Influence of phenanthrene and fluoranthene on the degradation of fluorene and glucose by Sphingomonas sp. strain LB126 in chemostat cultures

Since bacteria degrading polycyclic aromatic hydrocarbon compounds (PAHs) in polluted soils are generally exposed to mixtures of PAHs, we examined the influence of simple PAH mixtures on the degradation activity of Sphingomonas sp. strain LB126. Fluorene serves as sole carbon and energy source for the strain LB126 and phenanthrene and fluoranthene are cometabolically degraded by this species. Chemostat cultures of the strain LB126 were used to study a potential inhibiting effect of phenanthrene and fluoranthene on the degradation of fluorene that was previously observed in batch cultures. We also looked at the effect of phenanthrene on the degradation of glucose in a chemostat culture to see if this effect was specific for the PAH-metabolic pathway or for the total metabolism of the strain. The co-substrates were supplied in a 5% to 30% fraction of fluorene. Phenanthrene and fluoranthene had no significant influence on growth. However, fluorene degradation was inhibited by both phenanthrene and fluoranthene. The effect of phenanthrene was about 10 times stronger than the effect of fluoranthene. Nevertheless, more than 95% removal of fluorene took place together with more than 95% removal of either phenanthrene or fluoranthene. The effect of phenanthrene on the strain LB126 could be ascribed to both toxicity and competitive inhibition, but the effect observed at steady state was due to competitive inhibition only. It appeared that the strain LB126 adapts to the toxicity of phenanthrene within five generations. The inhibitory effects observed previously in batch cultures of the strain LB126 should mainly be ascribed to the toxic effect of phenanthrene.