SIGNIFICANCE AND APPLICATION OF MICROBIAL TOXICITY TESTS IN ASSESSINGECOTOXICOLOGICAL RISKS OF CONTAMINANTS IN SOIL AND SEDIMENT

Micro-organisms are vital for soil fertility and for the degradation o f organic matter and pollutants in soils and sediments. Due to their f unction and ubiquitous presence micro-organisms can act as an environm entally very relevant indicator of pollution. Microbial tests should b e used discriminatory for the establishment of soil and sediment quali ty guidelines. This review gives an evaluation of microbial toxicity t ests and a novel method to derive quality guidelines. Long term microb ial tests are generally less sensitive than short term tests. The toxi c effects can be obscured by the activity of a few resistant micro-org anisms, when for example soil respiration is used as a sum parameter d uring a long incubation period. Mineralization tests with high substra te concentrations which enable growth, are less sensitive than similar tests with low concentrations of substrate. The latter tests are more relevant for natural ecosystems. The often applied microbial toxicity tests can be categorized as single species tests, biomass measurement s, carbon and nitrogen transformations, enzymatic tests and tests meas uring changes in microbial diversity. Comparisons between tests can on ly be indicative because the relative sensitivity depends on the toxic ants and soils used. The respiration rate per unit of biomass is a mor e sensitive indicator of toxic effects than the respiration rate or th e amount of biomass alone. The autotrophic nitrification and acetylene reduction tests can be sensitive when short incubation times are used . The nitrogen mineralization, denitrification and many enzymatic test s are often not very sensitive. The urease activity is a relatively se nsitive enzymatic test in many studies. The replacement of sensitive m icro-organisms by different resistant species can have serious ecologi cal consequences. Some species become extinct while others appear in b ulging numbers. Adaptation of a community to a pollutant must be consi dered as the very process which disturbs a polluted ecosystem. The res istant micro-organisms often fail to perform specific ecological funct ions. The occurrence of resistant species can be used as an sensitive and ecologically relevant indicator for deterioration from environment al pollution. Persistent toxic effects on the microflora can be caused by zinc, cadmium and copper at concentration levels lower than Europe an Community limits. Tests with anaerobic sediment processes were orde rs of magnitude more sensitive for some clorinated aliphatic compounds than aquatic toxicity tests. The addition of a few mg zinc per kg soi l can inhibit the more sensitive microbial processes (like chloroform or 4-chlorophenol degradation), whereas soil invertebrates and some pl ants are less sensitive to zinc. After the evaluation of the tests, a novel method is described to derive soil and sediment quality guidelin es using microbial toxicity tests. The results of single species tests with micro-organisms can be incorporated into the contemporary risk a ssessment method for higher organisms which is based on the extrapolat ion from single species tests to the protection of 95% of all species in an ecosystem. This method uses the No Observed Effect Concentration s (NOEC) of a number of toxicity tests to calculate a Hazardous Concen tration 5% (HC5). The HC5 is calculated from more than 5 NOEC values. In analogy the Effect Concentration 10% (EC10) can be used to calculat e the Dangerous Concentration 5% (DC5). The DC5 is calculated from mor e than 5 EC10 values. The DC5 should give protection to 95% of the mic robial processes. The DC5 of a number of pollutants are calculated and compared with the HC5 values from the literature. Microbial toxicity tests can be used for risk assessment because micro-organisms are amon g the most sensitive organisms for the effects of pollutants. (C) 1997 Elsevier Science Ltd.