# Propagation of Inoculum for _Haematococcus pluvialis_
## 1 Introduction  
Microalgae are microscopic unicellular organisms that can convert solar energy to chemical energy through photosynthesis [1]. Despite the small size, they are producers of 60% of the Earth's oxygen. To this date, 30,000 species of microalgae have been identified [2]. Few hundred species are being analyzed to determine their chemical composition and metabolite production abilities. Only a few strains are cultivated on an industrial scale to be used in the production of commercial products [3]. This group includes, among others: _Spirulina_, _Chlorella_, _Haematococcus_, _Dualiella_, _Botryococcus_, _Phaeodactylum_, _Porphyridium_, _Chaetoceros_, _Crypthecodimium_, _Isochrysis_, _Nanochloris_, _Nitzschia_, _Schizolchytrium_, _Tetraselmis_, and _Skeletonema_[4]. Microalgae are very resistant to environmental conditions. Due to their ability to utilize solar energy more efficiently, microalgae have the higher photosynthetic potential for producing valuable compounds or for energetic use when compared with higher plants [1]. In optimal cultivation conditions, they produce protein and biomass from 30 to 100 times faster than terrestrial plants [2]. Microalgae can be used to produce a wide range of metabolites, such as protein, lipids, carbohydrates, carotenoids, or vitamins [1]. These microorganisms are widely known as a valuable and renewable raw material used by various economy sectors, especially by food, feed, pharmaceutical and cosmetic industry. They are also used to produce bio-fertilizers and biofuels as well as for removing pollutants from wastewater [5; 6]. One could say that microalgae are natural bioreactors that enable the productionof various metabolites, which are considered to be highly valuable products. Microalgae cultivations are an interesting alternative to the conventional synthesis of the selected chemicals [2]. Unlike most vascular plants, microalgae cultivation does not require soil, and the cultivation installations take up limited space [7]. Cultivation ponds and tanks may be used in the desert, on rocky terrain and idle agricultural land. Production of microalgae will not deplete supplies of drinking water as it may take place in seawater, brackdish water, or wastewater [8]. Microalgae, used for commercial purposes, are currently being produced in two main types of cultivation systems: open and closed. The open system, which utilizes both natural and artificial ponds (raceway ponds), is the easiest microalgae cultivation method [9]. The artificial ponds are similar in structure to small ponds or shallow channels. Paddle wheels, among other measures, are used to force circulation of the growth medium [10]. In the open ponds, microalgae absorb all sunlight necessary for their growth. There is no need to use artificial lighting [11]. The essential advantage of open ponds is the fact that they are easy to build, and they enable to cultivate microalgae in their natural environment. The disadvantage of these types of tanks is the high possibility of contamination [2]. The second method of cultivating microalgae is based on using closed systems known as photobioreactors (PBR). They are specially designed transparent tanks in various sizes and shapes, intended for cultivating microalgae in the optimal conditions, automatically controlled by means of process monitoring equipment [11]. Photobioreactors solve the problem of evaporation and susceptibility to contamination, which are the biggest problems in the case of the open ponds systems [9]. Transparent walls of PBRs allow for the light to reach the inside of the photobioreactor. In comparison to the open ponds, photobioreactors allow for obtaining higher productivity of biomass as well as easier, and as a result, cheaper harvesting [2]. Unfortunately, the costs of cultivating microalgae in closed systems are high. For that reason, these systems are applied commercially mainly to produce high-value compounds [12].