Influence of various redox conditions on the degradation of microalgal triacylglycerols and fatty acids in marine sediments

Sediment cakes, supplemented with microalgal cells (Nannochloropsis salina), were incubated for 35 days under permanently oxic, oscillating (5d:5d changeover oxic/anoxic) and strictly anoxic conditions of oxygenation in diffusively ‘‘open’’ sedimentary systems. Total lipids (TLip) and triacylglycerols (TG) concentrations were monitored by thin layer chromatography-flame ionisation detection, whereas the concentrations of the main extractable (free+ester-bound) individual fatty acids (C16:0, C16:1, C18:1) were followed using gas chromatography-mass spectrometry. Under the three conditions of oxygenation, TOC, TLip and TG showed a sharp decrease in concentration during the early days of incubation and seemed to stabilise thereafter, defining an apparent non degradable fraction (GNR). The GNR content was systematically higher in the anoxic incubation than under the oxic and oscillating conditions. The ratio of the main hydrolysis products of TG versus TG [(Free fatty acids+Monoacylglycerols+1,2-Diacylglycerols)/TG], used as an indicator of the hydrolysis of TG, showed that the presence of oxygen in the sediments (oxic and oscillating conditions) stimulates the hydrolysis of TG and the subsequent degradation of their metabolites. Unlike TOC, TLip and TG, individual fatty acids (FA) showed a continuous concentration decrease until the end of the experiment, which was fitted with a simple first order model [G(t)=G0e_kt] to yield apparent degradation rate constants. The values observed under oscillating conditions (kFA=0.019 +/- 0.001 d_1) were intermediate to those observed during oxic (kFA=0.029 +/- 0.003 d_1) and anoxic (kFA=0.011 +/- 0.001 d_1) incubations, and no significant difference between individual FA could be observed. The production of saturated and monounsaturated C16 (and to a lesser extent C18) alkanols under oscillating and anoxic redox conditions suggested that (a part of) the dominant FA were reduced to the corresponding alcohols under anoxic conditions, following their release from acylglycerols

Fatty acids associated with the frustules of diatoms and their fate during degradation -- A case study in Thalassiosira weissflogii

International audienceDiatoms are major actors in the export of organic carbon out of the euphotic zone. Yet, the processes linking biogenic silica and carbon sedimentation fluxes to deep oceanic layers remain unclear. Analysing organic fractions in biominerals is challenging because efficient cleaning often led to structural alteration of organic molecules. Hence, although lipids are widely used as biogeochemical markers in ocean flux study, few studies have dealt with the lipids that are associated with frustules. In the present study, a protocol was set up to extract and quantify the fatty acids associated to the frustule of the diatom species Thalassiosira weissflogii. The protocol involves solvent extraction of diatom external lipids, followed by clean frustule dissolution by 4%NaOH during 1h at 95°C and subsequent solvent re-extraction of frustule-associated lipids. Results confirmed that this protocol was efficient first, to isolate the frustule from the rest of the cellular organic carbon and second to extract and quantify fatty acids (FA) associated to frustules of this species. FA composition of the frustules was significantly different from that of the whole cells consisting primarily of 14:0, 16:0 and 18:0 FA, as well as a smaller portion of 16:1 and 18:1 unsaturated FA. Frustule-associated FA constituted 7% of the total FA and 1.8% of the total POC. The 30 days T.weissflogii degradation/dissolution experiment suggested that frustule FA14:0 and 16:0 were mainly associated with the bSiO2 phase dissolving slowly as no degradation of this pool was measured despite 78% frustule dissolution. At the end of the degradation experiment, this pool constituted 5.8% of the remaining total POC suggesting an effective protection by the frustule through strong interaction with the biogenic silica which is consistent with the correlation observed at depth between Si and POC sedimentation fluxes

Fluorescence properties of dissolved organic matter in coastal Mediterranean waters influenced by a municipal sewage effluent (Bay of Marseilles, France)

