The effect of irradiance on algal biomarkers: towards the interpretation of stable isotopic signatures and implication for sea ice reconstructions

Gao, Simin (2022) The effect of irradiance on algal biomarkers: towards the interpretation of stable isotopic signatures and implication for sea ice reconstructions. Doctoral thesis, University of East Anglia.

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Source-specific highly branched isoprenoids (HBIs) have recently served as a binary or semi-quantitative biomarker to indicate the sea ice extent in the past. The type, thickness and overlying snow cover of the sea ice determine the light intensity underneath and can further have a significant impact on the productivity of photoautotrophic organisms. Since environmental water is the sole source of hydrogen for the biosynthesis of the biomarkers from these organisms, the hydrogen isotope ratio (2H/1H) of HBIs holds the potential to reveal more characteristics of sea ice. Based on the in situ irradiance underneath sea ice in polar regions, marine diatom Navicula salinicola, Pleurosigma intermedium and Rhizosolenia setigera were grown at varying light levels in laboratory conditions and harvested from exponential phase and stationary phase respectively to investigate the effect of light and growth phase on hydrogen and carbon isotope fractionation in HBIs and other algal lipids.

Gas chromatography-mass spectrometry (GC-MS) screening showed that P. intermedium produced C25 triene (C25:3) and tetraene (C25:4) HBIs, while pentaene (C25:5) was the only HBI detected in R. setigera. Minor diene (C25:2) and a triene with a new structure were found in samples from N. salinicola. Besides HBIs, fatty acids C14:0, C16:1, C16:0, and isoprenoid lipids phytol and squalene were detected in most samples. A remarkable decline in the ratio of C25:3 versus C25:4 was observed as irradiance increased. Hydrogen isotopic compositions of fatty acids, HBIs, phytol and squalene showed different directions in response to the increase of light. The relative proportion of NADPH and pyruvate originated from photosynthesis and the metabolic pathway for different lipid classes might account for this discrepancy. More than 20 per mil differences in δ13C of algal lipids in lowest and highest irradiance and a linear correlation between δ13C value and growth rate were observed. This is likely because of the limitation of environmental carbon transport into cells at high growth rate and of carboxylation by RuBisCO during photosynthesis at low growth rate.

The characterisation of hydrogen and carbon isotopic signatures of different algal lipid classes under varied light conditions will lead to a better understanding of the effect of light on diatom metabolism and biochemistry. 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.

Key words: Pleurosigma intermedium, Rhizosolenia setigera, HBIs, variable irradiance, stable hydrogen isotope, stable carbon isotope, MEP, MVA

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Environmental Sciences
Depositing User: Chris White
Date Deposited: 12 Jul 2023 14:51
Last Modified: 12 Jul 2023 14:51

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