Identification and semi-quantitative analysis of lipids in the diatom Porosira glacialis

Abstract

Background: In 2015, The University of Tromsø initiated a project with local industry involving cultivation of marine microalgae to reduce industrial CO2-emissions. Marine microalgae are primary producers with high capacity for carbon fixation into biomass rich in lipids. Their lipid composition is characterized by high content of PUFAs such as EPA (20:5) and DHA (22:6). The biomass can potentially be utilized as supplement to marine raw materials in fish feed for aquaculture industry. To maximize the yield of lipids rich in EPA and DHA, this thesis investigates cultivation in different lighting conditions and their influence on lipid composition and fatty acid content of EPA and DHA in marine microalgae.

Method: The diatom species, Porosira glacialis, was exposed to red, blue and white light conditions during four days of cultivation. After harvesting, the biomass was freeze-dried and homogenized. The lipids were extracted from the biomass by a modified Folch method (DCM/MeOH) and applied to High Performance Liquid Chromatography – Mass Spectrometry (HPLC-MS) for lipid analysis in positive and negative ionization mode. AcquireX was used for data dependent acquisition to generate data of identification and relative contents of lipids and fatty acids. Identification and semi-quantitative analysis of positively charged lipids were performed by LipidSearch software, whereas manual identification was performed for SQDG, a negatively charged lipid.

Results: The major content of EPA was seen in PC, where the cultures exposed to blue and white light showed significantly increased EPA content compared to red light. For the overall content of EPA, an even distribution was observed between the different lighting conditions. The major contents of DHA were seen in PC and PE, PC having significantly higher DHA content in red and white light and PE having significantly higher DHA content in blue and red light. For the overall content of DHA, there were only significant finding between red and blue light, with the culture in red light having the highest DHA content.

Conclusion: The blue and white lighting conditions seem promising to maximize the yield of lipids rich in EPA for P. glacialis. However, the findings are not as clear for DHA, but a suggestion might be red light before blue light to maximize the content of DHA.