Cyanobacteria are a major group of photosynthetic bacteria that can accumulate in surface water as so-called "blooms" in response to environmental factors such as temperature, light and certain nutrients such as N, P, and Fe. Some species of cyanobacteria produce toxins, causing a considerable danger for human and livestock health. As a consequence, monitoring of bloom formation and toxin production of drinking water supplies has become a major concern. To enable prediction and monitoring of cyanobacterial blooms, tools to detect nutrient bioavailability in water would be advantageous. A whole-cell biosensor was developed for monitoring nitrate (NO3-) bioavailability in aquatic ecosystems using the recombinant bioluminescent cyanobacterial strain Synechocystis PCC 6803 harboring an insertion of a luxAB-kmr fusion with nblA1 in its chromosomal DNA, leading to PnblA::luxAB-kmr. This reporter strain was designated N1LuxKm. Cells were immobilized in microtiter plates and showed a dose-dependent response to nitrate deprivation. The resultant CyanoSensor could detect nitrate in the 4-100 muM concentration range after a sample incubation time of 10 h under continuous illumination (50 muE m(-2) s(-1)). The optimal temperature for sensor operation was 29degreesC and the immobilized biosensor could be stored at 4degreesC in dark for about 1 month without significant loss of sensitivity.