Anaerobic Degradation of p-Alkylated Benzoates and Toluenes

New evidence for a hydroxylation pathway for anaerobic alkane degradation supported by analyses of functional genes and signature metabolites in oil reservoirs

Microbial degradation of recalcitrant alkanes under anaerobic conditions results in the accumulation of heavy oil fraction in oil reservoirs. Hydroxylation of alkanes is an important activation mechanism under anaerobic conditions, but the diversity and distribution of the responsible microorganisms in the subsurface environment are still unclear. The lack of functional gene polymerase chain reaction (PCR) primers and commercially available intermediate degradation chemical compounds are the major obstacles for this research. In this investigation, PCR primers for the ahyA gene (encoding alkane hydroxylase) were designed, evaluated, and improved based on the nucleotide sequences available. Using microbial genomic DNA extracted from oil-contaminated soil and production water samples of oil reservoirs, ahyA gene nucleotide sequences were amplified and retrieved successfully from production water sample Z3-25 of Shengli oilfield. Additionally, the signature biomarker of 2-acetylalkanoic acid was detected in both Shengli and Jiangsu oilfields. These results demonstrate that anaerobic hydroxylation is an active mechanism used by microorganisms to degrade alkanes in oxygen-depleted oil reservoirs. This finding expands the current knowledge of biochemical reactions about alkane degradation in subsurface ecosystems. In addition, the PCR primers designed and tested in this study serve as an effective molecular tool for detecting the microorganisms responsible for anaerobic hydroxylation of alkanes in this and other ecosystems.

Integrating Dispersants in Oil Spill Response in Arctic and Other Icy Environments

Future oil exploration and marine navigation may well extend into the Arctic Ocean, and government agencies and responders need to plan for accidental oil spills. We argue that dispersants should play an important role in these plans, since they have substantial logistical benefits, work effectively under Arctic conditions, and stimulate the rapid biodegradation of spilled oil. They also minimize the risk of surface slicks to birds and mammals, the stranding of oil on fragile shorelines and minimize the need for large work crews to be exposed to Arctic conditions.

Thermophilic archaea activate butane via alkyl-coenzyme M formation

The anaerobic formation and oxidation of methane involve unique enzymatic mechanisms and cofactors, all of which are believed to be specific for C₁-compounds. Here we show that an anaerobic thermophilic enrichment culture composed of dense consortia of archaea and bacteria apparently uses partly similar pathways to oxidize the C₄ hydrocarbon butane. The archaea, proposed genus 'Candidatus Syntrophoarchaeum', show the characteristic autofluorescence of methanogens, and contain highly expressed genes encoding enzymes similar to methyl-coenzyme M reductase. We detect butyl-coenzyme M, indicating archaeal butane activation analogous to the first step in anaerobic methane oxidation. In addition, Ca. Syntrophoarchaeum expresses the genes encoding β-oxidation enzymes, carbon monoxide dehydrogenase and reversible C₁ methanogenesis enzymes. This allows for the complete oxidation of butane. Reducing equivalents are seemingly channelled to HotSeep-1, a thermophilic sulfate-reducing partner bacterium known from the anaerobic oxidation of methane. Genes encoding 16S rRNA and methyl-coenzyme M reductase similar to those identifying Ca. Syntrophoarchaeum were repeatedly retrieved from marine subsurface sediments, suggesting that the presented activation mechanism is naturally widespread in the anaerobic oxidation of short-chain hydrocarbons.

Anaerobic Microbial Degradation of Hydrocarbons: From Enzymatic Reactions to the Environment

Hydrocarbons are abundant in anoxic environments and pose biochemical challenges to their anaerobic degradation by microorganisms. Within the framework of the Priority Program 1319, investigations funded by the Deutsche Forschungsgemeinschaft on the anaerobic microbial degradation of hydrocarbons ranged from isolation and enrichment of hitherto unknown hydrocarbon-degrading anaerobic microorganisms, discovery of novel reactions, detailed studies of enzyme mechanisms and structures to process-oriented in situ studies. Selected highlights from this program are collected in this synopsis, with more detailed information provided by theme-focused reviews of the special topic issue on 'Anaerobic biodegradation of hydrocarbons' [this issue, pp. 1-244]. The interdisciplinary character of the program, involving microbiologists, biochemists, organic chemists and environmental scientists, is best exemplified by the studies on alkyl-/arylalkylsuccinate synthases. Here, research topics ranged from in-depth mechanistic studies of archetypical toluene-activating benzylsuccinate synthase, substrate-specific phylogenetic clustering of alkyl-/arylalkylsuccinate synthases (toluene plus xylenes, p-cymene, p-cresol, 2-methylnaphthalene, n-alkanes), stereochemical and co-metabolic insights into n-alkane-activating (methylalkyl)succinate synthases to the discovery of bacterial groups previously unknown to possess alkyl-/arylalkylsuccinate synthases by means of functional gene markers and in situ field studies enabled by state-of-the-art stable isotope probing and fractionation approaches. Other topics are Mo-cofactor-dependent dehydrogenases performing O2-independent hydroxylation of hydrocarbons and alkyl side chains (ethylbenzene, p-cymene, cholesterol, n-hexadecane), degradation of p-alkylated benzoates and toluenes, glycyl radical-bearing 4-hydroxyphenylacetate decarboxylase, novel types of carboxylation reactions (for acetophenone, acetone, and potentially also benzene and naphthalene), W-cofactor-containing enzymes for reductive dearomatization of benzoyl-CoA (class II benzoyl-CoA reductase) in obligate anaerobes and addition of water to acetylene, fermentative formation of cyclohexanecarboxylate from benzoate, and methanogenic degradation of hydrocarbons.