Fluorescence Diagnostics for Lipid Status Monitoring of Microalgae during Cultivation

Abstract

To improve cultivation or extraction methods for maximum lipid yield, a fast and reliable quantitative determination of the lipid content of microalgae in cultivation medium is desirable. Here, we investigated the applicability of Nile red staining for a rapid quantification of neutral lipids in-vivo, without time-consuming intermediate steps like lyophilization, drying or solvent transfer of lipids, previously required in conventional lipid diagnostics. The studies were performed on Auxenochlorella protothecoides, reproducibly cultivated in a temperature and pH-controlled photobioreactor (PBR). The basic procedure of this method is to normalize the cell density of each sample to an optical density OD of 1.0 by employing a spectroscope. After staining of the normalized algal suspension, the relative fluorescence intensity at 580 nm is measured and compared with results acquired by an alternative lipid content determination method, i.e. the sulfo-phospho-vanillin (SPV) method. A linear relationship between in-vivo fluorescence intensity and the lipid content determined by the SPV method was found. It could be demonstrated that Nile red staining of algal cells and successive fluorescence measurement is suitable for a fast and reliable monitoring of the oil accumulation in algal cells during PBR cultivation. In the following measurement and calibration, procedures will be discussed in detail.