Effect of calcium on the surfactant tolerance of a fluoranthene degrading bacterium

Surfactants are known to increase the apparent aqueous solubility of polycy clic aromatic hydrocarbons (PAHs) and may thus be used to enhance the bioav ailability and thereby to stimulate the biodegradation of these hydrophobic compounds. However, surfactants may in some cases reduce or inhibit biodeg radation because of toxicity to the bacteria. In this study, toxicity of su rfactants on Sphingomonas paucimobilis strain EPA505 and the effect on fluo ranthene mineralization were investigated using Triton X-100 as model surfa ctant. The data showed that amendment with 0.48 mM (0.3 g l(-1)) of Triton X-100 completely inhibited fluoranthene and glucose mineralization and redu ced cell culturability by 100% in 24 h. Electron micrographs indicate that Triton X-100 adversely affects the functioning of the cytoplasmic membrane. However, in the presence of 4.13 mM Ca2+-ions, Triton X-100 more than doub led the maximum fluoranthene mineralization rate and cell culturability was reduced by only 10%. In liquid cultures divalent ions, Ca2+ in particular and Mg2+ to a lesser extent, were thus shown to be essential for the surfac tant-enhanced biodegradation of fluoranthene. Most likely the Ca2+-ions sta bilized the cell membrane, making the cell less sensitive to Triton X-100. This is the first report on a specific factor which is important for succes sful surfactant-enhanced biodegradation of PAHs.