Abstract: Nitrogen and phosphorus nutrients are the basic components of marine phytoplankton and key factors in controlling marine primary productivity. Previous studies have focused on the determination of nutrient chemical concentrations and neglected the detection of the actual bioavailability of nutrients in the water column. In order to achieve the detection of nitrogen and phosphorus nutrient bioavailable concentrations, this study screened two efficient and specific response to nitrogen and phosphorus bioavailable promoters of Synechococcus, drove the expression of bacterial luciferase reporter genes, constructed a microalgal cell bioreporter for the direct detection of nutrient bioavailability in environmental samples, and obtained the relationship curves between bioluminescence and standard nitrogen (\rmNO_3^- ) and phosphorus (\rmPO_4^3- ) nutrient concentrations, which can be used for the rapid detection of nitrogen and phosphorus bioavailability in water bodies. Meanwhile, the water samples of the South China Sea cruise were measured using the national standard method and the present method, and the phosphorus concentration measured by the national standard method was 0.17—3.00 μmol/L and the nitrogen concentration was 1.04—49.34 μmol/L, while the phosphorus concentration measured by the present method was 0.10—1.13 μmol/L, which was close to the results determined by the national standard method; the nitrogen concentration was 32.57—98.06 μmol/L, which was different from the results determined by the national standard, probably because the water samples also contained other forms of nitrogen. The study proposes a method to detect and quantify the bioavailability of seawater nitrogen and phosphorus using biological reporters, which is an important complement to the chemical concentration index of nutrients in water bodies and provides new directions and ideas for the detection of nutrient bioavailability of environmental samples.