## 1 Introduction  
Cyanobacteria and eukaryotic microalgae produce an extremely large number of currently largely unknown metabolites that could be potentially useful in some fields of biotechnology. Only about 10% of known cyanobacteria and eukaryotic microalgae have been isolated and cultured in the laboratory, and the biomass or metabolites of only about 10 species were commercially available to the end of the 20 th century [1]. Since then, the demand for new bioactive compounds and innovative products is constantly increasing. Microalgae for human and animal consumption have long been obtained from natural sources, and this method is still used by some commercial and healthcare companies. In this case, although there is no cost of culturing, adequate productivity and product quality are not ensured [1,2]. These problems can be eliminated by mass production in artificial systems. Obviously, the more complex the artificial system, the higher the production costs, and thus, the market price of the product. The main problem in the cultivation of photosynthesizing microorganisms, including algae, is providing the right amount of light energy since in photoautotrophic cultures, the other necessary factors (carbon source, nutrients, temperature) are more readily available in a controlled environment [1]. Algae cells can absorb light very efficiently, so light can be absorbed completely within a few mm in the culture, even at very low densities. Therefore, during the design of culturing systems, the main goal is keeping the thickness of the medium to a minimum and the surface area to a maximum [1,3,4]. Products from microalgae cultured in open ponds can usually only be used as dietary or feed supplements/additives since the exclusive presence of the algae strain, and the purity of the desired product cannot be guaranteed. This excludes biomass and products from markets where high purity is essential [5]. The application of open ponds for production is also hampered by the fact that in order to maintain the monoculture, usually extreme conditions (e.g., high salinity, unusual nutrient conditions) have to be established, which significantly reduces the number of microalgae species that can be taken into account. The need to culture microalgae living under average conditions in monoculture has led to the development of closed photobioreactors [6]. Photobioreactors can be most simply described as illuminated, closed vessels in which biomass is produced under controlled conditions. Algae are not in direct contact with the environment, so there is no possibility of unwanted gas exchange, contamination, or evaporation of the culturing medium [2]. The most important factor to consider during a photobioreactor design is the introduction of sufficient light and proper mixing, preferably in all parts of the device [7]. Two secondary problems also arise from the primary one: scaling up and sterility. Scaling up requires keeping a small layer thickness and a large surface area to receive sufficient light for the algae. Besides this, adequate mixing, sufficient gas supply, sterilizability, and adequate productivity should be maintained. Despite these difficulties, there are many experimental and a few industrially applied designs [8,9]. Nowadays the most common and most widely used types are tubular photobioreactors. Basically, these tubes are made of transparent material (glass or plastic) installed with a wide range of complementary equipment [10,11]. They have good light distribution [12] and are considered particularly suitable for outdoor mass production [11]. The tubes can be placed horizontally, vertically, tilted, or coiled in some form (cylinder, cone; [10,11,13,14,15,16,17]). The diameter of the tubes is usually up to 0.1 m due to the limited light transmission of dense cultures [10,11], and their length does not exceed 80 m in the presence of a degasser. The longer the tube, the more difficult the gas exchange, resulting in the loss of productivity [18].