Short-term Toxicity Testing for Mercuric Chloride, Cadmium Chloride, and Selenium in the Photobacterium Beneckea harveyi

Abstract

The contamination of naturally occurring bodies of water with toxic materials has become a major environmental problem. These natural resources may serve as the major source of potable water for population centers. To protect the general public against adverse health effects of environmental pollutants, the Environmental Protection Agency has as one of its duties the promulgation of standards which define the level of various chemicals permitted in drinking water. Such standards are established through a rigorous series of short-term and long-term toxicity tests. These tests are conducted to define adverse effects on different aquatic, mammalian, avian, and plant species to establish a no-effect level for toxic substances in the environment. A strategy for evaluating environmental contamination may involve testing a polluted water sample without fractionation, thus assessing its overall toxicity. This method does not identify the chemical(s) of greatest concern in the test mixture. The most toxic chemical(s) must be identified and removed since it is unreasonable to suggest that all chemicals be removed from the environment to an equal extent. The testing of chemicals separately is costly and time consuming if currently accepted protocols using mammals are selected. Thus, there is an urgent requirement to develop and validate short-term toxicity screening tests to complement the current toxicity testing strategy by providing a tier of screening prior to submitting chemicals to long-term mammalian tests. A battery of short-term screening tests is recommended because a single test system, especially an in vitro test, does not provide as comprehensive an assessment of toxicity as whole animal studies. It has been proposed that a battery of short-term tests might be predictive of chronic toxic effects observed in mammals (1). The need to develop and validate these short-term tests to assess toxicity has been recognized (2). Short-term test systems have been used to assess the toxicity of single compounds. Very little information, if any, is available on the ability of these tests to detect any interaction of biologic effects produced by a combination of chemicals. A chemical when tested alone will produce a certain level of toxicity. When a second chemical is added, the test system may reflect an additive effect. Alternatively, the level of toxicity may be less than that of the additive effect of both chemicals. There is yet a third possibility - a synergistic action. In this situation, the combined effect of the two chemicals is greater than that expected by a simple additive function of their individual levels of toxicity. Synergism of toxic effects among chemicals can only be accurately assessed by tests using biologic systems. Attempts to predict synergistic action by comparison of molecular structure (structure-activity studies) and other physical or chemical parameters have not proved useful for different classes of chemicals. Thus, the necessary reliance on biologic systems to test for interaction of toxic effects and the continued introduction of thousands of new chemicals into the environment each year indicate the need for short-term toxicity tests which are capable of quantitating toxic interactions. The biologic test system selected for this study is the photobacterium Beneckea harveyi. A photobacterial toxicity screening system was developed by Shiotsuka et al. (3), and the graded response of the bioluminescent activity was shown to be proportional to the level of the toxic chemical in the test medium. Thus, the observed decrease in luminescence may allow one to estimate the degree of water contamination by toxic chemicals. This system has not been used to study interaction of effects of several chemicals tested simultaneously. The chemicals selected for testing are mercuric chloride, cadmium chloride, and selenium. These chemicals were selected because of their ubiquity as environmental pollutants and because of their known toxicity to man and the environment in general. These chemicals are conducive to testing in the photobacterial test system because of their solubility in water.