Changes

[[File:GeoWikiWriteOff2021-Aljezen-Figure4.jpg|framed|{{Figure number|4}}shows the mechanism to activate hydrocarbons using Oxygen-Independent Hydroxylation strategy.]]

[[File:GeoWikiWriteOff2021-Aljezen-Figure4.jpg|framed|{{Figure number|4}}shows the mechanism to activate hydrocarbons using Oxygen-Independent Hydroxylation strategy.]]

The third pathway is carboxylation. Carboxylation of non-branched alkyl chains the need of the presence of an alkyl group attached to it (e.g. benzene and naphthalene). The exact carboxylation reaction mechanism is still debated, but it is agreed that carbon dioxide is attached directly to the aromatic or aliphatic hydrocarbon via a carboxylase enzyme, requiring the presence of iron and nitrate reducing conditions.<ref name="12Gluecketal">Glueck, S. M., Gümüs, W. M. F. Fabian, and K. Faber, 2010, https://pubs.rsc.org/en/content/articlelanding/2010/cs/b807875k Biocatalytic carboxylation]: Chemical Society Reviews, v. 39, no. 1, p. 313-328.</ref>

The third pathway is carboxylation. Carboxylation of non-branched alkyl chains the need of the presence of an alkyl group attached to it (e.g. benzene and naphthalene). The exact carboxylation reaction mechanism is still debated, but it is agreed that carbon dioxide is attached directly to the aromatic or aliphatic hydrocarbon via a carboxylase enzyme, requiring the presence of iron and nitrate reducing conditions.<ref name="12Gluecketal">Glueck, S. M., Gümüs, W. M. F. Fabian, and K. Faber, 2010, [https://pubs.rsc.org/en/content/articlelanding/2010/cs/b807875k Biocatalytic carboxylation]: Chemical Society Reviews, v. 39, no. 1, p. 313-328.</ref>

==How biodegradation of hydrocarbons is identified?==

==How biodegradation of hydrocarbons is identified?==

Crude oils that has been biodegraded typically has a lower API gravity, high sulfur content (%S) and vanadium & nickel contents than corresponding non-biodegraded crude oils [13]. Gas Chromatography (GC) analysis of crude oils is commonly used to identify and assess hydrocarbon biodegradation. GC utilizes chromatography to firstly separate individual hydrocarbon components within crude oil mixtures and a detector (either flame ionization or mass spectrometer) to measure the amount of the separated hydrocarbons which are as relative intensities thus allowing comparison between different peak intensities (e.g. low peak intensity will indicate low amounts and high peak intensity will indicate high amounts).  In general, microbes consume the simplest hydrocarbons first, i.e. normal alkanes starting from C4 then C5 then C6 and then of increasing chain length. At more advanced levels of biodegradation, branched hydrocarbons (i.e. alkyled) are biodegraded. The general sequence of hydrocarbon biodegradation, by compound class is shown this Figure 5 below.  

Crude oils that has been biodegraded typically has a lower API gravity, high sulfur content (%S) and vanadium & nickel contents than corresponding non-biodegraded crude oils.<ref name="13Moldowanetal">Moldowan, M. J., K. E. Peters, and C. C. Walters, 2007, Biodegradation parameters, ''in'' Biomarker guide: Volume 2, biomarkers and isotopes in petroleum systems and earth history (Second, Vol. 2, pp. 645–705). essay, Cambridge University Press.</ref>. Gas Chromatography (GC) analysis of crude oils is commonly used to identify and assess hydrocarbon biodegradation. GC utilizes chromatography to firstly separate individual hydrocarbon components within crude oil mixtures and a detector (either flame ionization or mass spectrometer) to measure the amount of the separated hydrocarbons which are as relative intensities thus allowing comparison between different peak intensities (e.g. low peak intensity will indicate low amounts and high peak intensity will indicate high amounts).  In general, microbes consume the simplest hydrocarbons first, i.e. normal alkanes starting from C4 then C5 then C6 and then of increasing chain length. At more advanced levels of biodegradation, branched hydrocarbons (i.e. alkyled) are biodegraded. The general sequence of hydrocarbon biodegradation, by compound class is shown in [[:File:GeoWikiWriteOff2021-Aljezen-Figure5.jpg|Figure 5]].

