Lipid Estimation of Surfactant-Extracted Microalgae Oil Using Nile Red

Nile red staining has been used as a lipid quantification technique in many microalgae growth and oil accumulation studies. However, its application in lysed microalgae cells is limited. Therefore, this study focused on lysed microalgae cells and utilized the Nile red staining technique to evaluate oil content and extraction. This study aims to provide a rapid and high-throughput alternative method particularly in the microalgae extraction screening process. Potential interferences such as chlorophyll, β-carotene, soluble protein, and phospholipids were evaluated. The hydrophobic Nile red dye was found to quench in water, therefore the fluorometric measurement has to be completed immediately or within 5 min of dye addition. The fluorescence intensity was also found to be Nile red concentration dependent. The optimum Nile red concentration of 656 ppb was used throughout the study. In microalgae samples containing chlorophyll and carotenoids (such as Nannochloropsis sp.), Nile red fluorescence intensity was significantly reduced in comparison to non-chlorophyll microalgae (Schizochytrium limacinum). Soluble proteins from defatted microalgae did not fluoresce significantly relative to lipids, therefore did not interfere with the method to a high degree. Comparing the optimized Nile red staining method with the gravimetric lipid quantification method, a good linear correlation was found in all three materials tested (soybean oil, Nannochloropsis sp., and Schizochytrium limacinum).