Investigations into the toxicity of persistent fractions of mineral oils

The paper deals with the question whether toxicity could be a reason for the incomplete microbial degradation of mineral oils in the laboratory and the environment, especially in soils contaminated over a long period. The luminescence test showed inhibition by the nondegradable residues as well as by the water phase from degradation experiments. This most frequently applied test reaches its limits in the case of substances with very low solubility in water. Conclusions relating to the impact on the microbial community of a contaminated ecosystem are not admissible. Neither nondegradable residues from raffinates nor extracts from long-term contaminated soils showed inhibiting effects on the dehydrogenase activity of Acetobacter methanolicus and a mineral oil-adapted consortium Inhibition of the respiration of the pure strain A. methanolicus and of the consortium growing on glucose and spindle oil respectively was not observed either. The residues evidently contain mainly highly stable compounds which withstand microbial attack. The toxicity of such compounds per se is unlikely. Moreover, toxicity investigations with aqueous phases from degradation experiments gave no indication of the formation of metabolites, which could be responsible for the stagnation of the process.