Degradation state of organic matter in surface sediments from the Southern Beaufort Sea: a lipid approach

摘要

For the next decades significant climatic changes should occur in the Arcticzone. The expected destabilisation of permafrost and its consequences forhydrology and plant cover should increase the input of terrigenous carbon tocoastal seas. Consequently, the relative importance of the fluxes ofterrestrial and marine organic carbon to the seafloor will likely change,strongly impacting the preservation of organic carbon in Arctic marinesediments. Here, we investigated the lipid content of surface sedimentscollected on the Mackenzie basin in the Beaufort Sea. Particular attentionwas given to biotic and abiotic degradation products of sterols andmonounsaturated fatty acids. By using sitosterol and campesterol degradationproducts as tracers of the degradation of terrestrial higher plant inputsand brassicasterol degradation products as tracers of degradation ofphytoplanktonic organisms, it could be observed that autoxidation,photooxidation and biodegradation processes act much more intensively onhigher plant debris than on phytoplanktonic organisms. Examination ofoxidation products of monounsaturated fatty acids showed that photo- andautoxidation processes act more intensively on bacteria than onphytodetritus. Enhanced damages induced by singlet oxygen (transferred fromsenescent phytoplanktonic cells) in bacteria were attributed to the lack ofan adapted antioxidant system in these microorganisms. The strong oxidativestress observed in the sampled sediments resulted in the production ofsignificant amounts of epoxy acids and unusually high proportions ofmonounsaturated fatty acids with a trans double bond. The formation of epoxy acidswas attributed to peroxygenases (enzymes playing a protective role againstthe deleterious effects of fatty acid hydroperoxides in vivo), while cis/trans isomerisationwas probably induced by thiyl radicals produced during the reaction ofthiols with hydroperoxides. Our results confirm the important roleplayed by abiotic oxidative processes in the degradation of marine bacteriaand do not support the generally expected refractory character ofterrigenous material deposited in deltaic systems.