Agnès Hirschler-Réa

Anaerobic n-alkane metabolism by a sulfate-reducing bacterium, Desulfatibacillum aliphaticivorans strain CV2803T.

ABSTRACT The alkane-degrading, sulfate-reducing bacterium Desulfatibacillum aliphaticivorans strain CV2803T, recently isolated from marine sediments, was investigated for n-alkane metabolism. The total cellular fatty acids of this strain had predominantly odd numbers of carbon atoms (C odd) when the strain was grown on a C-odd alkane (pentadecane) and even numbers of carbon atoms (C even) when it was grown on a C-even alkane (hexadecane). Detailed analyses of those fatty acids by gas chromatography/mass spectrometry allowed us to identify saturated 2-, 4-, 6-, and 8-methyl- and monounsaturated 6-methyl-branched fatty acids, with chain lengths that specifically correlated with those of the alkane. Growth of D. aliphaticivorans on perdeuterated hexadecane demonstrated that those methyl-branched fatty acids were directly derived from the substrate. In addition, cultures on pentadecane and hexadecane produced (1-methyltetradecyl)succinate and (1-methylpentadecyl)succinate, respectively. These results indicate that D. aliphaticivorans strain CV2803T oxidizes n-alkanes into fatty acids anaerobically, via the addition of fumarate at C-2. Based on our observations and on literature data, a pathway for anaerobic n-alkane metabolism by D. aliphaticivorans is proposed. This involves the transformation of the initial alkylsuccinate into a 4-methyl-branched fatty acid which, in addition to catabolic reactions, can alternatively undergo chain elongation and desaturation to form storage fatty acids.

Anaerobic oxidation of long-chain n -alkanes by the hyperthermophilic sulfate-reducing archaeon, Archaeoglobus fulgidus

The thermophilic sulfate-reducing archaeon Archaeoglobus fulgidus strain VC-16 (DSM 4304), which is known to oxidize fatty acids and n-alkenes, was shown to oxidize saturated hydrocarbons (n-alkanes in the range C10–C21) with thiosulfate or sulfate as a terminal electron acceptor. The amount of n-hexadecane degradation observed was in stoichiometric agreement with the theoretically expected amount of thiosulfate reduction. One of the pathways used by anaerobic microorganisms to activate alkanes is addition to fumarate that involves alkylsuccinate synthase as a key enzyme. A search for genes encoding homologous enzymes in A. fulgidus identified the pflD gene (locus-tag AF1449) that was previously annotated as a pyruvate formate lyase. A phylogenetic analysis revealed that this gene is of bacterial origin and was likely acquired by A. fulgidus from a bacterial donor through a horizontal gene transfer. Based on three-dimensional modeling of the corresponding protein and molecular dynamic simulations, we hypothesize an alkylsuccinate synthase activity for this gene product. The pflD gene expression was upregulated during the growth of A. fulgidus on an n-alkane (C16) compared with growth on a fatty acid. Our results suggest that anaerobic alkane degradation in A. fulgidus may involve the gene pflD in alkane activation through addition to fumarate. These findings highlight the possible importance of hydrocarbon oxidation at high temperatures by A. fulgidus in hydrothermal vents and the deep biosphere.

Anaerobic Oxidation of Fatty Acids and Alkenes by the Hyperthermophilic Sulfate-Reducing Archaeon Archaeoglobus fulgidus

ABSTRACT Archaeoglobus fulgidus oxidizes fatty acids (C4 to C18) and n-alk-1-enes (C12:1 to C21:1) in the presence of thiosulfate as a terminal electron acceptor. End products of metabolism were CO2 and sulfide. Growth on perdeuterated hexadecene yielded C15- to C17-labeled fatty acids as metabolites, thus confirming the ability of A. fulgidus to oxidize alkyl chains.

Characterization of three spiral-shaped purple nonsulfur bacteria isolated from coastal lagoon sediments, saline sulfur springs, and microbial mats: emended description of the genus Roseospira and description of Roseospira marina sp. nov., Roseospira navarrensis sp. nov., and Roseospira thiosulfatophila sp. nov.

