Optimisation of physicochemical factors for enhanced astaxanthin accumulation in newly isolated freshwater green...

Microalgae are a rich source of valuable products like astaxanthin, a health-promoting ketocarotenoid, but their carotenoid production is challenging due to demanding culture conditions. Consequently, an attempt was made to isolate and screen a new microalgae strain for astaxanthin production via a two-stage cultivation method. In the first stage (vegetative phase), a nutrient-rich medium was used to promote cell growth and biomass, and in the second stage, astaxanthin accumulation was stimulated by stress conditions (0.2% NaCl and 4.4 mM sodium acetate) with continuous light illumination. The effect of physicochemical factors on cell growth, chlorophyll, and carotenoid content was intensively investigated. The microalgal strain was genomically identified as Desmodesmus sp. PLM2 using 18S rRNA sequencing. Optimised conditions, i.e. 20 mM sodium nitrate, 3 mM glucose, 32 mM potassium chloride, pH 7, and temperature 27 °C yielded a maximum biomass of 4.7 g/L, total chlorophyll content of 27.13 µg mL^-1, and total carotenoid content of 7.88 µg mL^-1. Further, in the red stage, stress induction led to the accumulation of carotenoid in the PLM2 strain. A comprehensive identification of the compound was done using various techniques including UHPLC, FT-IR, and Raman spectroscopy, all of which indicated towards the presence of astaxanthin. The strain Desmodesmus sp. PLM2 produced 20.2 mg/L of astaxanthin as per the UHPLC chromatogram. This study showed that Desmodesmus sp. PLM2 can be grown in two stages using the ideal physicochemical conditions, which could significantly aid in the industrialisation of microalgae for astaxanthin production.