Phenanthrene degradation by microorganisms isolated from a contaminated stream

Autochthonous yeast and bacteria strains were isolated from a contaminated stream and studied for their potential to degrade phenanthrene, a three-ring polycyclic aromatic hydrocarbon (PAH), as their only source of carbon and energy. Rhodotorula glutinis and Pseudomonas aeruginosa were the prevailing microorganisms utilizing phenanthrene. Cells of these microorganisms were inoculated in liquid mineral basal medium at 25, 50, 100 and 200 mg phenanthrene liter−1 in ethanolic solution. To volatilize the ethanol the medium was dried for 40 min at 35°C with UV exposure. Almost complete phenanthrene degradation was observed during the 1-month incubation period. R. glutinis grew exponentially to a maximum of 9.5×106 colony-forming units (CFUs) ml−1, with a short lag time only at 200 mg liter−1. The highest density (1.4×107 CFU ml−1) without a lag time was obtained at 50 mg liter−1; lower cell numbers were observed at 25 and 100 mg liter−1. During the logarithmic phase the specific growth rate (μ) was 0.022 h−1 and the corresponding doubling time (td) was 31.5 h (50 mg liter−1). A reduction to one-tenth of cell yeast numbers was observed at the lowest PAH levels on the 28th incubation day and a decrease of 100-fold was observed at 200 mg liter−1. Higher μ-values (μ=d [ln x/x0]/dt), shorter td (td=[ln 2]/μ), and a maximum of 7.7–7.4×108 CFU ml−1 were observed in P. aeruginosa cultures at 50–100 mg liter−1, respectively, on the 22nd day. At 25 and 200 mg liter−1 lower bacteria numbers were obtained (2.1–3.0×108 CFU ml−1) on the 28th day. R. glutinis was as active as P. aeruginosa at growing on phenanthrene; the aromatic hydrocarbon degradation correlated directly to microbial density and biomass increase, the highest biomass reaching 238.0 and 50.0 mg liter−1 for the yeast and bacteria species, respectively.