Sulfur (S), nitrogen (N) and oxygen (O) heterocycles areamong the most potent environmental pollutants.Microbial degradation of these pollutants is attractingmore and more attention because such bioprocesses areenvironmentally friendly. The biotechnological potentialof these processes is being investigated, for example, toachieve better sulfur removal by immobilized biocatalystswith magnetite nanoparticles or by solvent-tolerantbacteria, and to obtain valuable intermediates fromthese heterocycles. Other recent advances have demonstratedthe mechanisms of angular dioxygenation ofnitrogen heterocycles by microbes. However, these technologiesare not yet available for large-scale applicationsso future research must investigate proper modificationsfor industrial applications of these processes. Thisreview focuses on recent progress in understanding howmicrobes degrade S, N and O heterocycles.

Dibenzothiophene (DBT) is the most excessive and refractory sulfur compound in fossil fuels. The methods for removing DBT, using bacteria, were twofold: the first one involved the destruction of the carbon skeleton; the second, the use of a sulfur-specific process of biodesulfurization, without cleaving the carbon ring. Because the second method does not degrade the value of the fuel, it is considered by most researchers to be the method of choice. Bacteria used for this study, were obtained from the soil collected from a field that contained waste water from a refinery. Using GC/MS, it was confirmed that the metabolic pathway used by this bacterium, involved a sulfur-specific process of biodesulfurization, named the '4S pathway'. This strain appears to have the ability to remove the organic sulfur from thiophenic compounds over a wide temperature range from 25 to 45℃. And the half time of the whole cells desulfurization activity was 32 days, three times more than Rhodococcus erythropolis IGTS8. With the excellent stability, it may have industrial application for biodesulfurization.

A carbazole-utilizing bacterium was isolated by enrichment from petroleum-contaminated soil. The isolate,designated Sphingomonas sp. strain XLDN2-5, could utilize carbazole (CA) as the sole source of carbon,nitrogen, and energy. Washed cells of strain XLDN2-5 were shown to be capable of degrading dibenzofuran(DBF) and dibenzothiophene (DBT). Examination of metabolites suggested that XLDN2-5 degraded DBF to2-hydroxy-6-(2-hydroxyphenyl)-6-oxo-2,4-hexadienic acid and subsequently to salicylic acid through the angulardioxygenation pathway. In contrast to DBF, strain XLDN2-5 could transform DBT through the ringcleavage and sulfoxidation pathways. Sphingomonas sp. strain XLDN2-5 could cometabolically degrade DBFand DBT in the growing system using CA as a substrate. After 40 h of incubation, 90% of DBT was transformed,and CA and DBF were completely removed. These results suggested that strain XLDN2-5 might be useful inthe bioremediation of environments contaminated by these compounds.

Biodegradation of carbazole was enhanced by the presence of a non-aqueous phase liquid (logKo/w≥3.1) at phase ratio of 1:1 (organic/aqueous). In a cyclohexane/aqueous phase system, the maximum specific degradation rate (3.34mg carbazole min−1g dry cell−1) was at an organic/aqueous ratio of 1:1. Pseudomonas sp. XLDN4-9 degraded 47% (w/w) of 1g carbazole l−1 in cyclohexane phase directly within 1h.