Evaluating the potential of chlorophyll as a tool for estimating phytosterol production in microalgae

Phytosterols are bioactive lipids reported to have several health benefits including cholesterol-lowering properties, anti-cancer, and antioxidant activities. Microalgae species are being explored as sustainable, potential sources of phytosterols. Since all microalgae species do not produce phytosterols at viable commercial quantities, many prospective candidates must be screened for their phytosterol content. The traditional means of bioprospecting microalgae candidates for phytosterols involves growing large culture volumes, total lipid extraction, lipid extract saponification, trimethylsilyl derivatization of unsaponifiables, and GCMS analysis. This method is labour intensive, time consuming, and expensive. We propose a faster and cheaper method to screen for phytosterols indirectly, using chlorophyll, which can be measured rapidly (10 min) and with high precision. Chlorophyll and phytosterols concentrations are both expected to be modulated by light intensity and so to co-vary. The hypothesized co-variance must be validated before using chlorophyll as a proxy for estimating phytosterol production. Therefore, the aim of this study was to measure the ratio of phytosterols to chlorophyll (Str/Chl) and determine the effect of light intensity on Str/Chl within and across different microalgal candidates. Cultures of 4 species (Phaeodactylum tricornutum, Chaetoceros muelleri, Nannochloropsis oculata, and Pavlova lutheri) were acclimated to three light levels (40, 80 and 190 µmol photons m^-2 s^-1), and harvested in exponential phase. Chlorophyll concentration, specific growth rate, and total phytosterols were measured. Preliminary results show that Str/Chl increases with light intensity across all the candidates tested. The maximum varied by 4x between Pavlova lutheri and Nannochloropsis oculata (2.80 vs. 0.64 g g^-1). The covariance and limited inter-specific variability suggest that chlorophyll could be a useful screening tool for estimating phytosterol production in microalgae.