An adequate mixture should provide a high concentration of biomass, enable the circula­tion of liquid, keep the cells in suspension, eliminate thermal stratification, optimize the dis­tribution of nutrients, improve gas exchange, and reduce the shading and photoinhibition of microalgae. Turbulent flow is essential for maximum production of microalgae in open ponds. In raceway cultures, velocities of 5.0 cm. s-1 are sufficient to eliminate thermal strat­ification and maintain most species of algae in suspension.

Several mixing systems are used in microalgal cultures, depending on the type of reactor. In open pond systems, paddlewheels are used to induce turbulent flow. In stirred-tank photobioreactors, impellers are used to mix the algal cultures. In tubular photobioreactors, mixing can be carried out directly or indirectly through airlift systems (Ugwu et al., 2008).

The main costs of growing microalgae arise from the mixing and mass transfer in cultures (using paddlewheels, impellers, and airlifts) because of the energy consumed. For the race­way pond, the mixing cost is €0.08 per kg DW (dry weight), for the tubular reactor it is €1.27, and for the flat panel reactor it is €3.10 per kg DW (Norsker et al., 2011).

The mechanical stirrers (paddlewheel) provide optimal efficiency of mixing and gas transfer, but they cause significant hydrodynamic stress. Gas injection (bubbling) by airlift or impellers causes low hydrodynamic stress, good transfer of gas, and a reasonable mixing efficiency (Richmond and Cheng-Wu, 2001).

In closed photobioreactors, where the mixing is carried out by impellers or airlift, the increase of the speed of the gas bubbles enlarges the diameter of the bubbles (Ugwu et al.,

2008) . The bigger the bubbles, the lower the exchanges of gases with the liquid.

A high concentration of oxygen produced by photosynthesis inhibits microalgal growth. The supply of gas with the turbulent labor regime in closed photobioreactors is one solution to reduce this negative effect. However, depending on the microalgal species, high turbulence can cause damage to cells due to stress and high energy consumption (Pires et al., 2012). Low mixing results in an accumulation of toxic compounds in stagnant areas. In open ponds, oxygen has low solubility and rapid outflow since the photobioreactors are low in height.

Open systems are currently still the preferable choice for microalgal production on a large scale, especially when they are designed to produce low-priced products, such as biofuels. However, due to the requirements of good manufacturing practice (GMP) guidelines, pro­duction of high-value products from microalgae for application in pharmaceuticals and cosmetics seems feasible only in well-controlled photobioreactors with closed system ope­rations. Therefore, several closed systems (photobioreactors) for microalgae cultivation are discussed here.

The term closed systems refers to photobioreactors that have no direct exchange of gases and contaminants between the cultivation systems and the outside environment. The necessary gas exchange is performed through a sterilized gas filter, to avoid contamination inside the culture system. Therefore, closed systems are characterized by the minimization of con­tamination over open systems. Besides the typical drawback of high equipment cost, closed- system photobioreactors do have several major advantages over open systems (Singh and Sharma, 2012): (1) Photobioreactors could minimize contamination and allow axenic algal cultivation of monocultures; (2) photobioreactors offer better control over conditions such as pH, temperature, light, CO2 concentration, and so on; (3) using photobioreactors leads to less CO2 loss and prevents water evaporation; (4) photobioreactors permit higher cell con­centrations; and (5) photobioreactors permit the production of complex biopharmaceuticals.

There are several types of closed systems designed and developed for the cultivation of microalgae, including vertical (tubular) columns, flat plate photobioreactors, and horizontal tubular photobioreactors. The detailed descriptions of those cultivation systems are provided here. In addition, their advantages and weaknesses are summarized and compared in Table 2.2.