A high sensitivity iron-dependent bioreporter used to measure iron bioavailability in freshwaters

A Nostoc sp. PCC 7120 iron bioreporter containing iron-regulated schizokinen transporter gene alr0397 promoter fused to the luxAB genes was examined to optimize its response to bioavailable iron. Dose-response relationships between luciferase activity and free ferric ion (Fe(3+) ) concentrations pFe (-lg [Fe(3+) ]) were generated by measuring luciferase activities of the bioreporter in trace metal-buffered Fraquil medium with various incubation times. The results were best demonstrated by sigmoidal curves (pFe 18.8-21.7, Fe(3+)  = 10(-18.8) -10(-21.7)  M) with the linear range extending from pFe 19.6-21.5 (Fe(3+)  = 10(-19.6) -10(-21.5)  M) after a 12-h incubation time. Optimal conditions for the use of this bioreporter to sense the iron bioavailability were determined to be: a 12-h exposure time, initial cell density of OD(730 nm)  = 0.06, high nitrate (100 μM), high phosphate (10 μM), moderate Co(2+) (0.1-22.5 nM), Zn(2+) (0.16-12 nM), Cu(2+) (0.04-50 nM), and wide range of Mn(2+) concentration (0.92-2300 nM). The applicability of using this iron bioreporter to assess iron availability in the natural environment has been tested using water samples from eutrophic Taihu, Donghu, and Chaohu lakes. It is indicated that the bioreporter is a useful tool to assess bioavailable iron in various water quality samples, especially in eutrophic lakes with high bioavailable iron.