# Biomolecules from Microalgae and Cyanobacteria: Applications and Market Survey
## 2 Microalgae
### Compositions and Technologies for Photoautotrophic Production of Microalgae and Cyanobacteria
#### 2.3.6 Technologies for Photoautotrophic Production of Microalgae and Cyanobacterial Biomass  
Given the great interests of algae and their broad spectrum of applications, the world is invited to push and encourage the production and cultivation of these organisms. Furthermore, until 2011, global algal exploitation remained marginal, with 15 million tons/year of macroalgae and 4 billion tons/year of terrestrial plants against 10,000 tons/year of microalgae [38]. New estimates show that between 2016 and 2018, the production of microalgae, including Cyanobacteria, bounded by 19,000 and 20,000 tons per year. By 2024, their expected commercial production will be 27,500 tons per year, with a market value of USD 1.1 billion [87]. The challenge nowadays will be to improve current cultivation methods to scale up this production. Macroalgae are cultivated in a natural marine environment, while microalgae and Cyanobacteria are cultivated in reactors, either in photoautotrophy (traceways or photobioreactors) or in heterotrophy [38]. These two production systems remain, in fact, the most practical, as mentioned next. Open ponds are the most exploited processes for the industrial culture of microalgae and Cyanobacteria due to their ease of use and low cost. These systems generally use natural light and vary according to their shape, the type of materials used, as well as the mixing system. They are mainly classified into two types: circular and \"raceway\" ponds [88]. The first open ponds, qualified as natural basins, were without agitation and generally had low cell productivity. In order to overcome this problem, mixing systems with a circular shape and centrally pivoted rotating agitator were then put in place for the production of biomass, animal feed (aquaculture), and dietary supplements based on _Arthospira/Spirulina_ (Cyanobacteria) or _Chlorella_ (Chlorophyta) [89]. Open raceway tanks later appeared, characterized by an elongated shape, and they are currently the most widely used artificial systems for microalgal cultivation. This design consists of a closed-loop flow channel with a typical culture depth of approximately 12 to 50 cm, in which the mixing and circulation of the microalgae culture are promoted by paddle wheels [90]. Light intensity is considered among the parameters that strongly influence the growth rates of microalgae in outdoor cultures. Moreover, with such a mixing system, microalgae have better access to sunlight, which can come only from the top. Besides light, dissolved CO\({}_{2}\) concentration is another important parameter for productivity in these systems. Indeed, atmospheric CO\({}_{2}\) uptake by microalgae and Cyanobacteria is not sufficient for optimal growth. In open reactors, supplementary to light and CO\({}_{2}\) availabilities, factors such as temperature and the mixing and availability of nutrients are involved in reaching optimal growth and are strongly affected by the culture depth [90]. Photobioreactors (PBRs) are closed systems that consist either of horizontally or vertically arranged tubes or vertically arranged panels, wherein microalgae or Cyanobacteria are grown. This technology offers better control of all culture parameters such as pH,temperature, oxygen, and agitation. They are well suited for microalgae sensitive to contamination, such as extremophilic microalgae. Depending on location and metabolite production, they may or may not be artificially illuminated. During the last decades, the marketing of industrial PBRs with different designs and efficiencies has increased the yields of biomass production. Several shapes are currently available, such as sleeves, tubular reactors, annular reactors, flat reactors, rigid panel type reactors, and \"airlift\" type reactors (bubble column) [88; 91]. Microalgae and Cyanobacteria can be harvested by mechanical, chemical, biological, and electrical methods. They are based on the principles of separation by the density difference or cells sizes.