Development of a Compact, Highly-sensitive and Low-cost Biological Monitoring Method using Protozoa for Detecting Toxicants in Aquatic Environment

Many attempts have been reported so far for monitoring water quality by using various kinds of living organisms. For example, aquatic insects and fish have been used in practice as monitor organisms to detect changes in water quality by detecting alteration in their swimming behavior. In these systems, problems are still remained to be solved as follows: 1) maintenance of the monitor organisms, 2) constructing hardware to detect movement of test organisms in high accuracy, and 3) low sensitivity and possible adaptation of the animals to the pollutants. To overcome these problems, development of a novel system of bio-monitoring was attempted by using protozoa as monitor organisms. Benthic protozoans are suitable for this purpose, because 1) most of the protozoan species can be cultured easily and inexpensively in a short time, 2) The space required to monitor their behavior is small, and 3) adaptation of the organisms to the aquatic environment does not usually occur, since protozoans are unicellular organisms. Basic investigations have demonstrated that heliozoans are most adequate protozoa, since they are highly sensitive to aquatic pollutants such as heavy metals [1], and their movement is restricted only in a planar layer [2] which allows us to use easier algorithm for automatic image analysis.

Heliozoans possess long and thin tentacles called axopodia radiating from the spherical cell body, in which bundles of microtubules are present as a supporting cytoskeleton. The heliozoans paralyze and capture prey organism with the aid of the axopodia [3]. The axopodia show frequent shortening and re-elongation, as their microtubules are highly sensitive to environmental factors including toxic chemical substances. We have previously examined the effect of various concentrations of heavy metal ions (zinc, lead, copper, mercury and cadmium) to the axopodia and found that the length of axopodia decreased in a concentration-dependent manner [1, 4]. Raphidiophrys contractilis is one of the members of heliozoa that inhabits fresh or brackish water. This species can be cultured with ease in a bacteria-free condition [5]. In response to various environmental factors, R. contractilis shows axopodial retraction in a reversible fashion. The axopodia re-elongate to the original length of about 100 µm in about 20 min, when the environmental condition is fully restored. The factors include harmful chemicals such as heavy metals, changes in pH and water temperature, etc. In this research, we attempted to utilize such features of R. contractilis for water quality monitoring.