Recently, the pollution caused by various hazardous chemicals has become a very serious problem. Currently, 200 million kinds of chemical substances are registered, and it is technically and costly very difficult or even impossible to analyse, identify each chemical individually and then evaluate their toxicity on the environment. On the other hand, bioassay has been getting a lot of attention where the toxicity of environmental toxicity is assessed based on the reaction of micro-organisms such as plankton without identifying each chemical individually. For this technique, a microscope observation is required to obtain critical features such as alive/dead status and swimming ability. With smaller microorganisms, microscope observation becomes more difficult due to the narrower focal depth of the imaging system. In our study, to overcome these difficulties, we proposed a novel technique for the micro-bioassay utilizing laser biospeckle in the diffraction field generated from plankton. Paramecium chilomonas of size 30-40 µm was exposed to different pH conditions from control 7.2 to gradually decreasing by 0.5 till 3.7. Results reveal the sensitivity of laser biospeckles in detecting the subtle changes in the swimming behavior, the health of the microbe with change in pH suggesting the potential for fast assessment the toxicity of an environment.
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