Microbial Degradation of Alkanes

Due to lack of functional groups as well as very low water solubility, aliphatic hydrocarbons exhibit both, low chemical reactivity and bioavailability for microorganisms. However, some microorganisms possess the metabolic capacity to use these compounds as carbon and energy sources for their growth (Berthe-Corti and Fetzner 2002).

A number of microbes including bacteria, fungi and yeasts have been reported to degrade alkanes using them as the source of carbon and energy (van Beilen et al. 2003; Wentzel et al. 2007). Bacteria with alkane degradation ability have also versatile metabolism to use other compounds in addition to alkanes as source of carbon (Margesin et al. 2003; Haryama et al. 2004). Use of bacteria in the degradation of alkane compounds has been extensively studied by Haryama et al. (2004). Many microbes have been reported for the degradation of aliphatic compounds, such as Arthrobacter sp., Acinetobacter sp., Candida sp., Pseudomonas sp., Rhodococcus sp., Streptomyces sp., Bacillus sp., Aspergillus japonicus, Arthrobacter sp., Acinetobacter sp., etc. In addition, some bacterial species are reported as highly specialized in degrading hydrocarbons and hence called hydrocarbonoclastic bacteria. They play a key role in the removal of hydrocarbons from the polluted environments (Head et al. 2006; Yakimov et al. 2007). Schneiker et al. (2006) found a marine bacterium (Alcanivorax borkumensis) capable of assimilating both linear or branched alkanes, but unable to metabolize aromatic hydrocarbons. Alcanivorax dieselolei, a g-proteobacterium, is also a member of the hydrocarbonoclastic bacteria and cannot assimilate sugars or amino acids as sources of energy and carbon. But it can utilize some organic acids and alkanes. Notably, the spectrum of alkanes utilized by A. dieselolei (C5-C36) (Liu and Shao 2005) is substantially broader than those of most other previously described alkane degraders (van Beilen and Funhoff 2007). Other alkane degrading bacterial genera are Thalassolitus (Yakimov et al. 2004), Ole-iphilus (Golyshin et al. 2002), Bacillus, Geobacillus (Marchant et al. 2006), Thermus (Meintanis et al. 2006) and Oleispira (Yakimov et al. 2003).

Acinetobacter sp. was found to be capable of utilizing n-alkanes of chain length C10-C40 as a sole source of carbon (Throne-Holst et al. 2007). Other bacterial genera, namely, Gordonia, Brevibacterium, Aeromicrobium, Dietzia, Burkholderia and Mycobacterium isolated from petroleum contaminated soil were proven to be potential degraders of hydrocarbons (Chaillan et al. 2004). Hexadecane degradation was observed by the bacteria, such as Pseudomonas putida, Rhodococcus erythroplotis and Bacillus thermoleovorans (Abdel-Megeed et al. 2010) and two bacterial strains; Flavobacterium sp. ATCC39723 and Arthrobacter sp. (Steiert et al. 1987). Hexadecane (HXD) is present in the aliphatic fraction of crude oil and is one of the major components of diesel (Chenier et al. 2003). Volke-Sepulveda et al. (2003) demonstrated that HXD biodegradation by Aspergillus niger was considerably higher in SSF (Solid state fermentation) than in submerged fermentation. Complete HXD conversion was achieved at a C/N ratio of 29 under SSF conditions (Stroud et al. 2008). Desulfatibacillum alkenivorans AK-01 is a mesophilic sulfate-reducer isolated from estuarine sediment which utilizes C13-C18 alkanes, 1-alkenes (C15 and C16) and 1-alkanols (C15 and C16) as growth substrates.

Thermophilic alkane degrading bacterium, Goebacillus thermoleovorans (previously Bacillus thermoleovorans) B23 was reported from a deep-subsurface oil reservoir in Japan (Kato et al. 2001). This strain effectively degraded alkanes at 70°C with the carbon chain longer than dodecane (C12). Since tetradecanoate and hexadecanoate or pentadecanoate and heptadecanoate were accumulated as degradation intermediates of hexadecane or heptadecane degradation, respectively, it indicated that the strain B23 degraded alkanes by a terminal oxidation pathway, followed by b-oxidation pathway. Recently, another long chain alkane degrading Geobacillus thermodenitrificans NG80-2 was also isolated from a deep sub -surface oil reservoir and its complete genome sequence was determined (Feng et al. 2007).

Some organisms adapted to cold environment are capable of degrading high molecular weight petroleum hydrocarbons. Whyte et al. (1998) reported that Rhodococcus sp. strain Q15 was able to degrade alkanes up to n-C21 as well as some branched alkanes in diesel, and could also grow on dotriacontane (n-C32). Rhodococcus strains capable of growing on eicosane (n-C20) have been reported by Bej et al. (2000). Studies on petroleum biodegradation in soils from cold regions have reported that lower-molecular weight n-alkanes and unsubstituted aromatic hydrocarbons are biodegraded preferentially over the relatively higher-molecular weight n-alkane compounds, isoalkanes, alkylated aromatic hydrocarbons, isopre-noids and the branched and cyclic hydrocarbons (Sanscartier et al. 2009).

Besides, many yeasts and fungi, are also known to thrive on alkanes (van Beilen et al. 2003). Among fungal genera, Amorphoteca, Neosartorya, Talaromyces and Graphium and yeast genera, Candida, Yarrowia and Pichia, isolated from oil-contaminated soil were found potential degraders of petroleum of petroleum hydrocarbons (Chaillan et al. 2004). Singh (2006) has reported a group of fungi, namely Aspergillus, Cephalosporium and Pencillium to be high degraders of crude oil hydrocarbons. Among yeast species, Candida lipolytica, Rhodotrula mucilaginosa, Geotrichum sp. and Trichosporam mucoides isolated from contaminated water were capable to degrade petroleum compounds effectively (Boguslawska-Was and Dabrowski 2001). New genera containing alkane degraders are constantly being identified, leading to a better understanding of ecosystems.

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