## 1 Introduction  
Microalgae are a diverse group of microorganisms that combine properties of higher plants with those of some prokaryotes [1, 2]. They are ubiquitous, living either in aquatic (fresh and seawater) and terrestrial environments [3]. Microalgae occur as unicellular or simple colonial organisms, usually displaying rapid growth rates [4] and photosynthetic efficiencies between 9-10%, in general significantly higher than those of terrestrial plants [5, 6, 7]. They are key members of the biosphere, as they play an important role in the global carbon and nitrogen cycles. By using light energy to produce organic matter, algal photosynthesis contributes to around 50% of the total planetary primary production [8, 9]. Cyanobacteria, previously called \"blue-green algae\", are even able to fix atmospheric nitrogen (N\({}_{2}\)) into organic nitrogen and are the major contributors to global biological nitrogen fixation [10, 11]. Being a biodiverse resource, microalgae can produce a wide range of value-added molecules such as proteins, lipids, carbohydrates, pigments and other compounds for various productapplications [12; 13]. Of these, carotenoids, phycobiliproteins, and polyunsaturated fatty acids (PUFA) such as eicosapentaenoic (EPA) and docosahexaenoicenoic (DHA) acids are the most widely explored. Microalgae can be applied to food, feed, biofertilizer, and cosmetic industries [14]. They can also play a major role in the pharmaceutical industry as sources of novel biochemicals for the development of new drug leads to overcome the growing problem of antibiotic resistance in many bacterial pathogens [15], since some microalgae produce bioactive compounds with antibacterial activity [16]. Besides antimicrobial compounds, microalgae produce also antiprotozoan agents and anti-inflammatory, antioxidant and anticancer compounds [17]. The biomass of oleaginous microalgae is a sustainable, renewable and eco-friendly feedstock which can be a source for the production of biofuels and bioenergy [14; 18]. Microalgae can even be used in wastewater treatment and its photosynthetic cultivation is one of the most promising carbon dioxide (CO\({}_{2}\)) mitigation technologies [19]. The focus on research into the genus _Nannochloropsis_ has increased over the years [20]. These microalgae are considered to be oleaginous models because of their high lipid productivity and photosynthetic efficiency, having also a well-established genetic toolbox [21; 22; 23]. Several species of the genus _Nannochloropsis_ have been successfully cultivated autotrophically at large scale due to the relatively mature technology for outdoor large-scale cultivation of these eugtigmatophytes [22]. Despite their biodiversity, microalgae are still an underexploited biological resource. There are only a few long-standing companies that have been able to scale up their production of microalgal biomass in industrial facilities [12; 23; 24]. The economics of large scale facilities are often not favorable to short-term investment, because operational and capital costs are high and the payback is usually longer than expected [25]. Microalgae can be cultivated either in open or closed systems. Despite having several drawbacks, open systems are the most widely applied bioreactors in industrial microalgal processes, since they require lower capital and operational costs than closed systems [26]. Microalgal open production systems include circular and raceways ponds as well as cascade systems [27]. Among them, raceway ponds are the most commonly used open photobioreactor for commercial microalgal production [28]. These are shallow, hairpin-shaped, closed-loop flow channels, usually 0.15-0.30 m deep, internally lined with cement, polyvinyl chloride (PVC) or clay, with mixing and aeration provided through circulation by a rotating paddlewheel [29; 30; 31; 32]. A straight channel velocity of at least 0.2 m s\({}^{-1}\) is needed to ensure a velocity of 0.1 m s\({}^{-1}\), the minimum required to prevent sedimentation of microalgal biomass in the whole raceway [33].