Degradation Of SNO Heterocyclic Compounds By Sphingomonas Sp.

Heterocyclic compounds (nitrogen, oxygen, or sulfur-containing aromatic compounds) such as carbazole (CA), dibenzofuran (DBF) and dibenzothiophene (DBT) together with their degradation products have been detected in groundwater, seawater, sediments, and soil sites contaminated with spills of petroleum and wood-preserving wastes. These heterocyclic compound mixtures with other polycyclic aromatic hydrocarbons are widespread because of petroleum application in industrial processes, coal gasification and wood treatment with creosote. Heterocyclic compounds, DBT and its derivatives in particular, can persist for up to three years after an oil spill, while more susceptible compounds would have been biodegraded. Moreover, because some of these heterocyclic compounds are mutagenic and carcinogenic, ecosystems contaminated with these compounds may elicit serious health risks. Fundamental research examining the biodegradation of these recalcitrant compounds and the application of bacteria in bioremediation for reducing their concentrations in the environment has been made.Strain XLDN2-5 was isolated from petroleum-polluted soil by the enrichment method. The strain was able to utilize CA as the sole carbon and nitrogen source and degrade DBF and DBT co-metabolically. XLDN2-5 was rod-shaped and catalase positive. It accumulated poly-β-hydroxybutyrate and gave positive results for citrate and glucose utilization and growth on MacConkey agar. XLDN2-5 gave negative results for methyl red test, fluorescent on King medium, urease production, Voges-Proskauer test, oxidase test and yielded negative results for H₂S and indole production. The 16S rRNA sequence of strain XLDN2-5 showed 99% homology to that of Sphingomonas yanoikuyae. Therefore, strain XLDN2-5 was assigned to the genus Sphingomonas.Washed cells of XLDN2-5 were shown to be capable of degrading DBF and DBT. Examination of metabolites suggested that XLDN2-5 degraded DBF to 2-hydroxy-6-(2-hydroxyphenyl)-6-oxo-2,4-hexadienic acid and subsequently to salicylic acid through the angular dioxygenation pathway. In contrast to DBF, strain XLDN2-5 could transform DBT through the ring cleavage and sulfoxidation pathways. Sphingomonas sp. XLDN2-5 could simultaneously degrade CA, DBF and DBT in the growing system. After 40 h incubation, 90% of DBT was transformed and CA and DBF were completely removed. These results suggested that strain XLDN2-5 might be useful in the bioremediation of environments contaminated by these compounds. The (lH)-4-quinolinone detected in the degradation of CA had a similar structure to the chromone identified in DBF degradation by XLDN2-5. This result suggested that angular dioxygenation might have occurred when XLDN2-5 degraded CA and DBF. Although anthranilic acid, (AA) was not detected as a metabolite in the degradation of CA by this bacterium, the isolate XLDN2-5 did use AA as a growth substrate.Benzothiophene (BT), a simple form of PASHs, expresses high water solubility and persistence which contribute to an increasing dominance of the down gradient of the plume. Several high-molecular-weight sulfur-containing aromatic compounds including benzo[b]naphtha[1,2-d]thiophene (BN12T) were detected by GC-MS together with 2-mercaptobenzoic acid. BN12T was formed by Diels-Alder-type condensation of two molecules of benzothiophene-1-oxide produced from BT, with the subsequent dehydrogenation and deoxygenation. Because the BN12T is among the most persistent contaminants, aerobic transformation of BT to more persistent products in the environment might profound implication for human health and for regulation of these compounds.We further studied degradation of alkyl substituted BT and DBT. Alkyl substituted BT and DBT were more persistent than BT and DBT, and difficult degraded by XLDN2-5. When 3-methyl-BT (3MBT) was added at a final concentration of 0.3 mM. the degradation of 3MBT and CA were prevented. Morever. 0.3 mM 3MBT could lead to the inactivation of the bacteria.