Lipoxygenase-induced autoxidative degradation of terrestrial particulate organic matter in estuaries: A widespread process enhanced at high and low latitude

International audienceThere exists a substantial amount of research on abiotic (e.g. photochemical) degradation pertaining to organic matter (OM) in the marine realm. While recent research has shown its importance in the degradation of terrestrial particulate OM (TPOM), the mechanisms involved in the induction of autoxidation in estuaries remain unclear. In this study, we propose for the first time the involvement of lipoxygenase (LOX) activity in the induction of autoxidation in mixed waters. The observation of unusual profiles of palmitoleic acid oxidation products and the presence of jasmonic acid in suspended particulate matter (SPM) collected close to the Rhône River, as well as in samples from the Mackenzie and Amazon rivers, is attributed to strong LOX activity. We show the role played by salinity in the induction of this LOX activity and provide an explanation for the differences in estuarine autoxidation level. At high latitude, lower temperatures and irradiance favor photooxidative damage to higher plant debris and, consequently, hydroperoxide production. High hydroperoxide content strongly contributes to LOX activation in mixed waters. The high resulting LOX activity enhances alkoxyl radical production and thus autoxidation. On the contrary, at low latitude, photooxidative effects are limited, and riverine autoxidation is favored. The higher hydroperoxide content of TPOM may, as a consequence, thereby also contribute to a high level of LOX activity and autoxidation in estuaries. In temperate zones, land and riverine photooxidative and autoxidative damage is limited, unlike estuaries where we observed significant LOX-induced and autox-idative damage.

Anaerobic bacterial degradation of pristenes and phytenes in marine sediments does not lead to pristane and phytane during early diagenesis

International audienceAnaerobic degradation of mixtures of isomeric pristenes and phytenes by sedimentary bacterial communities was investigated. These isoprenoid alkenes were quickly metabolized without formation of the corresponding alkanes. Identification of several bacterial metabolites allowed us to confirm the key role played by hydration in the metabolism of alkenes. Despite the increasing production of 5alpha-stanols during incubation, attesting to the involvement or biohydrogenation, this process did not act significantly on the double bond of pristenes and phytenes. Their resistance is attributed to the lack of binding polar group(s) to anchor the substrate to the enzyme and thus allow the double bond to reach the reductive catalytic site. It therefore seems that microbial hydrogenation of pristenes and phytenes does not account for the presence of pristane and phytane in recent sediments during early diagenesis

Autoxidative and Photooxidative Reactivity of Highly Branched Isoprenoid (HBI) Alkenes

Autoxidation of several mono-, di-, tri- and tetra-unsaturated highly branched isoprenoid (HBI) alkenes was induced in organic solvents using a radical initiator and enhancer, and their degradation rates were compared to those of classical phytoplanktonic lipids (mono-unsaturated fatty acids, sterols and chlorophyll phytyl side-chain). Autoxidation of two HBI trienes was also investigated in senescent and highly photodegraded diatom cells, collected in the Antarctic, using Fe2+ ions as radical inducers. Autoxidation rates of HBI alkenes were found to increase with the number of tri-substituted double bonds, as expected. Further, HBI trienes possessing one bis-allylic position (where hydrogen abstraction is highly favoured) were found to be particularly reactive towards autoxidation and degraded at similar rates compared to polyunsaturated fatty acids in diatom cells. By comparison of the autoxidation products of the most reactive tri-unsaturated HBI with the corresponding photooxidation products, some specific tracers of these two types of abiotic degradation processes were identified. The lack of reactivity of the mono-unsaturated HBI IP25 and a structurally similar di-unsaturated HBI towards autoxidative degradation supports the good preservation of these biomarkers in marine sediments

EIMS Fragmentation and MRM quantification of ferulic and p-coumaric acid TMS derivatives in deposited atmospheric particles.

