## 1 Introduction  
Microalgal biomass is currently considered as a promising resource for valuable biochemicals and biofuels [1, 2]. Especially, astaxanthin (3,3\({}^{\prime}\)-dihydroxy-\(\beta\)-carotene-4,4\({}^{\prime}\)-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]. Although 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 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].  
_H. pluvialis_ cell wall is difficult to destroy due to the considerable thickness and rigidity that are associated with its multiple layers formed during the transformation from green vegetative cells to astaxanthin-rich red cysts [10; 11]. Apart from astaxanthin recovery, a bioreinery process that can co-produce other cellular components, such as lipids, proteins, and carbohydrates, can significantly reduce the cost of large-scale _H. pluvialis_-based astaxanthin production [12; 13]. Accordingly, efforts to develop an environmental-friendly and cost-effective cell disruption process from _H. pluvialis_ is practically in progress. Various chemical and supercritical fluid extractions of astaxanthin from _H. pluvialis_ biomass have been extensively studied [14; 15]. However, the chemical treatment is potentially harmful with the excessive use of strong organic solvent (s) resulting in a lower stability and quality of astaxanthin [8]. The supercritical CO\({}_{2}\) extraction process requires longer operation time and, generally, the use of co-solvent(s) and/or physicochemical pretreatment. Furthermore, this method is very expensive for large-scale biorefining applications [8; 14].  
High-pressure homogenization has been proposed as a relatively simple and importantly scalable cell disruption method for industrial-scale microalgal biorefining processes among various mechanical techniques, including bead milling, high-pressure homogenization, ultrafine grinding, autoclave, and sonication [8; 12]. However, although there have been several studies on oleaginous _Chlorella_ and _Nanochlorosis_ species, especially for biodiesel production, application on harder aplanospore-forming species (e.g., _H. pluvialis_) is relatively limited [16].  
In this study, the efficiency of astaxanthin recovery according to cellular disruption level of _H. pluvialis_ cyst cells was investigated by controlling high pressures (10,000-30,000 psi) and passages (1-3) of a French press cell as a model high-pressure homogenization method. During the mechanical cell disruption, the cell types were classified as intact, leaky, and ruptured based on optical microscopy. The size distribution of cellular particles and spatial distribution of intracellular biomolecules (e.g., astaxanthin, lipids, proteins, carbohydrates, and chlorophyll) were also characterized while using a Coulter counter and staining-based fluorescence microscopy techniques, respectively. Specific insights on their morphological changes and distributions will be beneficial for efficient scale-up and biorefinery applications. Finally, the technical feasibility of a fast instrumental Coulter counter was investigated as an alternative to the conventional time-consuming microscopic cell classification as an indicator of the algal cell disruption level [17].