Lipid content, composition and dynamics in northern marine diatoms. An experimental study using different analytical methods

Abstract

In this study, the fluorophore Nile Red was used as a probe for lipid accumulation during nutrient stress in the diatom Coscinodiscus concinnus, and to quantify lipid concentration fluorometrically in 5 monocultures of northern diatoms. The results from the fluorometrical quantification were validated by solvent extraction and HPLC-MS analysis in Bergen. In addition, 17 monocultures of 7 species of arctic diatoms were analyzed for lipid class and fatty acid composition by HPLC-MS in Prague.Nile Red was shown to be an effective probe for visualizing the physiological state of C. concinnus. The quantification procedure using Nile Red yielded high correlation coefficients in the lipid standards cod liver oil and α-phosphatidylcholine (R2 = 0.995 and 0.985, respectively), but was in all likelihood unable to quantify the true polar and neutral lipid concentration of the diatom samples, possibly due to fluorescence from an unknown source. Solvent extraction in Bergen determined a total lipid concentration of 2.15-7.32% per dry weight, but unidentified contaminations probably caused an over-estimation of the lipid content. The HPLC-MS performed in Bergen produced results that indicated hydrolysis of the sample or unknown contaminations and were not included in the study. The HPLC-MS analysis in Prague revealed large variations in the composition of the main lipid classes (TAG, PC, PG and PS) both within and between species. Large contributions of LPG and LPC were also observed. The composition of fatty acids showed less variation within species, and was similar for all species with the exception of T. gravida. The main fatty acids were C16:0, C16:1, C18:0, C18:1 (n-9), C20:5 (n-3) and C22:6 (n-3).