Abstract
In this study, confocal Raman microspectroscopy was used to detect lipids in microalgae rapidly and non-destructively. Microalgae cells were cultured under nitrogen deficiency. The accumulation of lipids in Scenedesmus obliquus was observed by Nile red staining, and the total amount of lipids accumulated in the cells was measured by gravimetric method. The signals from different microalgae cells were collected by confocal Raman microspectroscopy to establish a prediction model of intracellular lipid content, and surface scanning signals for drawing pseudo color images of lipids distribution. The images can show the location of pyrenoid and lipid accumulation in cells. Analyze Raman spectrum data and build PCA-LDA model using four different bands (full bands, pigments, lipids, and mixed features). Models of full bands or pigment characteristic bands were capable of identifying S. obliquus cells under different nitrogen stress culture time. The prediction accuracy of model of lipid characteristic bands is relatively low. The correlation between the fatty acid content measured by the gravimetric method and the integral Raman intensity of the oil characteristic peak (1445 cm−1) measured by Raman spectroscopy was analyzed. There was significant correlation (R2 = 0.83), which means that Raman spectroscopy is applicable to semi-quantitative detection of microalgal lipid content.
Highlights
The world energy crisis is intensified continuously nowadays, which raises interest in looking for renewable energy resources
The total lipid content of microalgae was 20.54% before stress culture; this value increased to 30.77% at the ninth day of stress culture (Fig. 1)
The above analysis results show that, with the capability of synthesizing large number of long-chain fatty acids, S. obliquus can be used as potential algae for biodiesel production
Summary
The world energy crisis is intensified continuously nowadays, which raises interest in looking for renewable energy resources. Among the existing renewable energy sources, biodiesel is the most widely used variety which have the fastest speed of development. Several methodologies used for quantitative analysis of microalgae lipids, such as gas chromatography–mass spectrometry (GC–MS), are not just time-consuming and ruinous, and requires sophisticated sample preparation process which produces hazardous chemical waste. GC–MS provide precise analysis of lipid composition, it cannot provide the detailed information of lipid metabolism. Studies showed that the lipid content of one cell was significantly correlated with the fluorescence intensity of Nile Red combined with the lipid inside the cell [7, 8]
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