International audienceRationale: Ferulic and p-coumaric acids are important biological and structural components of plant cell walls and possess antioxidant and antimicrobial properties. These phenolic acids are widespread in environmental samples. However, when they are present at very low concentrations or in very complex lipid extracts, their identification and quantification can be challenging.Methods: The electron ionization mass spectrometry (EI-MS) fragmentation pathways of ferulic and p-coumaric acid trimethylsilyl (TMS) derivatives were investigated. These pathways were deduced by (i) low-energy collision-induced dissociation (CID) gas chromatography (GC)/EI-MS/MS, (ii) accurate mass measurement, and (iii) 13C labelling. These compounds were then characterized and quantified in multiple reaction monitoring (MRM) mode in total lipid extracts of deposited atmospheric particles using highly specific transitions based on the main fragmentation pathways elucidated.Results: Low-energy CID-MS/MS analyses, accurate mass measurement and 13C labelling enabled us to elucidate EI-MS fragmentations of ferulic and p-coumaric acid TMS derivatives. Some specific fragmentations proved useful for subsequent characterization and quantification of these compounds. As an application of some of the described fragmentations, trace amounts of these phenolic acids were characterized and quantified in MRM mode in wet- and dry-deposited atmospheric particles containing low proportions of organic matter.Conclusions: EI-MS fragmentations of ferulic and p-coumaric acid TMS derivatives exhibit specific fragment ions that can be very useful for the quantification of trace amounts of both phenolic acids in environmental samples

Origin and degradation of lipids in aeolian particles from a coastal area of the north-western Mediterranean Sea

The lipid content of eight samples of aeolian particles collected on the Frioul Islands (north-western Mediterranean Sea) during the year 2010 was investigated. Lipid analyses indicated a significant contribution of terrestrial higher plant debris to some of the samples, while meat cooking residues were also found. Specific degradation products of sterols and monounsaturated fatty acids were employed to assess the importance of biotic and abiotic degradation processes on land and during aeolian transport. Degradation of higher plant lipids on land appears to involve Type II (i.e., singlet oxygen-mediated) photooxidation during senescence and bacterial degradation processes in soils, while during aeolian transport this terrestrial material mainly undergoes autoxidation and/or Type I (i.e., radical-mediated) photooxidation and is relatively well preserved towards bacterial degradation. In contrast, bacteria play a significant role in the degradation of meat cooking residues in these particles

Effect of UV and Visible Radiation on Optical Properties of Chromophoric Dissolved Organic Matter Released by Emiliania huxleyi

International audiencePhotodegradation is a natural process that strongly a↵ects the chromophoric fraction of dissolved organic matter (DOM), especially in surface water of the oceans. In the euphotic zone, the concentration and quality of DOM are mostly dependent on primary production by phytoplankton. The e↵ect of photodegradation on algal DOM has not been investigated as much as on terrestrial DOM. In this study, we explored the e↵ect of di↵erent spectral regions (i.e., full sun spectrum, visible light, 295-800 nm, 305-800 nm, and 320-800 nm) on algal exudates by Emiliania huxleyi, a ubiquitous coccolithophore. The optical properties (absorption and fluorescence) of algal DOM were investigated before and after irradiation with the di↵erent spectral regions. The absorption and fluorescence spectra were compared before and after irradiation. The results showed an increase in the e↵ect of photobleaching with increasing irradiation energy for all of the absorbance indices. Similarly, the protein-like fluorescence decreased at increasing irradiation energy. The humic-like fluorescence, which was the most a↵ected, did not show a linear trend between photobleaching and irradiation energy, which suggested that irradiation mainly determined a change in these molecules' quantum yield

Abiotic degradation of highly branched isoprenoid alkenes and other lipids in the water column off East Antarctica

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Transfer of singlet oxygen from senescent irradiated phytoplankton cells to attached heterotrophic bacteria: Effect of silica and carbonaceous matrices

The effect of silica and carbonaceous matrices (charged mineral surfaces) in phytoplankton cells on the transfer of singlet oxygen from irradiated phytodetritus to their attached bacteria was investigated under controlled laboratory conditions. Our results indicate that a silica matrix (i.e. as in diatom frustules) inhibits the transfer of singlet oxygen and limits the induced photodegradation of cis-vaccenic acid (a fatty acid generally considered as specific to bacteria). In contrast, a carbonaceous matrix (i.e. as in coccoliths) does not seem to inhibit the transfer probably due to the release of coccoliths upon cell death. As a consequence, bacteria associated with phytodetritus from diatoms should be in a healthy state and biodegradation of organic matter associated with these particles should be favoured. These results should contribute to a better understanding of photosensitized degradation processes and to a better estimation of the balance between degradation and preservation of organic material during sedimentation in seawater

