Alkanes anaerobic degradation

There is evidence for the anaerobic degradation of alkanes to COj, plausibly under conditions of sulfate reduction. In experiments with sediment slurries from contaminated marine areas, was recovered from " C-hexadecane (Coates et al. 1997), and was inhibited by molybdate that is consistent with the involvement of sulfate reduction. Under sulfate-reducing conditions was produced from C[14,15]octacosane (CagHjg) (Caldwell et al. 1998). Different mechanisms have been elucidated for the anaerobic degradation of higher alkanes, and both occurred simultaneously in a sulfate-reducing consortium (Callaghan et al. 2006) ... 

CH3(CH2) CH2CH2CH3 CH3(CH2) CH-CH2CH3 FIGURE 7.12 Anaerobic degradation of -alkanes. 

Coates JD, J Woodward, J Allen, P Philip, DR Lovley (1997) Anaerobic degradation of polycyclic aromatic hydrocarbons and alkanes in petroleum-contaminated marine harbor sediments. Appl Environ Microbiol 63 3589-3593. 

Anaerobic degradation of cycloalkanes has seldom been reported. The pathway used for the degradation of ethylcyclopentane by a sulfate-reducing enrichment is analogous to the fumarate pathway used for -alkanes (Part 1 of this chapter) with the formation of 3-ethylcyclopentanecar-boxylate followed by ring fission to 3-ethylpentan-l,5-dioate (Rios-Hamandez et al. 2003). 

Wilkes H., Rabus R. Fischer Th., Armstroff A., Behrends A., Pierik A. J., and Widdel F. (2001). The anaerobic degradation of n-alkanes as pure substrates and in crude oil first direct insights into the molecular mechanism of the initial C-H-activation and the subsequent degradation... 

The degradation of alkynes has been the subject of sporadic interest during many years, and the pathway has been clearly delineated. It is quite distinct from those used for alkanes and alkenes, and is a reflection of the enhanced nucleophilic character of the alkyne C C bond. The initial step is hydration of the triple bond followed by ketonization of the initially formed enol. This reaction operates during the degradation of acetylene itself (de Bont and Peck 1980), acetylene carboxylic acids (Yamada and Jakoby 1959), and more complex alkynes (Figure 7.18) (Van den Tweel and de Bont 1985). It is also appropriate to note that the degradation of acetylene by anaerobic bacteria proceeds by the same pathway (Schink 1985b). 

So, C. M., and L. Y. Young, Initial reactions in anaerobic alkane degradation by a sulfate reducer, strain AK-10 , Appl. Environ. Microbiol., 65, 5532-5540 (1999). 

A petroleum-degrading strain HD-1 was able to grow autotrophically by using CQ2 as sole carbon source. Inside the cell there accumulated alkane/alkene besides fatty acids or poly-p-hydroxyalkanoic acid. Fatty acids and fatty aldehydes were shown to be reduced to alkane/alkene by anaerobic incubation with the HD-1 cell lysate. 

An unusual pathway has been proposed for the degradation of n-alkanes to the carboxylic acids by a Pseudomonas sp. under anaerobic conditions this involves initial dehydrogenation and hydrox-ylation followed by successive oxidations (Figure 6.7) (Parekh et al. 1977). 

From an environmental point of view, it therefore appears that the complete degradation of chlorinated alkenes and alkanes will often require the operation of both anaerobic and aerobic steps for example, partial or complete dehalogenation may occur under anaerobic conditions, and aerobic degradation of the partially dechlorinated metabolites such as dichlo-romethane (La Roche and Leisinger 1991) and vinyl chloride (Castro et al. 1992a,b Hartmans and de Bont 1992) may then subsequently take place. The combination of the two activities in a single strain has been exploited in the genetically constructed strain of P. putida and has been discussed above. 

One potential limitation of the ENA approach is that MTBE is mineralized by only a few specialized bacteria and mainly imder aerobic conditions. Co-metabolic biotransformation of MTBE by aerobic, alkane-degrading bacteria has also been reported. Although several studies have demonstrated anaerobic biodegradation, anaerobic MTBE degradation rates are very low compared to aerobic rates. 

Obvious depletion of n-alkanes and other paraffins, classically regarded as indicative of early biodegradation, is not observed in examined samples. However, Kuparuk viscous oils show slight to extreme selective depletion in long-chain alkyl aromatic (LCAA) hydrocarbon families (e.g. alkylbenzenes and alkyltoluenes). This is interpreted as indicative of an early stage of anaerobic microbial degradation that likely destabilized the oil to promote subsequent precipitation of asphaltenes as tar. 

The biological degradation of alkane sulfonates occurs somewhat more easily than the degradation of alkylbenzene sulfonates. Alkane sulfonates are toxico-logically harmless and ecologically safe. From recent investigations, it was found that even under anaerobic conditions adapted strains of bacteria are able to desulfonate alkane sulfonates. 

Hess A, Zarda B, Hahn D, Haner A, Stax D, HOhener P, Zeyer J (1997) In situ analysis of denitrifying toluene-and m-xylene-degrading bacteria in a diesel fuel-contaminated laboratory aquifer column. Appl Environ Microbiol 63 2136-2141 Rabus R, Wilkes H, Schramm A, Harms G, Behrends A, Amann R, Widdel F (1999) Anaerobic utilization of alkylbenzenes and -alkanes from crude oil in an enrichment culture of denitrifying bacteria affiliating with the B-Subclass of Pro-teobacteria. Environ Microbiol 1 145-157... 

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