RAPID METHODS TO ASSESS THE EFFECTS OF CHEMICALS ON MICROBIAL ACTIVITY IN SOIL

Fluorescein diacetate (FDA) hydrolysis and an automated bacterial impedance technique (RABIT) were compared to the Pseudomonas putida growth inhibition assay (PGI) to assess their suitability as rapid methods to screen for effects of chemicals on soil microbial activity. RABIT was rapid, was the simplest, and was most convenient of the three methods to perform, but it was an order of magnitude less sensitive to toxicity than FDA or PGI. Further modification of the method may result in a most useful tool for screening purposes. The FDA hydrolysis method was both simple and rapid to perform. It gave comparable results to the PGI method for 3,5-dichlorophenol and an amphoteric surfactant, but FDA was an order of magnitude more sensitive to the anionic and nonionic and an order of magnitude less sensitive to the cationic surfactants tested. This apparent conflict emphasizes that the toxicity assessment of a chemical in the environment should not rely entirely on inherent toxicity tests. For terrestrial toxicity testing, interactions between chemical and soil should be taken into account in order to predict correctly the chemical's impact on the environment. For example, adsorption of cationics onto soil particles results in significantly higher concentrations of the chemical being needed in vivo to cause a toxic effect than that predicted by inherent toxicity tests. The use of soil suspensions in the FDA assay, rather than pure cultures of microorganisms, is a more realistic, but still indirect, test of a chemical's toxicity in soil. When the assay is calibrated against a standard method, such as PGI using uncharged reference chemicals, this method can provide a simple and rapid tool to assess the relative effects of chemicals on soil microbial activity. (C) 1994 by John Wiley & Sons, Inc.