[[File:GeoWikiWriteOff2021-Aljezen-Figure5.jpg|framed|{{Figure number|5}}shows the PM scale for biodegradation. The shaded area at the end of the bars represent the qualitative extent of the partial removal of thecompound within a class.<ref name="15Larteretal">Larter, S., H. Huang, J. Adams, B. Bennett, and L. R. Snowden, 2012, [https://www.sciencedirect.com/science/article/abs/pii/S0146638012000083 A practical biodegradation scale for use in reservoir geochemical studies of biodegraded oils]: Organic Geochemistry, v. 45, p. 66-76.</ref>]]

==Biodegradation scales==

==Biodegradation scales==

In order to assess the level of crude oil biodegradation, a scale has been developed by Peters and Moldowan (PM) [13]. The scale is called the PM scale. The PM scale ranges from 1 to 10, with 10 to be the most altered i.e. most biodegraded. This scale is efficient and can be evaluated in conjunction with observation of the corresponding gas chromatography traces of crude oil samples. This scale illustrates very clearly that microbes favor consumption of simple hydrocarbons starting from n-alkanes and proceed to heavy aromatics at higher levels of biodegradation (fig.5).

In order to assess the level of crude oil biodegradation, a scale has been developed by Peters and Moldowan.<ref name="13Moldowanetal" /> The scale is called the PM scale. The PM scale ranges from 1 to 10, with 10 to be the most altered i.e. most biodegraded. This scale is efficient and can be evaluated in conjunction with observation of the corresponding gas chromatography traces of crude oil samples. This scale illustrates very clearly that microbes favor consumption of simple hydrocarbons starting from n-alkanes and proceed to heavy aromatics at higher levels of biodegradation ([[:File:GeoWikiWriteOff2021-Aljezen-Figure5.jpg|Figure 5]]).

[[File:GeoWikiWriteOff2021-Aljezen-Figure5.jpg|thumbnail|Figure 5 shows the PM scale for biodegradation. The shaded area at the end of the bars represent the qualitative extent of the partial removal of thecompound within a class. (Steve et al, 2012) [15]]]

Other authors have proposed their own biodegradation scales, e.g. Another biodegradation scale is Wenger et al. although that scale only describes the oil based on very slight, slight, moderate, heavy and severe biodegradation levels. The scale is based on the presence or absence of specific compound classes ([[File:GeoWikiWriteOff2021-Aljezen-Figure6.jpg|Figure 6]]). The main problem with these scales is that, there are insufficient changes observable in chemical classes at very high biodegradation levels (heavy to severe) thus biodegradation between PM 5-8 on the PM scale and heavy to severe on Wenger et al. scale.

[[File:GeoWikiWriteOff2021-Aljezen-Figure6.jpg|thumbnail|Figure 6 shows the Wenger 2002 scale with the corresponding PM scale and the key compounds altered or removed. (Steve et al., 2012)<ref name="15Larteretal" />]]

 

[[File:GeoWikiWriteOff2021-Aljezen-Figure6.jpg|thumbnail|Figure 6 shows the Wenger 2002 scale with the corresponding PM scale and the key compounds altered or removed. (Steve et al., 2012) [15]]]

==What are the effects of biodegradation on crude oil economically?==

==What are the effects of biodegradation on crude oil economically?==

==Biodegradation of soil or marine hydrocarbon-contaminated environments:==

==Biodegradation of soil or marine hydrocarbon-contaminated environments:==

Despite the fact that biodegraded crude oils can be a disadvantage for crude oil economics, hydrocarbon biodegradation can be implemented as a useful process to deal with hydrocarbon spills in marine and/or soil environments. Many studies have monitored oil biodegradation in the natural environment [14], and oil spill remediation strategies include procedures to actively stimulate microbes to biodegrade spilt oil in the environment.  Most reported remediation strategies attempt to introduce known hydrocarbon-consuming bacteria to these environments. Numerous studies report the use of surfactants along with these bacteria in order to break large oil droplets to smaller micelles to increase the contact area, thus, increase the rate of biodegradation [14].  

Despite the fact that biodegraded crude oils can be a disadvantage for crude oil economics, hydrocarbon biodegradation can be implemented as a useful process to deal with hydrocarbon spills in marine and/or soil environments. Many studies have monitored oil biodegradation in the natural environment<ref name="14Lawniczaketal">Ławniczak, Ł., M. Woźniak-Karczewska, A. P. Loibner, H. J. Heipieper, and Ł. Chrzanowski, 2020, Microbial degradation of hydrocarbons—basic principles for bioremediation: A review: Molecules, v. 25, no. 4, p.856.</ref>, and oil spill remediation strategies include procedures to actively stimulate microbes to biodegrade spilt oil in the environment.  Most reported remediation strategies attempt to introduce known hydrocarbon-consuming bacteria to these environments. Numerous studies report the use of surfactants along with these bacteria in order to break large oil droplets to smaller micelles to increase the contact area, thus, increase the rate of biodegradation.<ref name="14Lawniczaketal" />

==References==

==References==