## 1 Introduction  
Microalgae are gaining prominence in the food, feed, and healthcare industry owing to their capacity to produce distinctive value-added products and biologically active compounds. As a result, the United Nations' \"Sustainable Development Goals\" (SDGs) advocate microalgae biotechnology platforms for the synthesis of industrially significant bioproducts [1]. Microalgae produce a myriad of naturally occurring bioactive compounds, prominently carotenoids and other lipids [2]. Likewise, microalgal carotenoids can exhibit a plethora of health benefits against viral, microbial, and cancer activities, in addition to the advantages of antioxidants and vitamin A precursors [3]. Microalgal carotenoids serve as natural pigments in addition to their bioactive molecular attributes in nutraceuticalals, food, feed, and cosmetics. This has led to a rise in consumption that has driven the pursuit for novel natural origins for carotenoids [4]. In recent years, the demand for natural and health protective compounds has grown tremendously. The US Food and Drug Administration has approved the application of numerous microalgae species in food and health [5]. Internationally, the carotenoids demand is, however, predicted to thrive with an average acquire profit of 3.9% [6]. Nonetheless, the industrialization of microalgae biotechnology remains difficult due to high operating and processing costs [7]. Numerous studies have been carried out in recent years to use stressors to reduce the expense of algal biotechnology. Nevertheless, commercialization is yet to be achieved because of the lower yield of value-added compounds and high cultivation costs [8]. Despite the lower production levels of intercellular compounds, a combination of abiotic factors can be optimized to obtain enhanced microalgae bioactive compounds. For maximal biomass production, microalgae require nutrient-sufficient conditions, although the basal amount of lipids can be adjusted by altering nutritional factors and cultivation conditions [9]. Previous research has identified light, nutrients, temperature, and pH as abiotic stressors that have a direct impact on microalgae development. Intercellular carotenoid synthesis increases during light stress to protect cells from photoinhibition [10]. Consequently, different microalgae species require different levels of light for growth and biomolecule synthesis [11]. Since the nutrients in the growth medium have a major influence on the development of microalgae, optimizing an appropriate growth media is critical for industrial scale-up. Nitrogen is an essential nutrient for biomolecule synthesis (protein, lipids, chlorophyll, and nucleic acids), metabolism, and cell growth [12]. Nitrogen and phosphorus insufficiency alters the carbon flux protein, triggering higher lipid deposition in microalgae [13]. Moreover, a lack of phosphorus increases lipid content, which causes triacylglycerlycerols (TAG) to build up during microalgal growth. The high production of biomass, lipid, and carotenoid necessitates the nutritional composition to be optimized [14]. Response surface methodology (RSM) is a computational and statistical design targeted at evaluating challenges associated with multiple independent factors with the intent to achieve the maximum response [15]. RSM was applied to optimize the variables predicated on the intricate dynamic interplay of multiple variables on biomass, lipids, and carotenoid production. It is widely known that stressful conditions could improve lipid content [16]. Nonetheless, little is known about the physiological and molecular components of the underlying processes. Recently, the ability of _Monoraphidium_ sp. FXY-10 to produce oil was reported by employing fulvic acid coupled with a two-stage culture, although its carotenoid-producing capability was not studied [17]. _Haematococcus_ sp. is a popular microalga for the commercial production of astaxanthin. Numerous studies have shown that _Monoraphidium_ GK12 possesses the ability to substitute _Haematococcus_ sp.