Contrasting effects of a nonionic surfactant on the biotransformation of polycyclic aromatic hydrocarbons to cis-dihydrodiols by soil bacteria

The biotransformation of the polycyclic aromatic hydrocarbons (PAHs) naphth alene and phenanthrene was investigated by using two dioxygenase-expressing bacteria, Pseudomonas sp. strain 9816/11 and Sphingomonas yanoikuyae B8/36 , under conditions which facilitate mass-transfer limited substrate oxidati on. Both of these strains are mutants that accumulate cis-dihydrodiol metab olites under the reaction conditions used. The effects of the nonpolar solv ent 2,2,4,4,6,8,8-heptamethylnonane (HMN) and the nonionic surfactant Trito n X-100 on the rate of accumulation of these metabolites were determined. H MN increased the rate of accumulation of metabolites for both microorganism s, with both substrates. The enhancement effect was most noticeable with ph enanthrene, which has a lower aqueous solubility than naphthalene. Triton X -100 increased the rate of oxidation of the PAHs with strain 9816/11 with t he effect being most noticeable when phenanthrene was used as a substrate. However, the surfactant inhibited the biotransformation of both naphthalene and phenanthrene with strain B8/36 under the same conditions. The observat ion that a nonionic surfactant could have such contrasting effects on PAH o xidation by different bacteria, which are known to be important for the deg radation of these compounds in the environment, may explain why previous re search on the application of the surfactants to PAH bioremediation has yiel ded inconclusive results. The surfactant inhibited growth of the wild-type strain S. yanoikuyae B1 on aromatic compounds but did not inhibit B8/36 dio xygenase enzyme activity in vitro.