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Recently, bioassay has been getting much attention as it can comprehensively evaluate water toxicity without identifying the individual chemical component. In this technique, a microscope observation is required to know the critical features of the targeted microorganisms. However, as the size of the microorganism becomes smaller, observation becomes more difficult due to the narrower focal depth of the imaging system. Here, to overcome these difficulties, a novel biospeckle technique is used for the microbioassay utilizing the biospeckle in the diffraction field generated from the microorganisms. Paramecium caudatum (200-300µm) and Euglena gracilis(50-60µm) were used as a microorganism. Dynamic biospeckle patterns were captured using a CCD camera to evaluate the swimming activity of microorganisms under varying water toxicity levels induced by heavy metal pollutants (Zn(NO₃)₂·6H₂O and FeSO₄·7H₂O). To quantitatively evaluate the swimming activity of microorganisms, the crosscorrelation function between the initial frame as a reference frame and the subsequent frames was measured, and the correlation time was evaluated as a measure of their swimming ability. Results show a concentration dependent effect of Zn on both species, leading to decreased swimming ability. Conversely, Fe exhibited varying effects on Paramecia and Euglena, with the latter displaying tolerance at lower concentrations but a notable response at higher concentrations. The advantage of the method is that owing to the non-imaging system, an enormous number of planktons can be processed. This allows for an immediate and statistically significant estimation of their swimming ability in response to environmental pollution.
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