Abstract. Three new spirilloid phototrophic purple nonsulfur bacteria were isolated in pure culture from three different environments: strain CE2105 from a brackish lagoon in the Arcachon Bay (Atlantic coast, France), strain SE3104 from a saline sulfur spring in the Pyrenees (Navarra, Spain) , and strain AT2115 a microbial mat (Tetiaroa Atoll, Society Islands). Single cells of the three strains were spiral-shaped and highly motile. Their intracellular photosynthetic membranes were of the vesicular type. Bacteriochlorophyll a and carotenoids of the normal spirilloxanthin series were present as photosynthetic pigments. Optimal growth occurred under photoheterotrophic conditions and in the presence of 0.5–4% w/v NaCl. These features are similar to those described for Roseospiramediosalina. Comparative sequence analysis of their 16S rRNA genes placed these strains within the α-subclass of Proteobacteria, in a cluster together with Roseospira mediosalina and Rhodospira trueperi. They form a closely related group of slightly to moderately halophilic spiral-shaped purple nonsulfur bacteria. However, the three new isolates exhibited some differences in their physiology and genetic characteristics. Consequently, we propose that they are members of three new species within the genus Roseospira, Roseospira marina sp. nov., Roseospira navarrensis sp. nov., and Roseospira thiosulfatophila sp. nov., with strains CE2105, SE3104, and AT2115 as the type strains, respectively. As a consequence, an emended description of the genus Roseospira is also given.

Anaerobic 1-Alkene Metabolism by the Alkane- and Alkene-Degrading Sulfate Reducer Desulfatibacillum aliphaticivorans Strain CV2803T

ABSTRACT The alkane- and alkene-degrading, marine sulfate-reducing bacterium Desulfatibacillum aliphaticivorans strain CV2803T, known to oxidize n-alkanes anaerobically by fumarate addition at C-2, was investigated for its 1-alkene metabolism. The total cellular fatty acids of this strain were predominantly C-(even number) (C-even) when it was grown on C-even 1-alkenes and predominantly C-(odd number) (C-odd) when it was grown on C-odd 1-alkenes. Detailed analyses of those fatty acids by gas chromatography-mass spectrometry after 6- to 10-week incubations allowed the identification of saturated 2- and 4-ethyl-, 2- and 4-methyl-, and monounsaturated 4-methyl-branched fatty acids with chain lengths that correlated with those of the 1-alkene. The growth of D. aliphaticivorans on (per)deuterated 1-alkenes provided direct evidence of the anaerobic transformation of these alkenes into the corresponding 1-alcohols and into linear as well as 10- and 4-methyl-branched fatty acids. Experiments performed with [13C]bicarbonate indicated that the initial activation of 1-alkene by the addition of inorganic carbon does not occur. These results demonstrate that D. aliphaticivorans metabolizes 1-alkene by the oxidation of the double bond at C-1 and by the subterminal addition of organic carbon at both ends of the molecule [C-2 and C-(ω-1)]. The detection of ethyl-branched fatty acids from unlabeled 1-alkenes further suggests that carbon addition also occurs at C-3. Alkylsuccinates were not observed as potential initial intermediates in alkene metabolism. Based on our observations, the first pathways for anaerobic 1-alkene metabolism in an anaerobic bacterium are proposed. Those pathways indicate that diverse initial reactions of 1-alkene activation can occur simultaneously in the same strain of sulfate-reducing bacterium.

The anaerobic hydrocarbon biodegrading bacteria: An overview

Abstract Hydrocarbons are widespread in our environment. The number of bacteria known to oxidize hydrocarbons in the absence of oxygen has considerably increased during the last ten years. Anaerobic bacteria have been shown capable of utilizing hydrocarbons not only in consortia but also in pure cultures. The results obtained in the framework of MATBIOPOL project on anaerobic hydrocarbon degradation by denitrifying bacteria and by enrichment cultures maintained under methanogenic conditions are exposed together with the present knowledge on hydrocarbon biodegradation.

Anaerobic biodegradation of pristane by a marine sedimentary bacterial and/or archaeal community

Pristane was incubated in anaerobic sediment slurries under conditions promoting or limiting nitrate reduction. Pristane was efficiently degraded (85% after 6 months) by the mixed microbial community when nitrate reducing processes were avoided. The biodegradation of pristane was accompanied by the abundant production of methane which indicated that methanogenic Archaea were involved in the degradation of the substrate. Comparison of the biodegradation rate of pristane with that of unsaturated isoprenoid alkenes indicated that, in Recent anoxic sediments, acyclic isoprenoid alkanes can also undergo relatively rapid biotransformations and, therefore, can no longer be considered as recalcitrant biomarkers.

Anaerobic oxidation of n-alkenes by sulphate-reducing bacteria from the genus Desulfatiferula: n-ketones as potential metabolites.

