Abstract—A novel method for biological monitoring to detect toxic substances in water was developed by using the protozoan Raphidiophrys contractilis as an indicator organism. In this system (named HELIOSENSOR), the adhesion of R. contractilis to the substratum was used as a measure of the health of the living organisms. A flow-through type chamber was designed for toxicity testing, in which cells that had been damaged by harmful materials were flushed away by the water flow. The number of protozoa was continuously monitored with a digital camera. The test results revealed that this monitoring system has high durability and efficiency compared with other bio-monitoring systems, enabling us to make a quicker and easier detection of toxic substances. This system showed particularly high sensitivity to heavy metals such as mercury, arsenic, lead and cadmium. Due to high sensitivity (ex. ~ 10^-7 M for Hg²⁺), fast response time (<20 min) and small size (30×14×20 cm), this system has distinct advantages over other conventional biomonitoring systems using multicellular animals such as fish and crustaceans.

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