International audienceFluorescent dissolved organic matter (FDOM) in coastal marine waters influenced by the municipal sewage effluent (SE) from Marseilles City (France, north-western Mediterranean Sea) has been characterised. Samples were collected eleven times from September 2008 to June 2010 in the Bay of Marseilles along a coast-open sea transect from the SE outlet in the South Bay and at the Mediterranean Institute Observation site in the central Bay. Fluorescence excitation-emission matrices combined with parallel factor analysis (PARAFAC) allowed the identification of two protein-like (tyrosine C1, with excitation maxima (lEx) and an emission maximum (lEm) of ,230, 275/306 nm; tryptophan C2, lEx/lEm,230, 270/346 nm) and three humic-like components (marine humic C3, lEx/lEm 280/386 nm; C4, lEx/lEm 235, 340/410 nm; C5, lEx/lEm 255, 365/474 nm). From the SE outlet to the central Bay, a gradient appeared, with decreasingFDOM intensities, decreasing dissolved organic carbon, particulate carbon, nutrients and faecal bacteria concentrations and increasing salinity values. This gradient was associated with decreasing abundances in protein-like fluorophores and rising abundances in humic-like (C3 and C5) materials. This shift in FDOM composition illustrated the decrease in wastewater inputs and the increase in marine sources of DOM along the transect. FDOM data showed that the Marseilles SE spread up to 1500m off the outlet, but it did not reach the central Bay. Tryptophan-like material was the dominant fluorophore in the SE and displayed the highest correlations with biogeochemical parameters (organic carbon, phosphates, faecal bacteria). Therefore, it is proposed to use its fluorescence intensity to detect and track SE inputs in the Marseilles coastal marine waters

A Glider-Compatible Optical Sensor for the Detection of Polycyclic Aromatic Hydrocarbons in the Marine Environment

This study presents the MiniFluo-UV, an ocean glider-compatible fluorescence sensor that targets the detection of polycyclic aromatic hydrocarbons (PAHs) in the marine environment. Two MiniFluos can be installed on a glider, each equipped with two optical channels (one PAH is measured per channel). This setup allows the measurement of up to 4 different fluorescent PAHs: Naphthalene, Phenanthrene, Fluorene and Pyrene. Laboratory tests on oil products (Maya crude oil and Diesel fuel) as well as on marine samples near industrial areas (urban harbor and offshore installations) revealed that the concentration of the four PAHs targeted accounted for 62–97% of the total PAH concentration found in samples (∑16 PAHs determined by standard international protocols). Laboratory tests also revealed that for marine applications, the calibration on Water Accommodated Fraction (WAF) of crude oil is more appropriate than the one on pure standards (STD). This is because PAH fluorescence is constituted in large part of alkylated compounds that are not considered with STD calibration. Results from three glider deployments with increasing levels of complexity (a laboratory trial, a field mission in non-autonomous mode and a fully autonomous mission) are also presented. During field deployments, the MiniFluo-glider package was able to detect concentration gradients from offshore marine waters toward the head of a Mediterranean harbor (< 80 ng L−1) as well as hydrocarbon patches at the surface waters of an oil and gas exploitation field in the North Sea (< 200 ng L−1, mainly Naphthalene). It is suggested that using only the WAF calibration, the concentration derived with the MiniFluo agrees within one order of magnitude with the concentration determined by Gas Chromatography coupled with Mass Spectrometry (overestimation by a factor 7 on average). These performances can be improved if the calibration is made with a WAF with PAH proportions similar to the one find in the environment. Finally, it is shown that the use of in situ calibration on water samples collected during the glider deployment, when possible, gives the best results

Si–C interactions during degradation of the diatom Skeletonema marinoi

International audienceWhile a relationship between ballast and carbon in sedimenting particles has been well-documented, the mechanistic basis of this interaction is still under debate. One hypothesis is that mineral ballast protects sinking organic matter from degradation. To test this idea, we undertook a laboratory experiment using the diatom Skeletonema marinoi to study in parallel the dissolution of one of the most common mineral ballasts, biogenic silica (bSiO2), and the associated degradation of organic matter. Three different models were applied to our results to help elucidate the mechanisms driving bSiO2 dissolution and organic compound degradation. Results of this modeling exercise suggest that the diatom frustule is made up of two bSiO2 phases that dissolve simultaneously, but at different rates. In our experiments, the first phase was more soluble (View the MathML source) and made up 31% of the total bSiO2. In this phase, bSiO2 was mainly associated with membrane lipids and the amino acids glutamic acid, tyrosine, and leucine. The second phase was more refractory (View the MathML source), and contained more neutral lipid alcohols and glycine. Until it dissolved, the first bSiO2 phase effectively protected much of the organic matter from degradation: particulate organic carbon (POC) degradation rate constants increased from 0.025 to 0.082 d−1 after the total dissolution of this phase, and particulate organic nitrogen (PON) degradation rate constants increased from 0.030 to 0.094 d−1. Similar to POC and PON, the total hydrolyzable amino acids (THAA) degradation rate constant increased from 0.054 to 0.139 d−1 after dissolution of the first bSiO2 phase. The higher THAA degradation rate constant is attributed to a pool of amino acids that was produced during silicification and enclosed between the two silica phases. This pool of amino acids might come from the incorporation of silica deposition vesicles into the diatom wall and might not be directly associated with bSiO2. In contrast, most lipid degradation was not prevented by association with the more soluble bSiO2 phase, as the average lipid degradation rate constant decreased from 0.048 to 0.010 d−1 after 17 d of degradation. This suggests that most lipids were associated with rather than protected by silica, except pigments that appeared resistant to degradation, independently from silica dissolution. When the only organic compounds remaining were associated with the second bSiO2 phase, degradation rate constants decreased greatly; concentrations changed only slightly after day 25