Two alkene-degrading sulphate-reducing bacteria from the genus Desulfatiferula (Desulfatiferula olefinivorans strain LM2801(T) and Desulfatiferula sp. strain BE2801) were investigated for their 1-alkene metabolism. Their total cellular fatty acids were predominantly C-even when they were grown on C-even 1-alkene (1-hexadecene), whereas a mixture of fatty acids with C-odd or C-even carbon chains predominated when cells were grown on C-odd 1-alkene (1-pentadecene). This is consistent with the fatty acid composition of other sulphate-reducing strains previously reported to grow on n-alkenes. Linear and 3-OH-fatty acids appear to be the main fatty acids produced by the two Desulfatiferula strains. The analysis of their neutral lipids led to identifying several n-alkanols and n-ketones with the same number of carbon atoms as the alkene growth substrate and with functionality located between C-1 and C-5. Growth of strains LM2801(T) and BE2801 on (per) deuterated 1-alkenes provided direct evidence of their anaerobic transformation to corresponding 1-alkanols, n-ketones and linear (3-OH-) fatty acids. These results demonstrate that Desulfatiferula strains oxidize a 1-alkene by oxidation of the double bond at C-1, but also at C-2 to C-5 (after eventual isomerization of the double bond) yielding the corresponding C-2 to C-5 n-ketones (via the corresponding n-alkanols). The formation of specific 3-OH-fatty acids by elongation of shorter chain fatty acids was also demonstrated. Based on our observations, pathways for anaerobic 1-alkene metabolism in sulphate-reducing bacteria from the genus Desulfatiferula are proposed. They indicate that n-ketones can constitute new metabolites of the biodegradation of n-alkenes in anaerobic environments.

Growth of the Obligate Anaerobe Desulfovibrio vulgaris Hildenborough under Continuous Low Oxygen Concentration Sparging: Impact of the Membrane-Bound Oxygen Reductases

Although obligate anaerobe, the sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough (DvH) exhibits high aerotolerance that involves several enzymatic systems, including two membrane-bound oxygen reductases, a bd-quinol oxidase and a cc(b/o)o3 cytochrome oxidase. Effect of constant low oxygen concentration on growth and morphology of the wild-type, single (Δbd, Δcox) and double deletion (Δcoxbd) mutant strains of the genes encoding these oxygen reductases was studied. When both wild-type and deletion mutant strains were cultured in lactate/sulfate medium under constant 0.02% O2 sparging, they were able to grow but the final biomasses and the growth yield were lower than that obtained under anaerobic conditions. At the end of the growth, lactate was not completely consumed and when conditions were then switched to anaerobic, growth resumed. Time-lapse microscopy revealed that a large majority of the cells were then able to divide (over 97%) but the time to recover a complete division event was longer for single deletion mutant Δbd than for the three other strains. Determination of the molar growth yields on lactate suggested that a part of the energy gained from lactate oxidation was derived toward cells protection/repairing against oxidative conditions rather than biosynthesis, and that this part was higher in the single deletion mutant Δbd and, to a lesser extent, Δcox strains. Our data show that when DvH encounters oxidative conditions, it is able to stop growing and to rapidly resume growing when conditions are switched to anaerobic, suggesting that it enters active dormancy sate under oxidative conditions. We propose that the pyruvate-ferredoxin oxidoreductase (PFOR) plays a central role in this phenomenon by reversibly switching from an oxidative-sensitive fully active state to an oxidative-insensitive inactive state. The oxygen reductases, and especially the bd-quinol oxidase, would have a crucial function by maintaining reducing conditions that permit PFOR to stay in its active state.

Diversity of Magnetotactic Bacteria from a French Pristine Mediterranean Area

Magnetotactic bacteria synthesize intracellular magnetite and/or greigite magnetosome crystals. They play a significant role in both iron and sulfur cycles in sedimentary aquatic environments. To get insight into the bio-geochemical contribution of MTB, more studies concerning their ecology and their distribution in diverse habitats are necessary. The MTB community of an oil-industry polluted area of the French Mediterranean coast has been previously investigated. Here, we investigate the MTB community from coastal sediments of a Mediterranean pristine area using optical and transmission electron microscopy and phylogenetic analysis based on 16S rRNA gene sequences. A particularly high diversity of MTB was observed, with cocci phylogenetically distributed across the order Magnetococcales, including a novel cluster with sequences from the Mediterranean Sea designated as “Med group”, and novel morphotypes.