Heavy metal effects on the biodegradation of fluorene by Sphingobacterium sp. KM-02 isolated from PAHs-contaminated mine soil

Polycyclic aromatic hydrocarbon compounds (PAHs) are widely distributed in the environment and occur ubiquitously in fossil fuels as well as in products of incomplete combustion and are known to be strongly toxic, often with carcinogenic and mutagenic properties. Fluorene is one of the 16 PAHs included in the list of priority pollutants of the Environmental Protection Agency. The fluorene-degrading bacterial strain Sphingobacterium sp. KM-02 was isolated from PAHs-contaminated soil near an abandoned mine impacted area by selective enrichment techniques. Fluorene added to the Sphingobacterium sp. KM-02 culture as sole carbon and energy source was 78.4% removed within 120 h. A fluorene degradation pathway is tentatively proposed based on mass spectrometric identification of the metabolic intermediates 9-fluorenone, 4-hydroxy-9-fluorenone, and 8-hydroxy-3,4-benzocoumarin. Further the ability of Sphingobacterium sp. KM-02 to bioremediate 100 mg/kg fluorene in mine soil was examined by composting under laboratory conditions. Treatment of microcosm soil with the strain KM-02 for 20 days resulted in a 65.6% reduction in total amounts. These results demonstrate that Sphingobacterium sp. KM-02 could potentially be used in the bioremediation of fluorene from contaminated soil. Mine impacted area comprises considerable amounts of heavy metals such as cadmium, lead, mercury, arsenic, and copper. Although some of these metals are necessary for biological life, excessive quantities often result in the inhibition of essential biological reactions via numerous pathways. A number of reports collectively show that various metals, such as Al, Co, Ni, Cu, Zn, Pb, and Hg at a range of concentrations have adverse effects on the degradation of organic compounds. However, at present there is only limited information on the effect of individual heavy metals on the biological degradation of polyaromatic hydrocarbons (PAHs) including fluorene. Moreover, heavy metal effects were not considered during biodegradation in mine impacted areas. The heavy metal effects on the degradation of fluorene by Sphingobacterium sp. KM-02 was determined in liquid cultures. The results showed that 10 mg/L cadmium, mercury and copper not only affected the growth of KM-02 with fluorene but also the ability of resting cells to degrade this compound. Growth and degradation were strongly inhibited by mercury, even at 1 mg/L, while the inhibitory effect of cadmium and copper at the same concentration or at 5 mg/L were negligible. In contrast, arsenic and lead did not affect degradation or growth, even at very high concentrations of 100 mg/L. Subsequent analyses additionally revealed that concentrations of arsenic and lead remained unchanged following incubation, while those of cadmium, mercury and copper decreased significantly. These data suggest the potential inhibition of fluorene degradation in mine soil, the major source of PAHs degradation, but which also would limit the applicability of a slurry-based fermentation reactor for PAHs degradation. Therefore, further study should be performed to elucidate whether these conditions are effectively imitating those of contaminated mine impacted soil, which are very complicated chemical and physical phenomena.