The thermodynamic landscape of methanogenic PAH degradation.
Methanogenic degradation of polycyclic aromatic hydrocarbons (PAHs) has long been considered impossible, but evidence in contaminated near surface environments and biodegrading petroleum reservoirs suggests that this is not necessarily the case. To evaluate the thermodynamic constraints on methanogenic PAH degradation we have estimated the Gibbs free energy values for naphthalene, phenanthrene, anthracene, pyrene and chrysene in the aqueous phase, and used these values to evaluate several possible routes whereby PAHs may be converted to methane. Under standard conditions (25 °C, solutes at 1 M concentrations, and gases at 1 atm), methanogenic degradation of these PAHs yields between 209 and 331 kJ mol(-1). Per mole of methane produced this is 27-35 kJ mol(-1), indicating that PAH-based methanogenesis is exergonic. We evaluated the energetics of three potential PAH degradation routes: oxidation to H(2)/CO(2), complete conversion to acetate, or incomplete oxidation to H(2) plus acetate. Depending on the in situ conditions the energetically most favourable pathway for the PAH-degrading organisms is oxidation to H(2)/CO(2) or conversion into acetate. These are not necessarily the pathways that prevail in the environment. This may be because the kinetic theory of optimal length of metabolic pathways suggests that PAH degraders may have evolved towards incomplete oxidation to acetate plus H(2) as the optimal pathway.
School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK. Jan.Dolfing@ncl.ac.uk
Jan
J
Dolfing
Aiping
A
Xu
Neil D
ND
Gray
Stephen R
SR
Larter
Ian M
IM
Head
D001711:Biotransformation; D002245:Carbon Dioxide / Q000378:metabolism; D006859:Hydrogen / Q000378:metabolism; D008697:Methane / Q000737:chemistry / Q000378:metabolism*; D011084:Polycyclic Aromatic Hydrocarbons / Q000737:chemistry / Q000378:metabolism*; D013816:Thermodynamics
D011084:Polycyclic Aromatic Hydrocarbons; D002245:Carbon Dioxide; D006859:Hydrogen; D008697:Methane
J Hazard Mater. 2008 May 1;153(1-2):123-7
17869417
Biotechnol Prog. 2005 May-Jun;21(3):682-8
15932243
Microb Ecol. 2001 Feb;41(2):83-89
12032612
Water Sci Technol. 2004;50(9):237-44
15581018
Environ Sci Technol. 2002 Jun 1;36(11):2429-35
12075800
Chemosphere. 2002 Aug;48(7):717-24
12201202
Trends Biotechnol. 2002 Jun;20(6):243-8
12007492
ISME J. 2008 Apr;2(4):442-52
18079730
Bacteriol Rev. 1977 Mar;41(1):100-80
860983
Nature. 2003 Nov 20;426(6964):344-52
14628064
Nature. 2008 Jan 10;451(7175):176-80
18075503
Biotechnol Lett. 2006 Mar;28(6):425-30
16614909
Trends Microbiol. 2006 May;14(5):213-9
16621570
J Mol Microbiol Biotechnol. 2008;15(2-3):93-120
18685265