The effect of irradiance on lipids of highly branched isoprenoids (HBIs) producing diatom culture of Pleurosigma intermedium: towards stable isotope proxies for the paleo sea-ice reconstructions

Source-specific highly branched isoprenoids (HBIs) have been recently served as a binary or semi-quantitative biomarker to indicate the sea ice extent in the past. Since the light intensity controlled by overlying snow cover and sea ice thickness has a significant impact on the productivity of photoautotrophic organisms and environmental water is the sole source of the hydrogen for the biosynthesis of these organisms, the hydrogen isotope ratio (²H/¹H) of HBIs holds the potential to reveal more characteristics of sea ice. In this study, based on the observation of natural settings underneath sea ice, diatom Pleurosigma intermedium were grown at irradiances from 20 to 300 μmol m^-2 s^-1 in laboratory conditions and harvested from exponential phase and stationary phase respectively to investigate the effect of light and growth phase on hydrogen isotope fractionation in HBIs. Gas chromatography-mass spectrometry (GC-MS) screening showed that a triene (C25:3) and a tetraene (C25:4) C25 HBI alkene were detected in all samples from varying irradiances. A remarkable decline of the ratio of C25:3/C25:4 from higher to lower irradiances was observed. However, there was no significant change in the concentration of C14 (myristic), C16:1 (palmitoleic) and C16 (palmitic) fatty acids with varying light intensity. In addition, terpenoids such as phytol, squalene and range of sterols were also be identified. Published studies on phytol, fatty acid and sterol from Thalassiosira pseudonana and alkenones from Emiliania huxleyi have shown dramatic changes in hydrogen isotope fractionation and concluded that the source of nicotinamide adenine dinucleotide phosphate (NADPH) and the operation of acetogenic pathway, plastidic methylerythritol phosphate (MEP) and/or cytosolic mevalonic acid (MVA) of lipids are the key factors controlling ²H/¹H fractionation. The integration of molecular distribution of HBIs, fatty acids and terpenoids in Pleurosigma intermedium together with our ongoing work on their ²H/¹H and ¹³C/¹²C compositions will lead to a better understanding of diatom metabolism and biochemistry under different light conditions. This knowledge will be instrumental to a more robust interpretation of stable isotope data from environmental samples and thus will contribute to further developing HBI biomarkers as a tool for estimating not only the absence/presence of sea ice but also the ice type, thickness, and snow cover.