Abstract
Abstract. For the next decades significant climatic changes should occur in the Arctic zone. The expected destabilisation of permafrost and its consequences for hydrology and plant cover should increase the input of terrigenous carbon to coastal seas. Consequently, the relative importance of the fluxes of terrestrial and marine organic carbon to the seafloor will likely change, strongly impacting the preservation of organic carbon in Arctic marine sediments. Here, we investigated the lipid content of surface sediments collected on the Mackenzie basin in the Beaufort Sea. Particular attention was given to biotic and abiotic degradation products of sterols and monounsaturated fatty acids. By using sitosterol and campesterol degradation products as tracers of the degradation of terrestrial higher plant inputs and brassicasterol degradation products as tracers of degradation of phytoplanktonic organisms, it could be observed that autoxidation, photooxidation and biodegradation processes act much more intensively on higher plant debris than on phytoplanktonic organisms. Examination of oxidation products of monounsaturated fatty acids showed that photo- and autoxidation processes act more intensively on bacteria than on phytodetritus. Enhanced damages induced by singlet oxygen (transferred from senescent phytoplanktonic cells) in bacteria were attributed to the lack of an adapted antioxidant system in these microorganisms. The strong oxidative stress observed in the sampled sediments resulted in the production of significant amounts of epoxy acids and unusually high proportions of monounsaturated fatty acids with a trans double bond. The formation of epoxy acids was attributed to peroxygenases (enzymes playing a protective role against the deleterious effects of fatty acid hydroperoxides in vivo), while cis/trans isomerisation was probably induced by thiyl radicals produced during the reaction of thiols with hydroperoxides. Our results confirm the important role played by abiotic oxidative processes in the degradation of marine bacteria and do not support the generally expected refractory character of terrigenous material deposited in deltaic systems.
Highlights
River-dominated shelves are some of the most important sites of organic carbon (OC) burial in the marine environment (Berner, 1982; Hedges and Keil, 1995)
This assumption is supported by the penetration depth of oxygen, which may reach 2–4 cm in this zone (Magen, 2007) and which may contribute to oxic degradation of the settled organic matter well deeper than the sediment-water interface
These oxygen demands, which were associated with high production rates of metabolites (e.g. NO−2, PO34−, NH+4 ), are indicative of an intense biodegradation activity in these sediments. These results are in good agreement with the weak degradation state and the cis / trans ratio of vaccenic acid measured at these stations (Fig. 5) attesting to the presence of non-stressed bacteria in a good healthy state and very active. Lipids and their degradation products were quantified in eight samples of surface sediments collected in the Beaufort Sea
Summary
River-dominated shelves are some of the most important sites of organic carbon (OC) burial in the marine environment (Berner, 1982; Hedges and Keil, 1995). Several studies demonstrated that terrestrial organic matter (OM) was more degraded in coastal sediments than in river suspended particulate matter (Ingalls et al, 2003; Unger et al, 2005a, b) and that the reactivity of the sedimentary OM is influenced by its origin and by several factors, such as water column depth, redox conditions, microbial activity, mineral composition and sediment physical characteristics (Alkathib et al, 2012; Niggemann et al, 2007; Hedges et al, 1997). We studied the degradation of suspended particulate matter (SPM) from the Mackenzie River to the Beaufort Sea by using specific lipid degradation tracers (Rontani et al, 2012a). Lipids of terrestrial vascular plants, which are well preserved in SPM of the Mackenzie River, appeared to be extensively degraded by bacterial and especially autoxidative degradative processes in the water column of the Beaufort Shelf, while planktonic lipids were only weakly affected. In order to explain the specific induction of autoxidative processes on vascular plant-derived material, a mechanism involving metal ion-catalysed homolytic cleavage of photochemically produced hydroperoxides resulting from the senescence of higher plants on land was proposed
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