## 1 Introduction  
Synthetic carotenoids are ubiquitously available since they can be rapidly produced using inexpensive chemicals, therefore reducing the production costs associated with cultivating, harvesting, and extracting from a living organism [1]. Although synthetic carotenoids, including synthetically produced astaxanthin (S-AX), are more abundant on the market, and faster and cheaper to produce, their health-promoting and antioxidant properties are not as effective as their natural counterparts, hence making them a less desirable product. Astaxanthin is a powerful natural antioxidant. It can be extracted from krill, yeasts, marine bacteria, and microalgae (e.g., _Chloorella zofingiensis_, _Chlorococcus_ sp.) [2, 3]. The most common source of natural astaxanthin (N-AX) on the market is derived from the freshwater microalgae _Haematococcus pluvialis_[4]. The natural form of astaxanthin has a stronger antioxidant activity. The final product composition also differs in the two forms: natural astaxanthin (N-AX) can potentially carry other carotenoids (i.e., \(\beta\)-carotene, lutetium, and canhaxanthin) [5], bringing additional value to the market, while the \(\lx@sectionsign\)AX contains synthetic astaxanthin and potentially synthesises intermediates with unknown effects [6, 7]. For this reason, S-AX is still not currently permitted for human consumption [8],however, it is approved by the European Food Safety Authority (EFSA) and the United States Food and Drug Administration (US FDA) as an aquaculture feed additive. The enhanced performance of natural astaxanthin compared to synthetic variants has made it a much more in-demand and valuable product. Currently, the market value of astaxanthin, depending on products' purity, varies from $2500 for S-AX and >$7000 kg for N-AX derived from _H. pluvialis_[8; 9; 10]. The global market demand for both forms is estimated to exceed $880 million by 2026 and a compound annual growth rate (CAGR) of 8.2% by 2027 [11]. Furthermore, N-AX production accounts for less than 5%, of which only less than 1% is _H. pluvialis_ derived [12]. The high market value is caused by the scarcity of natural astaxanthin due to its difficult production process. _Haematococcus pluvialis_ has a complex life cycle, undergoing several life stages before encysting and accumulating astaxanthin [13]. It involves different morphotypes: a green vegetative motile cell; a green vegetative palmella cell; a palmella cell with initial astaxanthin accumulation in transition to an aplanospore; and finally, an aplanospore cell with accumulated astaxanthin [14]. _H. pluvialis_ industrial-scale cultivation commonly employs a two-step process: first, it aims to achieve high biomass yields; and secondly, the biomass is exposed to a stressor (e.g., high light) to induce carotenogenesis and hence the accumulation of astaxanthin [13]. In this first step, _H. pluvialis_ is conventionally grown in autrophy to reduce the risk of biological contamination, and to facilitate better operator control of the transition between green and the red phases. There has been considerable interest in the research and development of the green phase of _H. pluvialis_ during the past ten years due to the high value of N-AX and the challenges involved in _H. pluvialis_ cultivation. In fact, due to its complexity, any small modification in the cultivation cycle that results in faster growth rates or higher overall biomass/astaxanthin yield can represent a significant economic improvement. The long vegetative growth contributes to the high production costs, especially in terms of nutrient supply. Among nutrients, nitrogen is needed not only for cell growth but also to produce the enzymes that catalyse astaxanthin synthesis and accumulation in _H. pluvialis_. Microalgae can uptake and assimilate different nitrogen sources (NS), i.e., nitrate, nitrite, ammonium, and urea; nitrogen assimilation utilising urea requires a different set of enzymes from nitrate, nitrite, and ammonium, which share the same metabolic pathway.