Luminometry and PCR‐based monitoring of gene‐tagged cyanobacteria in Baltic Sea microcosms

The cyanobacterium Synechocystis 6803 was tagged by chromosomal integration of the firefly luciferase gene, lue, resulting in the modified strain Synechocystis 6803‐luc. The tagged cells were monitored in Baltic Sea microcosms both by detection of the luc gene by PCR amplification and by measurement of luc gene expression (bioluminescence) in total protein extracted from sediment and water. A new method was developed for isolation and concentration of total protein from sediment for optimization of luciferase quantitation. The detection limit for Synechocystis with a chromosomal luc insertion by bioluminescence was in the order of 4 × 103 cells per g sediment, a considerable improvement in sensitivity over previous methods. Another improvement was to use an internal luciferase standard to correct for quenching of light output by impurities in the samples. Baltic sea microcosms were inoculated with Synechocystis 6803‐luc, and the luc DNA and luciferase protein specific to the tagged cells were monitored over time. A decrease in luminescence in the microcosm water was observed, simultaneously with an increase in luminescence in the sediment, suggesting settling of the luc‐tagged cells in the sediment layer.