Changes

The normal alkane (C₁-C₈ ''n''-alkane) in [[:File:GeoWikiWriteOff2021-Aljezen-Figure2.jpg|Figure 2]] will react with oxygen with the help of monooxygenase enzyme produced by living organisms (e.g. bacteria), and converts the normal alkane to an alcohol and oxidize iron from Fe²⁺ to Fe³⁺. Under aerobic conditions, simple aromatics such as benzene, xylene and toluene can be degraded.  Typically, this requires 3mg/L of dissolved oxygen to degrade 1 mg/L of these aromatics (i.e. 3:1 ratio).  If the dissolved oxygen content is lower than 3:1 then the biodegradation rate is slower.

The normal alkane (C₁-C₈ ''n''-alkane) in [[:File:GeoWikiWriteOff2021-Aljezen-Figure2.jpg|Figure 2]] will react with oxygen with the help of monooxygenase enzyme produced by living organisms (e.g. bacteria), and converts the normal alkane to an alcohol and oxidize iron from Fe²⁺ to Fe³⁺. Under aerobic conditions, simple aromatics such as benzene, xylene and toluene can be degraded.  Typically, this requires 3mg/L of dissolved oxygen to degrade 1 mg/L of these aromatics (i.e. 3:1 ratio).  If the dissolved oxygen content is lower than 3:1 then the biodegradation rate is slower.

Compared to aerobic degradation, anaerobic degradation is considered to proceed much slower This is because anaerobic pathways require more energy, i.e. they are energetically unfavorable. There are three different pathways that anaerobic microbes can utilize to biodegrade hydrocarbons. All three pathways require inserting an oxidizing group into the molecule, which makes it more active and thus easier to transform to microbial-consumable products such as fatty acids. The first pathway is called fumarate addition and this pathway is used by bacteria to activate C3 to C20 alkanes and alkyl-substituted aromatics such as xylene and toluene. The mechanism proceeds via addition to the double bond by terminal or pre-terminal alkyl group from the alkanes or alkyl-substituted aromatics then followed by the removal of a carbon dioxide molecule ([[File:GeoWikiWriteOff2021-Aljezen-Figure3.jpg|Figure 3).<ref name="7Fuchsetal">Fuchs, G., M/ Boll, and J. Heider, 2011, [https://www.nature.com/articles/nrmicro2652 Microbial degradation of aromatic compounds—from one strategy to four]: Nature Reviews Microbiology, v. 9, p.803–816.</ref><ref name="8Bianetal">Bian, X. Y., S, M. Mbadinga, Y. F. Liu, S. Z. Yang, J. F. Liu, R. Q. Ye, J. D. Gu, and B. Z. Mu, 2015, [https://www.nature.com/articles/srep09801 Insights into the anaerobic biodegradation pathway of n-alkanes in oil reservoirs by detection of signature metabolites]: Scientific Reports, v. 5. article no. 9801.</ref>  

Compared to aerobic degradation, anaerobic degradation is considered to proceed much slower This is because anaerobic pathways require more energy, i.e. they are energetically unfavorable. There are three different pathways that anaerobic microbes can utilize to biodegrade hydrocarbons. All three pathways require inserting an oxidizing group into the molecule, which makes it more active and thus easier to transform to microbial-consumable products such as fatty acids. The first pathway is called fumarate addition and this pathway is used by bacteria to activate C3 to C20 alkanes and alkyl-substituted aromatics such as xylene and toluene. The mechanism proceeds via addition to the double bond by terminal or pre-terminal alkyl group from the alkanes or alkyl-substituted aromatics then followed by the removal of a carbon dioxide molecule ([[:File:GeoWikiWriteOff2021-Aljezen-Figure3.jpg|Figure 3]]).<ref name="7Fuchsetal">Fuchs, G., M/ Boll, and J. Heider, 2011, [https://www.nature.com/articles/nrmicro2652 Microbial degradation of aromatic compounds—from one strategy to four]: Nature Reviews Microbiology, v. 9, p.803–816.</ref><ref name="8Bianetal">Bian, X. Y., S, M. Mbadinga, Y. F. Liu, S. Z. Yang, J. F. Liu, R. Q. Ye, J. D. Gu, and B. Z. Mu, 2015, [https://www.nature.com/articles/srep09801 Insights into the anaerobic biodegradation pathway of n-alkanes in oil reservoirs by detection of signature metabolites]: Scientific Reports, v. 5. article no. 9801.</ref>  

[[File:GeoWikiWriteOff2021-Aljezen-Figure3.jpg|thumbnail| Figure 3 shows the mechanism to activate hydrocarbons using Fumarate addition strategy.]]

[[File:GeoWikiWriteOff2021-Aljezen-Figure3.jpg|framed|{{figure number|3}}shows the mechanism to activate hydrocarbons using Fumarate addition strategy.]]

Oxygen-independent hydroxylation is another pathway that selected microbes can utilize to activate hydrocarbons in the absence of oxygen. This is illustrated below using ethylbenzene as an example (Figure 4). The process will start with the attack of a hydroxyl group (-OH) to the closest carbon atom to the ring with the help of ethylbenzene dehydrogenase enzyme, forming (S)-1-phenylethanol. Then a molecule of carbon dioxide will react with (S)-1-phenylethanol with the help of (S)-1-phenylethanol dehydrogenase enzyme to form benzoylacetate. The last two steps of the process involve reaction with a thiol group that contains co-enzyme A (CoA). The first thiol attack will convert benzoylacetate to benzoylacetyl-CoA. The second thiol attack will cleave the benzoylacetyl-CoA to benzoyl-CoA and acetyl-CoA (fig.4) [7][9][10][11].

Oxygen-independent hydroxylation is another pathway that selected microbes can utilize to activate hydrocarbons in the absence of oxygen. This is illustrated below using ethylbenzene as an example (Figure 4). The process will start with the attack of a hydroxyl group (-OH) to the closest carbon atom to the ring with the help of ethylbenzene dehydrogenase enzyme, forming (S)-1-phenylethanol. Then a molecule of carbon dioxide will react with (S)-1-phenylethanol with the help of (S)-1-phenylethanol dehydrogenase enzyme to form benzoylacetate. The last two steps of the process involve reaction with a thiol group that contains co-enzyme A (CoA). The first thiol attack will convert benzoylacetate to benzoylacetyl-CoA. The second thiol attack will cleave the benzoylacetyl-CoA to benzoyl-CoA and acetyl-CoA (fig.4) [7][9][10][11].