Microalgal toxicity tests

Most microalgal toxicity tests procedures recommend the use of initial cellular concentrations of 104 cells mL 1. This cellular concentration should be selected because it is the minimum cellular concentration that can be measured in haematocytometers (Neubauer chambers). Furthermore, natural cellular concentrations in non-polluted conditions (in marine environments) are often below the concentration mentioned. The importance of cellular density at the beginning of the test has been demonstrated for certain toxicants [43]. The lower the cellular concentration, the higher the sensitivity of the test, at least for certain types of xenobiotics, such as heavy metals. 

Accordingly, the use of flow cytometry can improve the design of toxicity bioassays, as the detection limit of this apparatus includes cellular concentrations equal to those of microalgal populations found in natural conditions. Comparison of compositions utilised in some known toxicity tests for microalgaes are shown in Table 7.1.1. 

Environment Canada recently developed an evaluation system based on effluent toxicity testing, capable of ranking the environmental hazards of industrial effluents [185]. This so-called Potential Ecotoxic Effects Probe (PEEP) incorporates the results of a variety of small-scale toxicity tests into one relative toxicity index to prioritize effluents for sanitation. In the index no allowance has been made for in-stream dilution, therefore the acmal risk for environmental effects is not modeled. The tests performed on each effluent are the following bacterial assay [V.fisheri (P. phosphoreum), Microtox], microalgal assay S. capricornutum) crustacean assay (C. dubiay, and bacterial genotoxicity test E. coli, SOS-test). 

In principle, any battery of bioassays can be employed, but small-scale toxicity tests are preferred because of their performance output (Wells et al., 1998). It is highly desirable that bioassays used were part of the initial bioassay battery (prerequisite step) that proved to be sufficiently sensitive in the WASTOXHAS approach. Examples include the Microtox light inhibition test (Vibrio fischeri) and the microalgal growth inhibition assay (Selenastrum capricornutum ) that were found suitable for two tested wastes (see Section 7). 

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