## 1 Introduction
These agents are important bioactive components in antiaging serums, sunblock creams, and beauty products, and are also provided as oral dietary supplements at a recommended dose of 4 mg per day. They also have anti-gastric, anti-cancer, anti-inflammatory, anti-diabetic, anti-obesity, ocular-protective, and immune response-boosting properties, and many studies are currently underway to explore their immense potential for biological applications [17; 18; 21; 22; 23]. Natural sources of astaxanthin include marine bacteria, fungi, higher plants (_Adonis_ species), krill, and microalgae, such as _Chlorococcus_ spp., _Chorella zofingiensis_, _H. pluvialis_, _Dunaliella salina_, etc. Among these, the green alga _H. pluvialis_ is regarded as the most abundant source of natural astaxanthin on the market [17; 19; 21; 22; 24; 25]. _Haematococcus pluvialis consists of_ green ovoid-spherical biflagellate cells containing chloroplasts; they are motile and swim freely in the initial green vegetative stage. In the subsequent intermediate stage, the cells turn orange-green within 7-10 days, and astaxanthin begins to accumulate with the development of a thick red cell wall containing the encrystent system. Following these stages, the red stage appears within 11-14 days under unfavourable growth conditions, in which the cell size increases, flagella are lost, and orange-red astaxanthin droplets accumulate rapidly in the perinuclear cytoplasm [15; 22]. Under such unfavourable conditions, _H. pluvialis_ accumulates up to 4% dry weight (DW) of astaxanthin for resistance against oxidative stress and photoinhibition [9]. Astaxanthin accumulation in a shorter time has also been attempted using high-light irradiation and specific inhibitors of cytochrome P450 enzymes. Although these results have not been conclusive, further research may lead to further clarification [26]. Generally, large-scale cultivation of microalgae is achieved using one of two types of systems: open systems, in which microalgae are cultured in an open environment, and closed systems, in which they are grown in photobioreactors or closed vessels [27]. Open-pond cultivation allows the extensive production of microalgal biomass in a natural environment. These ponds are categorised as raceway ponds, unmixed ponds, shallow lagoons, circular ponds, and inclined ponds based on their architectural features and modes of operation. Among these types, raceway ponds are the most popular due to their simplicity, low cost, ease of operation and maintenance, low power demands, and utilisation of solar energy [28; 29; 30]. Various systems are employed for stirring and blending the culture and avoiding sedimentation in open raceway ponds; these include paddle-wheels, propellers, impellers, air lifts, Archimedes screws, and water jets. Some ponds also feature a clear plastic cover that reduces their vulnerability to contamination from airborne dust particles. Covering the ponds with plastic allows them to retain heat in colder climates, whereas cooling systems allow temperature adjustment in hotter climates [30]. However, open-pond systems require physiochemical supervision and are susceptible to the risk of biological contamination, mainly from birds, as well as drastic climatic changes, heavy metal and microplastic pollution, and biomass yield and water losses due to evaporation. Absolute isolation from the external environment, continuous water supply to compensate for water loss, high cell density, contamination avoidance, and harvest system improvements can minimise the drawbacks and maximise the operational effectiveness of open-pond systems for natural astaxanthin production at large scales [1; 6; 29; 31]. Photo bioreactors or closed microalgae cultivation systems also have diverse designs (e.g., tubular, fermenter-like, or laminar reactors). The benefits of closed systems include less contamination, reduced carbon dioxide loss, a regenerative cultivation environment, better regulation of the cultivation parameters, and improved yield.