Abstract
Anaerobic degradation of alkanes in hydrocarbon-rich environments has been documented and different degradation strategies proposed, of which the most encountered one is fumarate addition mechanism, generating alkylsuccinates as specific biomarkers. However, little is known about the mechanisms of anaerobic degradation of alkanes in oil reservoirs, due to low concentrations of signature metabolites and lack of mass spectral characteristics to allow identification. In this work, we used a multidisciplinary approach combining metabolite profiling and selective gene assays to establish the biodegradation mechanism of alkanes in oil reservoirs. A total of twelve production fluids from three different oil reservoirs were collected and treated with alkali; organic acids were extracted, derivatized with ethanol to form ethyl esters and determined using GC-MS analysis. Collectively, signature metabolite alkylsuccinates of parent compounds from C1 to C8 together with their (putative) downstream metabolites were detected from these samples. Additionally, metabolites indicative of the anaerobic degradation of mono- and poly-aromatic hydrocarbons (2-benzylsuccinate, naphthoate, 5,6,7,8-tetrahydro-naphthoate) were also observed. The detection of alkylsuccinates and genes encoding for alkylsuccinate synthase shows that anaerobic degradation of alkanes via fumarate addition occurs in oil reservoirs. This work provides strong evidence on the in situ anaerobic biodegradation mechanisms of hydrocarbons by fumarate addition.
Highlights
Some cycloalkanes, iso-alkanes, aromatic hydrocarbons and polycyclic aromatic hydrocarbons have been reported being degraded anaerobically via addition to fumarate[26,29,32,33,34,35,36]
We used a multidisciplinary approach that combines metabolite profiling and functional gene assays to investigate the specific biochemical mechanism in production fluids of twelve oil reservoirs obtained from three distinct oilfields in China
Alkylsuccinates have been found in anaerobic enrichment cultures amended with either alkanes or crude oil[15,18] and in environmental samples obtained from oil-contaminated sites, but scarcely reported in samples originating from oil reservoirs
Summary
In the fumarate addition pathway, n-alkanes are initially activated by addition to the double bond of fumarate at the subterminal[21,25,26,27,28,30] or terminal[31] (with propane) carbon, producing 2-(1-methylalkyl)succinates (or 2-alkylsuccinates). This process is catalyzed by alkylsuccinate synthase. We used a multidisciplinary approach that combines metabolite profiling and functional gene (assA/masD) assays to investigate the specific biochemical mechanism in production fluids of twelve oil reservoirs obtained from three distinct oilfields in China. Our results, combined with the earlier works[17,43] illustrate that anaerobic degradation of alkanes via the fumarate addition pathway appears to be a common initial activation strategy in different oil reservoir systems
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