With the rapid development of nuclear energy and technology, more nuclear power plants are being built and operated worldwide. Due to the need for large amounts of cooling water, most nuclear power plants are located near coastal or inland rivers, which increases the risk of radioactive pollution in surrounding waters. Traditional detection methods of radionuclides in water often have some problems such as tedious detection processes and high detector costs. They also cannot reflect the toxic effect of radionuclides on organisms and do not meet the needs of in situ rapid detection of radionuclide pollution in water. Algae fluorescence induction kinetics technology has been widely used in toxicity detection of heavy metals, pesticides, and other pollutants in water in recent years due to its simplicity, rapidity, and non-destructive characteristics. However, it is still unknown whether the algae fluorescence induction kinetic technique can be applied to the field rapid detection of radionuclide toxicity in water. Therefore, we study the response rules and characteristics of algae rapid chlorophyll fluorescence induction kinetic line and commonly used photosynthetic fluorescence parameters to the short-term toxicity stress of three common radionuclides. In this way, we determine the feasibility of the application of algae fluorescence induction kinetic technique to the field detection of radionuclide toxicity in water.

Chlorella pyrenoidosa, a common freshwater green algae, is used as the test organism, and three typical radionuclide pollutants strontium (⁹⁰Sr), cesium (¹³⁷Cs) and cobalt (⁶⁰Co) are studied. By using the fluorescence induction kinetics method, we study the toxicity response rules and characteristics of fast chlorophyll fluorescence induction kinetics (OJIP) curve, maximum photochemical quantum yield (F_v/F_m, where F_v is variable fluorescence, and F_m is maximal fluorescence) and photosystem II performance parameters (PI_ABS) under 180 min short-term stress of ⁹⁰Sr, ¹³⁷Cs and ⁶⁰Co. The feasibility of the application of algae fluorescence induction kinetics technique to the rapid detection of radionuclide toxicity in water is determined. By establishing two photosynthetic fluorescence parameters and three radionuclides dose-response curves, we analyze the 20% effect concentration (EC₂₀) and 50% effect concentration (EC₅₀) of F_v/F_m and PI_ABS to further compare the toxicity response of F_v/F_m and PI_ABS to three radionuclides. Finally, we select the best toxicity response index which can be used for sensitive detection of radionuclide toxicity in water.

The three radionuclides ⁹⁰Sr, ¹³⁷Cs, and ⁶⁰Co can damage the photosynthetic system of chlorella pyrenoidosa and inhibit the activity of photosystem II and electron transfer, thereby inhibiting the photosynthesis of chlorella pyrenoidosa and finally leading to changes in the OJIP curve (Fig. 1). Therefore, microalgae fluorescence dynamics technology can be applied to the rapid detection of radionuclide toxicity in water. By studying the response characteristics of the photosynthetic fluorescence parameter F_v/F_m to the toxicity of three radionuclides ⁹⁰Sr, ¹³⁷Cs, and ⁶⁰Co, we find that the inhibition degree of the three radionuclides to F_v/F_m is continuously enhanced within 180 min, and the toxic response of F_v/F_m to three radionuclides is activity concentration- and time-dependent (Fig. 2). The adjusted coefficient of determination (adj-R²) obtained by Logistic fitting curves between F_v/F_m inhibition rate and nuclides activity concentration are all greater than 0.9, indicating that the three radionuclides and F_v/F_m have good Logistic dose-response relationships (Fig. 3). Therefore, F_v/F_m can be well used in the detection and evaluation of radionuclide toxicity. Within 180 min of exposure, the photosynthetic fluorescence parameter PI_ABS has activity concentration- and time-dependent toxicity response to the three radionuclides (Fig. 4). The adj-R² obtained by the Logistic fitting curves between PI_ABS inhibition rate and nuclides activity concentration are all greater than 0.9, indicating that the three radionuclides and PI_ABS also have good Logistic dose-response relationships (Fig. 5). Therefore, PI_ABS can be used in the detection and evaluation of radionuclide toxicity. In addition, by comparing the EC₂₀ and EC₅₀ values obtained based on the two photosynthetic fluorescence parameters, we find that the EC₂₀ (Fig. 6) and EC₅₀ values (Fig. 7) obtained based on F_v/F_m of the three radionuclides are greater than those obtained based on PI_ABS. It shows that the photosynthetic fluorescence parameter PI_ABS has more sensitive response characteristic to radionuclide toxicity than F_v/F_m.

Under short-term exposure for 180 min, three typical radionuclides ⁹⁰Sr, ¹³⁷Cs, and ⁶⁰Co have significant toxic effects on the photosynthesis of chlorella pyrenoidosa, resulting in a significant change in the shape of OJIP curves and significant inhibition of photosynthetic fluorescence parameters F_v/F_m and PI_ABS of chlorella pyrenoidosa. Thus, the fluorescence induction kinetics of microalgae can be used for rapid detection of radionuclide toxicity in water. The two photosynthetic fluorescence parameters F_v/F_m and PI_ABS obtained based on fluorescence induction kinetics have good Logistic dose-response curves with ⁹⁰Sr, ¹³⁷Cs, and ⁶⁰Co. They have a certain time-dependent response to the toxicity of ⁹⁰Sr, ¹³⁷Cs, and ⁶⁰Co. Therefore, F_v/F_m and PI_ABS can be used as toxicity response indexes to detect radionuclide toxicity in water based on fluorescence kinetics. By comparison, the response sensitivity of PI_ABS to the toxicity of three radionuclides under short-term exposure for 180 min is significantly better than that of photosynthetic fluorescence parameter F_v/F_m. This indicates that PI_ABS is the best toxicity response index for sensitive detection of radionuclide toxicity in water based on microalgae fluorescence kinetic technique. Our study provides a method basis for the rapid detection of radionuclide toxicity in the water environment and a new idea for emergency monitoring and early warning of nuclear leakage accidents in inland nuclear power plants. It has practical significance for ensuring the safety of water ecological environment.