There are two main types of microalgae cultivation systems: open ponds and closed photobioreactors (Moheimani 2012; Moheimani et al. 2011).

1.2.1.1 Closed Photobioreactors

Closed algal cultures (photobioreactors) are not exposed to the atmosphere and are covered with a transparent material or contained within transparent tubing. Pho­tobioreactors have the distinct advantage of preventing evaporation (Dodd 1986; Moheimani et al. 2011). Culturing microalgae in these kinds of systems have the added benefit of reducing the contamination risks, limiting the CO2 losses, creating reproducible cultivation conditions, and flexibility in technical design (Jeffery and Wright 1999). Closed and semi-closed photobioreactors are mainly used for pro­ducing high-value algal products (Becker 1994). In closed photobioreactors, the main challenge is being less economical than open ponds (Borowitzka 1996; Moheimani and McHenry 2013; Moheimani et al. 2013c; Pulz and Scheibenbogen 1998). A number of researchers have endeavoured to overcome a number of the limitations in closed including:

• reducing the light path (Borowitzka 1996; Janssen et al. 2002; Miron et al. 1999)

• solving shear (turbulence) complexity (Barbosa et al. 2003; Borowitzka 1996; Miron et al. 2003)

• reducing oxygen concentration (Acien Fernandez et al. 2001; Kim and Lee 2001; Rubio et al. 1999; Weissman et al. 1988), and

• temperature control system (Becker 1994; Borowitzka 1996; Carlozzi and Sacchi 2001; Morita et al. 2001; Rubio et al. 1999; Zhang et al. 1999).

Currently, the main disadvantages of closed systems are the high cost of con­struction, operation both for energy (pumping and cooling) and maintenance [such as cleaning and sterilization (Borowitzka 1996)], and scaling up difficulties (Grima et al. 2000; Janssen et al. 2002; Miron et al. 1999). However, if these difficulties can be overcome, these controlled closed systems may allow commercial mass pro­duction of an increased number of microalgal species at a wider number of locations.