Mechanistic investigation in Co-biodegradation of phenanthrene and pyrene by Candida tropicalis MTCC 184

Polycyclic aromatic hydrocarbons like phenanthrene and pyrene are found ubiquitously in terrestrial and aquatic ecosystems. This study has attempted to investigate mechanistic features of simultaneous degradation of phenanthrene and pyrene by the yeast Candida tropicalis. Batch experiments comprising various combinations of initial phenanthrene and pyrene concentrations have been analyzed vis-a-vis kinetic model for cell growth that takes into account self- and cross-inhibition of both substrates. The values of interaction parameters of inhibition revealed strong competitive cross-inhibition between two substrates, due to which biomass yield with dual substrates was reduced significantly. Inhibition induced by pyrene on cell growth was higher than phenanthrene. GC-MS analysis of intermediate metabolites revealed two parallel pathways, first triggered by intracellular cytochrome P450 monooxygenase enzyme, and second initiated by dioxygenase enzymes. The former pathway yielded sulfate conjugate and dihydrodiols, while the latter resulted in acids and aldehydes that enters the TCA cycle.