# Caroenoid Production from Microalgae: Biosynthesis, Salinity Responses and Novel Biotechnologies
## 1 Introduction  
Carotenoids are a class of terpenoid pigments with C40 backbones and are health-promoting for human daily diets. Up until now, over 1100 carotenoids have been discovered, and they exist in various species [1], especially in aquatic creatures, microorganisms and terrestrial plants. Carotenoids are very common in our daily lives; tomato fruit is rich in lycopene (pink red), maize corn is abundant with zeaxanthin (yellow) and carrots and _Dunaliella salina_ are well known for producing \(\beta\)-carotene (orange) [1]. Carotenoids ingested by humans can work as precursors of Vitamin A [2], reduce free radicals [2] and repair damaged retina [3]. Moreover, they can also reduce the risks of some diseases as they have anti-cancer, anti-inflammatory and anti-obesity properties [4]. Hence, carotenoids have been widely used in the food, feed, cosmetic and pharmaceutical industries. With the development of modern biotechnologies and market concern for food safety, the demand for carotenoids from natural sources is increasing remarkably. Compared with plant-derived carotenoids, those from microorganisms are more efficient, have lower cost and are not limited by regions and seasons. However, the carotenoids market is still mainly occupied by chemically synthetic products (80-90%), with a much lower portion of natural sources (10-20%) [5]. Thus, choosing low-cost, profitable and safe microorganisms for carotenoid production is drawing extensive interest. Microalgae are diverse and abundant. They can adapt to different cultivation conditions, grow rapidly and accumulate a high amount of desirable bioproducts, such as fattyacids, proteins and carotenoids. The global algae market is expected to reach 970 million U.S. dollars by the end of 2025 [6] and that of carotenoids is expected to reach 2 billion U.S. dollars by 2026, mainly including food and beverages (26.1%), pharmaceuticals (9.2%), cosmetics (6.5%) and dietary supplements (23.5%) [7]. Some species, such as _Chlorella_ and _Spirulina_, have GRAS status and are well accepted as health foods [8]. Microalgae have versatile metabolic modes and tend to accumulate different metabolites. Different trophic modes (autotrophy, mixotrophy, heterotrophy), culture conditions (light, pH, dissolved oxygen) and nutrient availability (repletion or depletion) will lead carbon skeletons to have significantly different cell compositions. For primary metabolites, light and adequate nutrients are essential for active growth. On the contrary, adverse stresses, such as nutrient deficiency, high light and salt stress, are key factors that arouse the defense mechanisms of microalgae and accumulate secondary metabolites to survive. Salt stress is a common stress factor in natural environments, especially for freshwater microalgae. Salt stress can lead to oxidative damage, chlorophyll degradation and inhibit photosynthesis and growth [9]. To adapt to salt stress, microalgae have evolved a survival strategy to guarantee the balance of growth and stress responses. During the adaptation to stress, some microalgae may become cysts and accumulate secondary metabolites, such as astaxanthin and triacylglycerored [10]. However, serious salt stress can be fatal to microalgae when the cells are in low concentration or poor viability. Thus, two-stage cultivation, balancing cell density and metabolites accumulation, is a popular strategy in the practical production of high-value compounds from microalgae [11]. Due to the shortage of freshwater resources, overcoming the salt tolerance problem of freshwater microalgae is also of invaluable significance for microalgae cultivation in seawater. This review comprehensively introduces the production of high-value carotenoids and their beneficial effects on human health. The latest research results are summarized, and biotechnology topics are focused on, such as yields of microalgae-derived carotenoids and microalgal cultivation strategies. Meanwhile, the contribution of salt stress on the growth and carotenoid accumulation of microalgae is also discussed.