Fourth International Conference on the Molecular Biology and Pathogenesis of the Clostridia. Abstracts of poster presentations.

Abstract

Micro-organisms are vital for soil fertility and for the degradation of organic matter and pollutants in soils and sediments. Due to their function and ubiquitous presence micro-organisms can act as an environmentally very relevant indicator of pollution. Microbial tests should be used discriminatory for the establishment of soil and sediment quality guidelines. This review gives an evaluation of microbial toxicity tests and a novel method to derive quality guidelines.Long term microbial tests are generally less sensitive than short term tests. The toxic effects can be obscured by the activity of a few resistant micro-organisms, when for example soil respiration is used as a sum parameter during a long incubation period. Mineralization tests with high substrate 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 measurements, carbon and nitrogen transformations, enzymatic tests and tests measuring changes in microbial diversity. Comparisons between tests can only be indicative because the relative sensitivity depends on the toxicants and soils used. The respiration rate per unit of biomass is a more sensitive indicator of toxic effects than the respiration rate or the 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 tests are often not very sensitive. The urease activity is a relatively sensitive enzymatic test in many studies.The replacement of sensitive micro-organisms by different resistant species can have serious ecological consequences. Some species become extinct while others appear in bulging numbers. Adaptation of a community to a pollutant must be considered as the very process which disturbs a polluted ecosystem. The resistant micro-organisms often fail to perform specific ecological functions. The occurrence of resistant species can be used as an sensitive and ecologically relevant indicator for deterioration from environmental pollution. Persistent toxic effects on the microflora can be caused by zinc, cadmium and copper at concentration levels lower than European Community limits.Test with anaerobic sediment processes were orders of magnitude more sensitive for some clorinated aliphatic compounds than aquatic toxicity tests. The addition of a few mg zinc per kg soil can inhibit the more sensitive microbial processes (like chloroform or 4-chlorophenol degradation), whereas soil invertebrates and some plants are less sensitive to zinc.After evaluation of the tests, a novel method is described to derive soil and sediment quality guidelines using microbial toxicity tests. The results of single species tests with micro-organisms can be incorporated into the contemporary risk assessment method for higher organisms which is based on the extrapolation from single species tests to the protection of 95% of all species in an ecosystem. This method uses the No Observed Effect Concentrations (NOEC) of a number of toxicity tests to calculate a Hazardous Concentration 5% (HC5). The HC5 is calculated from more than 5 NOEC values. In analogy the Effect Concentration 10% (EC10) can be used to calculate the Dangerous Concentration 5% (DC5). The DC5 is calculated from more than 5 EC10 values. The DC5 should give protection to 95% of the microbial 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 among the most sensitive organisms for the effects of pollutants.