## 1 Introduction  
Numerous recent publications have dealt with the large potential of microalgae to become future producers of energy, food and bioactive compounds [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]. The underlying reasons for this are that they are nutritious in terms of lipid and protein and the fact that they \"grow\" by binary fission (division), making them capable of doubling their biomass every day. They can also thrive in both fresh and ocean water, and as photoautotrophs they are efficient CO\({}_{2}\) sequestration agents [13]. Microalgae thus have large potential in carbon capture and sequestration (CCS) as well as in carbon capture and utilization (CCU) processes [14, 15, 16]. Other promising and sometimes successful microalgae applications are (in operation, tested or planned) production of biogas, utilization of wastewater as inorganic nutrient source or production of biofuel [17, 18, 19]. Industrial mass production of microalgae has been pursued for more than a century and was probably first suggested and demonstrated by [PERSON] [20] with monocultures of _Chlorella vulgaris_. The \"father\" of photosynthesis, [PERSON] [21], also applied _Chlorella_ sp. in his seminal CO\({}_{2}\) assimilation experiment. After World War 2, interest in microalgae cultivation increased, and a book on the subject stated: \"... [the authors] hope that this book will result in increased progress toward the large-scale culture of algae\" [22]. Over the years commercial production has gained momentum in Europe, Asia, Japan, Australia and India [23], but it is mainly a few small green and blue-green species from the genera _Chlorella_, _Spirulina_, _Dunialella_, _Aphanizomenon_, _Haematococcus_, _Cryptecodimium_ and _Shizochutrium_ that are cultivated. The global annual production is difficult to estimate but probably adds up to ca. 23,000 tons [24; 25]. Considering the announced potential of microalgae to be a future source for biofuel and feed, this is highly meagre compared to the world soy production that is 360 \(\times\) 10\({}^{6}\) tons. Microalgae hence amounts to only ca. 0.05% of world soy production. Thus, mass cultivation of microalgae compared to other land-based crops must still be considered in its infancy. The reasons for this may be diverse, but the bottom line is that the production is expensive. Microalgae are present at much higher concentrations in PBRs than in their natural environments. The annual photoautotrophic production along, e.g., the coast of Norway, amounts to 100-200 g C m\({}^{-2}\)[26]. This equals an average production rate of merely 0.0001 g L\({}^{-1}\) day\({}^{-1}\) (DW). Compared to the 300-6000 times higher (0.03-0.6 g L\({}^{-1}\) day\({}^{-1}\) (DW) production levels aimed at in industrial PBRs [5; 8; 27], this illustrates some of the challenges one meets when attempting to mass cultivate microalgae for industrial purposes. Production is often hampered by complex and expensive production processes [28; 29]. This especially relates to energy use (e.g., pumping, illumination, complexity of reactors and de-watering) but also to contamination, lack of temperature control, troubles with gas exchange (CO\({}_{2}\) and O\({}_{2}\)), contamination and illumination [30; 31]. [PERSON] [32] stated that the production of 1 ton of _Chlorella_ would cost about USD 520 and can therefore not compete with inexpensive proteinaceous plant materials like, e.g., soy and bean meal. This is still a relevant statement as high costs hinder production of large volumes. Microalgae therefore appear on the world market as small volume niche products [33; 34], i.e., as pharmaceuticals, bioactives, nutraceuticalals, vitamins, enzymes and health food [6]. We stress that here we have focused on photoautotrophic algae and not so-called heterotrophic algae that utilize organic resources [35]. Compared to the large amounts of microalgae species that exist, i.e., perhaps more than 1,000,000 in total including 200,000 diatoms [36; 37], very few species have been tested out and exploited commercially.