenetics, and the presented bacterial biosensor design is transferable to other bioluminescent reporters with particular interest for environmental monitoring.