A battery of bioreporters of nitrogen bioavailability in aquatic ecosystems based on cyanobacteria

Cyanobacteria are the main primary producers and are responsible for the blooms and eutrophication processes caused by excess nutrients in surface waters. The aim of this paper is to use cyanobacteria to monitor the presence and bioavailability of different chemical species of nitrogen in freshwater. Cyanobacteria have mechanisms which can detect the presence of nutrients in their environment and can activate or repress specific genes, or operons, depending on nutrient bioavailability. Therefore, monitoring the expression of these genes can facilitate measurement of the availability of nutrients. To achieve this we have constructed self-bioluminescent reporter strains of the filamentous nitrogen-fixing cyanobacterium Nostoc sp. PCC 7120 expressing promoters of genes responsive to nitrogen fused to the luxCDABE operon. Three promoters were selected to direct the expression of luxCDABE: The first, the glnA promoter, is activated in the absence of combined nitrogen. We found that it responded linearly to the addition of known amounts of combined N in the range 50–500μM NH4+ or NO3−. The second, the nirA operon promoter, turns on in the presence of nitrate being inhibited by ammonium. The bioreporter responded linearly in the range of 10–100μM NO3. The third, the gifA promoter, is activated in the presence of ammonium, responding linearly in the range 100–600μM NH4+. We also used a previously described strain of Synechococcus elongatus PCC 7942 expressing glnN (glutamine synthetase type III) fused to luxAB. We found that the glnN promoter responded linearly to the addition of known amounts of N in the range 50–500μM NH4+ or NO3−. These cyanobacterial bioreporters were tested in real environmental samples (i.e. river waters) which confirmed their validity and showed a broad spectrum response. They are therefore useful in the detection of both total N-bioavailability and specific nitrogen species. •Four novel cyanobacterial N bioavailability bioreporters have been characterized.•Bioreporter A. GLA and S. GLN strains are able to detect total N bioavailability.•Bioreporter A. NIR and S. GIF strains are able to detect specific N species.•The bioreporters have been successfully tested in fresh water samples.