Abstract
Haematococcus pluvialis accumulates astaxanthin, which is a high-value antioxidant, during the red cyst stage of its lifecycle. The development of a rigid cell wall in the cysts hinders the recovery of astaxanthin. We investigated morphological changes and cell disruption of mature H. pluvialis cyst cells while using high-pressure homogenization for astaxanthin extraction. When treated with French-press-cell (pressure, 10,000–30,000 psi; passage, 1–3), the intact cyst cells were significantly broken or fully ruptured, releasing cytoplasmic components, thereby facilitating the separation of astaxanthin by ethyl acetate. Fluorescence microscopy observations using three different fluorescent dyes revealed that a greater degree of cell breakage caused greater external dispersion of astaxanthin, chlorophyll, lipids, proteins, and carbohydrates. The mechanical treatment resulted in a high cell disruption rate of up to 91% based on microscopic cell typing and Coulter methods. After the ethyl acetate extraction, the astaxanthin concentration significantly increased by 15.2 mg/L in proportion to the increase in cell disruption rate, which indicates that cell disruption is a critical factor for solvent-based astaxanthin recovery. Furthermore, this study recommends a synergistic combination of the fast instrumental particle-volume-distribution analysis and microscope-based morphologic phenotyping for the development of practical H. pluvialis biorefinery processes that co-produce various biological products, including lipids, proteins, carbohydrates, chlorophyll, and astaxanthin.
Highlights
Microalgal biomass is currently considered as a promising resource for valuable biochemicals and biofuels [1,2]
Three different fluorescent probes of Calcofluor white, Nile red, and FITC were used under fluorescence microscopy, respectively, in order to simultaneously visualize the cell wall and cytoplasmic polysaccharides, neutral lipids, and proteins of H. pluvialis cyst cells
The intact encysted cells showed a continuous blue color on their cell walls, because the H. pluvialis cell wall is mostly composed of mannan and cellulose polysaccharides [21,22]
Summary
Microalgal biomass is currently considered as a promising resource for valuable biochemicals and biofuels [1,2]. Astaxanthin (3,30 -dihydroxy-β-carotene-4,40 -dione), which is a secondary ketocarotenoid compound, has been widely applied in feed, cosmetic, and pharmaceutical industries, owing to its excellent antioxidant, anti-inflammatory, and anti-cancer properties [3,4,5]. Synthetic astaxanthin is extensively used as feed additives in aquaculture, natural astaxanthin is preferred over synthetic form for human consumption in view of its mixture of isomers and safety. Sci. 2020, 10, 513 concerns [6,7]. Several commercial-scale facilities currently produce natural astaxanthin from green microalga Haematococcus pluvialis having high astaxanthin content (~4%, w/w) [8,9]
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