Search

A number of bacterial strains previously isolated from living cells of the marine haptophyte Emiliania huxleyi and identified from their 16S r-DNA profiles as Dietzia maris sp. S1, Stappia sp. AG2, Nocardioides sp. S3, Sphingomonas sp. AG6, Oceanicaulis alexandrii sp. AG4, O. alexandrii sp. AG7 and Micrococcus sp. AG10 were cultured for examination of their fatty acid (FA) compositions. The total fatty acid (TFA) contents of the different strains ranged from 0.3% to 4% dry wt., which suggests that the use of a single value when converting FA content to estimates of bacterial biomass may have a significant error. A variety of FA distributions was observed, illustrating the diverse patterns in bacteria from marine ecosystems. Some species showed a dominance of saturated and monounsaturated straight chain FAs, while in others iso - and anteiso -branched FAs were the major constituents. The specificity and possible modes of biosynthesis of the unusual constituents, 11-methyloctadec-12-enoic and 10,13-epoxy-11-methyloctadeca-10,12-enoic acids, found in some species, are discussed. Deuterium labelling allowed us to demonstrate that these two compounds arise from the methionine-mediated methylation of cis -vaccenic acid, which supports the previously proposed role of this monounsaturated FA as the precursor of bacterial furan FAs. The strain Sphingomonas sp. AG6 contains C 14 –C 16 2-hydroxy acids (strongly dominated by 2-hydroxytetradecanoic acid), which are components of sphingolipid structures linked to sphingosine by amide bonds. 3-Hydroxyacids ranging from C 11 to C 18 were detected in Stappia sp. AG2, O. alexandrii sp. AG4, O. alexandrii sp. AG7 and Micrococcus sp. AG10. These are classical components of cell wall lipopolysaccharides. C 32 –C 37 mycolic acids exhibiting an unusually high degree of unsaturation and dominance of odd numbered main chains (>78% of the FAs had a C 21 main chain) were detected in the strain D. maris sp. S1. The data provide further examples of the close association of different types of bacteria with geochemically important algal species.

Sterol and alkenone compositions in suspended particle and surface sediment samples collected in the Northwestern Mediterranean Sea during the MEDFLUX program were used to evaluate the relative importance of biotic and abiotic degradation processes on marine organic matter. Alkenone concentrations decreased much more rapidly (~ 500 fold) between 5 and 800 m than Δ 5 -sterols (~ 100-fold) or POC (~ 100-fold). The diverse functional groups attached to the stable tetracyclic carbon skeleton of Δ 5 -sterols appeared to be useful for estimating the relative effects of biotic vs. abiotic (photooxidation and autoxidation) degradation. Products of abiotic degradation predominated over products of biotic degradation in suspended particles in the NW Mediterranean. For alkenones, the U 37 K′ index increased from 0.43 to 0.55 with increasing water depth, and a good correlation between variations of U 37 K′ and concentrations of specific Δ 5 -sterol autoxidation products points to selective autoxidation of alkenones in suspended particles. Stereomutated alkenones (with cis double bonds) were detected in the surface sediment, allowing us to estimate that stereomutation resulted in a + 0.05 increase in U 37 K′ . Therefore, abiotic degradation may be another factor effect on alkenone-derived paleothermometry.

Electron ionization mass spectra of several monounsaturated methyl-branched fatty acid methyl and trimethylsilyl esters were examined. These spectra exhibited some intensive fragment ions, whose formation could be explained after double-bond migration to methylidene position. This preferential migration (substantiated by deuterium labeling) acts significantly in the case of monounsaturated fatty acid methyl and trimethylsilyl esters possessing a methyl branch localized between the penultimate and the C(4) positions (relative to the ester group), whatever the position of the double-bond. Allylic cleavage and gamma-hydrogen rearrangement of the ionized methylidene group thus formed afforded very interesting fragment ions, which could be particularly useful to determine branching positions of monounsaturated methyl-branched fatty acid methyl and trimethylsilyl esters without additional treatment.