Dynamic of bacterial communities attached to lightened phytodetritus

International audienceThe effects of singlet oxygen ((1)O2) transfer to bacteria attached on phytodetritus were investigated under laboratory-controlled conditions. For this purpose, a nonaxenic culture of Emiliania huxleyi in late stationary phase was studied for bacterial viability. Our results indicated that only 9 ± 3 % of attached bacteria were alive compared to 46 ± 23 % for free bacteria in the E. huxleyi culture. Apparently, under conditions of low irradiance (36 W m(-2)), during the culture, the cumulative dose received (22,000 kJ m(-2)) was sufficiently important to induce an efficient (1)O2 transfer to attached bacteria during the senescence of E. huxleyi cells. At this stage, attached bacteria appeared to be dominated by pigmented bacteria (Maribacter, Roseobacter, Roseovarius), which should resist to (1)O2 stress probably due to their high contents of carotenoids. After subsequent irradiation of the culture until fully photodegradation of chlorophyll, DGGE analyses showed that the diversity of bacteria attached to E. huxleyi cells is modified by light. Photooxidative alterations of bacteria were confirmed by the increasing amounts of cis-vaccenic photoproducts (bacterial marker) per bacteria observed during irradiation time. Interestingly, preliminary chemotaxis experiments showed that Shewanella oneidensis considered here as a model of motile bacteria was attracted by phytodetritus producing or not (1)O2. This lack of repulsive effects could explain the high mortality rate of bacteria measured on E. huxleyi cells

Caractérisation de la nature des particules en suspension transportées lors des étiages du Rhône (France) - Projet CANADER

International audienceStudies conducted on the Rhône River (France) highlights that, in the current context of climate change, the frequency of lowflow situations are likely to increase throughout its basin. For these extreme periods, it is expected that the proportion of organic matter (OM) contained in suspended particles will increase. However, the fixation of several contaminants is strongly linked to OM, where its form favors their complexation/adsorption. This is confirmed by the results of the CANADER project conducted on the Rhône River and its main tributaries (Saône, Ardèche and Durance Rivers) where the concentrations in suspended solids of several contaminants (anthropogenic metals, mercury and radionuclides) tend to increase during low water periods. For several anthropogenic metals (Cr, Ni, Zn), concentrations sometimes exceed the probable effect concentration (PEC). The decrease on natural metals (from the earth's crust) coupled with the increase in methyl-mercury (suggesting bioaccumulation of concentrations in phytoplankton) shows that a significant proportion of the OM that transits at low flow is autochthonous. The first results of sterol analysis suggest that OM origins differ depending the Rhône River and its tributaries, and further work must be done to characterize the sources of these particles.Les études menées sur le Rhône soulignent que dans le contexte actuel de changement climatique les situations de faible débit et d’étiage seront probablement de plus en plus fréquentes. Pour ces périodes extrêmes, il est attendu que la proportion de matière organique (MO) contenue dans les particules en suspension augmente. Or, la fixation de certains contaminants est fortement liée à la MO car sa forme favorise la complexation/adsorption de ceux-ci. C’est ce que confirment les résultats du projet CANADER conduit sur le Rhône et ses principaux affluents (Saône, Ardèche et Durance) où les concentrations dans les matières en suspension de certains contaminants (métaux anthropiques, mercure et radionucléides) ont tendance à augmenter en période d’étiage. Pour certains métaux anthropiques (Cr, Ni, Zn), les concentrations dépassent parfois le seuil d’effet probable (PEC). Par ailleurs, la diminution des métaux naturels (issus de la croûte terrestre – Co Cs Rb et V) couplée à l’augmentation du méthylmercure (suggérant une bioaccumulation des concentrations dans le phytoplancton) montre qu’une proportion non négligeable de la MO qui transite à l’étiage est autochtone. Les premiers résultats de mesure des stérols suggèrent que l’origine de cette MO diffère selon le Rhône et ses affluents, et des travaux additionnels doivent être réalisés pour caractériser les sources de ces particules