Sequential photo- and autoxidation of diatom lipids in Arctic sea ice

We measured the concentrations of selected lipids and some of their degradation products in a time series of sea ice samples collected from Resolute Bay in the Canadian Arctic in 2012. The identification of specific tracers of photo- and autoxidation reactions provided evidence of both abiotic processes acting on organic matter, although none of the lipids appeared sensitive to significant biodegradation. Some differences in lipid reactivity were observed between the upper (3-10 cm) and lower (0-3 cm) sections of the cores, possibly as a result of increased exchange at the ice-water boundary for the latter. In terms of photodegradation, the phytyl side chain of chlorophyll and a tri-unsaturated highly branched isoprenoid (HBI) alkene were most affected, 24-methylenecholesterol (from diatoms) was more susceptible to photodegradation than brassicasterol (from diatoms and prymnesiophytes), while sitosterol and cholesterol, likely from a range of sources, were largely unaffected. With respect to autoxidation, the reactivity trend for the sterols was reversed, with significant autoxidation of sitosterol and cholesterol, but not of 24-methylenecholesterol or brassicasterol. The phytyl side chain of chlorophyll and a highly branched isoprenoid triene were also particularly susceptible to autoxidation. In contrast, the diatom fatty acid, C16:1, was not substantially altered by any degradation pathway. By measuring temporal changes in the proportions of specific tracers of each degradation process, we provide evidence that such abiotic reactions take place via sequential photooxidation and autoxidation, with homolytic cleavage of photochemically produced hydroperoxides leading to autoxidation of initial substrates and subsequent oxidation products. The observation of significant abiotic degradation of several lipids appears to take place despite the low temperature in sea ice (ca. 0 °C) and should be considered carefully alongside other removal processes involving organic matter in marine systems.