Burial and reactivity of sedimentary microalgal lipids in bioturbated Mediterranean coastal sediments

Abstract

International audienceThe fate of microalgal lipid biomarkers in marine coastal sediments when acted on by natural bioturbation processes (Carteau Bay, Gulf of Fos, Mediterranean Sea) was studied under laboratory conditions. Both dead phytoplanktonic cells (Nannochloropsis salina) and luminophores (inert fluorescent particulate tracers) were deposited at the surface of intact sediment cores which were then incubated for 22, 44 and 63 days. Sediment reworking and concentration profiles of specific lipid components of N. salina (n-alkenes, alkyl diols, sterols and fatty acids) were determined as a function of time and depth. The results show that, in the sediment investigated, bioturbation occurs essentially as a biodiffusive process and that it has a rapid and significant impact on the qualitative and quantitative record of sedimentary lipids. Whereas most of the biomarkers were detected in the entire reworked layer (0–6 cm) after 22 days, n-alkenes were never detected below 3 cm due to their low concentration and their high reactivity. For each individual lipid, the comparison of the amounts obtained from the inventories of biomarkers in the reworked zone, with the amount deposited initially at the sediment surface, allowed the determination of its extent and rate of degradation. These ranged from 72% to 99% and from 0.010 to 0.047 day-1, respectively, depending on the biomarker considered, with polyunsaturated fatty acids (PUFAs) and alkenes being degraded faster than the other components. Comparison with previous work suggests that, in biologically reworked sediments, the apparent reactivity of lipids is: (i) positively correlated with the biological mixing coefficient (Db) and, (ii) generally much higher than in non-bioturbated (anoxic) sediments. Our results also support the idea that degradation of lipids in reworked sediments involves the combined effects of aerobic and anaerobic degradation processes, but that biological mixing results in diagenetic properties more characteristic of completely oxidized conditions

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

“…The degradation rate constants of individual (free+ esterified) FA were consistent with those calculated for individual lipids in previous studies performed under either experimental (Grossi et al, 2003, and references therein) or natural (Canuel and Martens, 1996) conditions. They were, however, one to three orders of magnitude higher than those generally observed in strictly anoxic sediments (Sun and Wakeham, 1994;Grossi et al, 2001Grossi et al, , 2003.…”

“…They were, however, one to three orders of magnitude higher than those generally observed in strictly anoxic sediments (Sun and Wakeham, 1994;Grossi et al, 2001Grossi et al, , 2003. Also, for the same FA under permanently oxic conditions Sun et al (2002) observed higher (3-4 times) degradation rates compared to the present case.…”

“…Alcohols and fatty acids were converted to trimethylsilyl ether derivatives by reaction with bis-trimethylsilyl-trifluoroacetamide (BSTFA, Supelco) in pyridine (1:1 v/v; 30 min at 60 C), and were identified with a HP 5890 Series II Plus gas chromatograph coupled to a HP 5972 mass spectrometer (Grossi et al, 2001). Individual compounds were quantified with an external standard calibration (squalane and nonadecanoic acid for alcohols and acids respectively) by gas chromatography using a HP4890 GC/FID system equipped with a HP-5MS capillary column (30 mÂ0.25 mm i.d.Â0.25 mm film thickness; Grossi et al, 2003). …”

“…Although it has been demonstrated that bioturbation can have a significant impact on the burial and degradation of sedimentary lipid components (e.g. Gilbert et al, 1994Gilbert et al, , 1996Sun et al, 1999;Ingalls et al, 2000;Grossi et al, 2002Grossi et al, , 2003, the influence of redox oscillation on lipid degradation has been little studied. Using a diffusively ''open'' incubation sedimentary system that mimics varying conditions of oxygenation in bioturbated sediments, Sun et al (1993) showed that redox changes influenced the diagenetic pathways of chloropigments.…”

Biogeochemistry of fatty acids in a river-dominated Mediterranean ecosystem (Rhône River prodelta, Gulf of Lions, France): Origins and diagenesis

“…Off the Rhône River, surface and subsurface deposit-feeder species predominate, representing a link between continental POC and the marine food web (Harmelin-Vivien et al, 2009). Biogeochemical processes (burial and organic matter decomposition) are also directly affected by the benthic infauna that rework and irrigate the sediments (Grossi et al, 2003;Duport et al, 2007). However, high physical disturbance can decrease diversity (Hermand et al, 2008) and generally precludes macrofaunal biomass from dominating benthic communities in fluidized muds .…”