Rôle de l'activité hydrolytique bactérienne dans le cycle de la matière organique en milieu marin (focus sur les enzymes lipase)

Après une étude bibliographique rappelant l'importance de l'activité ectoenzymatique bactérienne dans le cycle du carbone organique en milieu marin, ce manuscrit, ciblé sur l'étude du modèle lipides - lipases, présente les étapes qui ont permis de mettre au point un protocole fournissant des mesures réalistes de l'activité lipase des bactéries marines en suivant la dégradation d'un substrat triglycéride radioactif : la [3H]-trioléine. Au cours d'une étude dans le lagon de Nouvelle-Calédonie, cette nouvelle technique a été comparée avec la technique traditionnelle qui utilise des analogues fluorescents, et s'est révélée être plus spécifique, sensible et reproductible. Puis, comparée aux mesures chimiques de l'hydrolyse d'un triglycéride au cours d'une expérience de biodégradation in vitro, elle s'est avérée refléter de manière significative l'activité lipase s'exerçant in situ. Suite à ces deux étapes de validation, ce protocole a été utilisé au cours d'une étude environnementale en Méditerranée nord-occidentale dans le cadre du projet PROOF-PROPECHE, notamment pour mettre en évidence le rôle joué par l'hydrolyse ectoenzymatique dans l'accumulation de carbone organique dissous au cours de la transition printanière du milieu vers l'oligotrophieAIX-MARSEILLE1-BU Sci.St Charles (130552104) / SudocSudocFranceF

Biogéochimie de la matière organique dans deux écosystèmes marins (Cas d'une structure physique unidimensionnelle verticale (mer Ligure) et d'une structure physique frontale (front Alméria-Oran) : Variations spatio-temporelles de la fraction lipidique à petite échelle)

AIX-MARSEILLE2-BU Sci.Luminy (130552106) / SudocAIX-MARSEILLE2-Stat.Mar. Endoume (130552206) / SudocSudocFranceF

Development of a field-portable fluorometer based on deep ultraviolet LEDs for the detection of phenanthrene- and tryptophan-like compounds in natural waters

International audienceIn recent years, portable or submersible fluorometers have become key tools in acquiring real time high frequency measurements of target dissolved organic matter (DOM) fluorophores in aquatic environments. We have developed an original field-portable fluorometer based on deep UV light-emitting diodes (LEDs) for simultaneous measurements of two DOM fluorophores in natural waters: phenanthrene- and tryptophan-like compounds. This fluorometer, called "the MiniFluo-UV" was tested on standard solutions and natural samples collected on the northwestern Mediterranean coast (vicinity of Marseille City, France). The MiniFluo-UV responded linearly to an increase in phenanthrene and tryptophan concentrations over the range 0.1-100 µg L-1. Limits of detection for these two fluorophores were 1.43 and 0.72 µg L-1, respectively. The sensor also responded linearly to an increase in measurement duration over the range 200-2000 ms, and the fluorescence intensity showed a power decrease from calibre C0 (integration capacitor of 3 pF) to calibre C3 (integration capacitor of 73 pF). Optimal parameters (duration of 600 ms, calibre C0) were used for measurements in natural water samples. We found very significant linear relationships between the phenanthrene and tryptophan equivalent concentrations measured with the MiniFluo-UV and the concentrations measured with the Hitachi laboratory spectrofluorometer on all natural water samples (R2 = 0.90 and 0.98, n = 150, p < 0.01). We conclude that the MiniFluo-UV fluorometer is a very relevant tool to detect and track sewage effluents and hydrocarbon contamination in marine coastal waters