Lipid concentrations, UV radiation doses, and attenuation coefficients in coastal surface waters of West Antarctica

The seasonal depletion of stratospheric ozone over the Southern Hemisphere allows abnormally high doses of ultraviolet radiation (UVR) to reach surface waters of the West Antarctic Peninsula (WAP) in the austral spring, creating a natural laboratory for the study of lipid photooxidation in the shallow mixed layer of the marginal ice zone. While the photooxidation of lipids under such conditions has been identified as a source of stress to microorganisms and short-chain fatty acids altered by photochemical processes have been found in sinking marine particle material in polar regions, little remains known about the biogeochemical impact of lipid photooxidation compared to the many other biological and abiotic processes that can transform particulate organic matter in the surface ocean. We combined results from field experiments in a model liposome system with diverse environmental data - including high-resolution, accurate-mass HPLC-ESI-MS analysis of lipid extracts and in situ measurements of ultraviolet irradiance - to address several unresolved questions about lipid photooxidation in the marine environment. In our experiments, we examined the photolability of various moieties of the intact polar diacylglycerol (IP-DAG) phosphatidylcholine (PC), a structural component of membranes in a broad range of microorganisms. We observed significant rates of photooxidation only when the molecule contained the C22:6 polyunsaturated fatty acid (PUFA) docosahexaenoic acid (DHA). As the DHA-containing lipid was oxidized, we observed the steady ingrowth of a diversity of oxylipins and oxidized IP-DAG; our results suggest both the intact IP-DAG the degradation products were amenable to heterotrophic assimilation. To complement our experiments, we used an enhanced version of a recent lipidomics discovery software package to identify the lipids in water column samples and in several diatom isolates. The galactolipid digalactosyldiacylglycerol (DGDG), the sulfolipid sulfoquinovosyldiacylglycerol (SQDG) and the phospholipids PC and phosphatidylglycerol (PG) accounted for the majority of IP-DAG in the water column particulate (>= 0.2 µm) size fraction; between 3.4 and 5.3 % of the IP-DAG contained fatty acids that were both highly polyunsaturated (i.e., each containing >= 5 double bonds). By applying a broadband apparent quantum yield (AQY) for photooxidation of PUFA-containing IP-DAG to these water column data, we estimated that 0.8 ± 0.2 µmol IP-DAG /m**2/d (0.5 ± 0.1 mg C/m**2/d) were oxidized by photochemical processes in the mixed layer. This rate represented 3 and 22 % of the total bacterial production observed immediately following the retreat of the sea ice. While production of such diverse oxidized lipids and oxylipins has been previously observed in terrestrial plants and mammals in response to biological stressors such as disease, we show here that a similar suite of molecules can be produced via an abiotic process in the environment and that the effect can be significant at ecosystem scale.

This collection includes the following datasets: Daily doses of ultraviolet-B (UVB; 290-315 nm) and ultraviolet-A radiation (UVA; 315-400 nm) recorded in situ at 0.6 m depth at Palmer Station, Antarctica; diffuse downwelling attenuation coefficients in Napierian form for Southern Ocean waters at wavelengths 290-700 nm; concentrations of intact polar diacylglycerol (IP-DAG) lipids measured in cultures of four Antarctic diatom isolates and in water column samples from the Drake Passage and Bellingshausen Sea; transmission spectra from 191-800 nm for various glass and polymer incubation containers; and wavelength-specific molar decadic absorption coefficients of various phosphatidylcholine lipid standards for wavelengths 200-500 nm.