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.

The dibenzothiophene (DBT) desulfurization pathway of a facultative thermophilic bacterium Mycobacterium sp. X7B was investigated. Metabolites were identified by gas chromatography-mass spectrometry, and the results showed that 2-hydroxybiphenyl, the end product of the previously reported sulfur-specific pathway (also called 4S pathway), was further converted to 2-methoxybiphenyl. This is the first strain to possess this ability and therefore, an extended 4S pathway was determined. In addition, the DBT-desulfurizing bacterium Mycobacterium sp. X7B was able to grow on DBT derivatives such as 4-methylDBT and 4,6-dimethylDBT. Resting cells could desulfurize diesel oil (total sulfur, 535 ppm) after hydrodesulfurization. GCflame ionization detection and GCatomic emission detection analyses were used to qualitatively evaluate the effect of Mycobacterium sp. X7B treatment on the content of the diesel oil. The total sulfur content of the diesel oil was reduced 86% using resting cell biocatalysts for 24h at 45℃.

A kinetic model of continuous treatment of waste water by Rhodopseudomonas palustris Y6 immobilized on soft fibre in a columnar bioreaction system was established. Good agreement was found between the model prediction and the experimental data from continuous operation [initial chemical oxygen demand (COD) concentration" 29.700g/l] of the system. The optimum operational conditions for the maximum COD reduction capacity were investigated from the model prediction and the experimental data. The waste water treatment process may significantly increase the waste reduction capacity because a large amount of active biomass for COD reduction is immobilized in the system, resulting in operation stability. The results presented here provide a useful basis for further scaling up and e

On an industrial scale, the production of pyruvate at a high concentration from the cheaper lactate substrate is a valuable process. To produce pyruvate from lactate by whole cells, various lactate-utilizing microorganisms were isolated from soil samples. Among them, strain WLIS, identified as Acinetobacter sp., was screened as a pyruvate producer. For the pyruvate preparation from lactate, the preparative conditions were optimized with whole cells of the strain. The cells cultivated in the medium containing 100mM of L-lactate showed the highest biotransformation efficiency from lactate to pyruvate. The optimized dry-cell concentration, pH, and temperature of reaction were 6g/L, pH 7.0-7.5, and 30℃, respectively. The influences of ethylenediaminetetraacetic acid (EDTA) and aeration on a biotransformation reaction were carried out under the test conditions. Under the optimized reaction conditions, L-lactate at concentrations of 200 and 500mM were almost totally stoichiometrically converted into pyruvate in 8 and 12h, respectively. About 60% of 800mM of L-lactate was transformed into pyruvate in 24h. This reduced conversion rate is probably due to the high substrate inhibition in biotransformation.

In this study, the glycerol metabolism by Klebsiella pneumoniae is stoichiometrically analyzed according to energy (ATP), reducing equivalent and product balances. The theoretical analysis reveals that a microaerobic condition is more perfect for the production of 1,3-propanediol (1,3-PD) from glycerol by K. pneumoniae than anaerobic and aerobic conditions. The yields of 1,3-PD, biomass and ATP to glycerol under microaerobic conditions depend not only on the molar fraction of reducing equivalent oxidized completely by molecular oxygen in tricarboxylic acid (TCA) cycle (δ), but also on the molar fraction of TCA cycle in acetyl-CoA metabolism. The maximum theoretical yield of 1,3-PD to glycerol could reach to 0.85mol/mol rather than 0.72mol/mol if all acetyl-CoA entered into TCA cycle instead of acetic acid pathway under anaerobic conditions. The yield of 1,3-PD is still higher than 0.72mol/mol in a range of δ between 0.11 and 0.48, which corresponds to respiratory quotient (RQ) between 11.34 and 2.66. In the same range of δ or RQ, the biomass under a microaerobic condition is more than that of an anaerobic culture. The experimental results of batch cultures demonstrate that microaerobic cultivations are favorable for cell growth, reduction of culture time and ethanol formation, and enhancement of volumetric productivity of 1,3-PD. In addition, no aeration could improve the yield of 1,3-PD to glycerol in comparison with that of an anaerobic or aerobic culture.

The microbial production of 1,3-propanediol (1,3-PD) by Klebsiella pneumoniae under micro-aerobic conditions was investigated in this study. The experimental results of batch fermentation showed that the final concentration and yield of 1,3-PD on glycerol under micro-aerobic conditions approached values achieved under anaerobic conditions. However, less ethanol was produced under microaerobic than anaerobic conditions at the end of fermentation. The batch micro-aerobic fermentation time was markedly shorter than that of anaerobic fermentation. This led to an increment of productivity of 1,3-PD. For instance, the concentration, molar yield, and productivity of 1,3-PD of batch microaerobic fermentation by K. pneumoniae DSM 2026 were 17.65g/l, 56.13%, and 2.94g l-1 h-1, respectively, with a fermentation time of 6 h and an initial glycerol concentration of 40g/l. Compared with DSM 2026, the microbial growth of K. pneumoniae AS 1.1736 was slow and the concentration of 1,3-PD was low under the same conditions. Furthermore, the microbial growth in fed-batch fermentation by K. pneumoniae DSM 2026 was faster under micro-aerobic than anaerobic conditions. The concentration, molar yield, and productivity of 1,3-PD in fed-batch fermentation under micro-aerobic conditions were 59.50g/l, 51.75%, and 1.57g l-1 h-1, respectively. The volumetric productivity of 1,3-PD under microaerobic conditions was almost twice that of anaerobic fedbatch fermentation, at 1.57 and 0.80g l-1 h-1, respectively.