Degradation of planktonic and bacterial lipids in sinking particles

Biogeosciences Discussions This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract The lipid content of seven samples of sinking particles collected with sediment traps moored at ∼100 m depth in summer and fall across the Canadian Beaufort Shelf (Arc-tic Ocean) was investigated. Our main goal was to quantify and characterize the biotic and abiotic degradation processes that acted on sinking material during these peri-5 ods. Diatoms, which dominated the phytoplanktonic assemblage in every trap sample , appeared to be remarkably sensitive to Type II (i.e. involving singlet oxygen) pho-todegradation processes in summer, but seemed to be relatively unaffected by biotic degradation at the same time. Hence, the relative recalcitrance of phytodetritus towards biodegradation processes during the Arctic midnight sun period was attributed 10 to the strong photodegradation state of heterotrophic bacteria, which likely resulted from the efficient transfer of singlet oxygen from photodegraded phytoplanktonic cells to attached bacteria. In addition, the detection in trap samples of photoproducts specific to wax ester components found in herbivorous copepods demonstrated that zooplank-tonic faecal material exported out of the euphotic zone in summer were as well affected 15 by Type II photodegradation processes. By contrast, sinking particles collected during the autumn were not influenced by any light-driven stress. Further chemical analyses showed that photodegraded sinking particles contained an important amount of intact hydroperoxides, which could then induce a strong oxidative stress in underlying sediments .