Mixing is of paramount importance in microalgal culture systems as it is directly linked to other key parameters such as light provision, gas transfer, and nutrient provision. Good mixing keeps the cells in suspension, eliminates thermal stratifica­tion, determines the light-dark regime by moving cells through an optical gradient, ensures efficient distribution of nutrients, improves gas exchange, reduces mutual shading at the center of the reactor, and decreases photo-inhibition at the surface (Ugwu et al., 2008). Mixing affects the mass transfer rates of dissolved nutrients and gases by reducing the boundary layer between the surface of cells and gas particles and the bulk liquid (Grobbelaar, 2009). The synergistic effect on several parameters at once means that mixing efficiency has a strong effect on growth rate.

One of the major differences between open and closed reactors is the degree of tur­bulence achieved. Higher turbulences are more easily achieved in closed PBRs with narrow tubes or plates. Mixing in open ponds is typically provided by a paddlewheel or rotating arm. In closed reactors, mixing can be achieved mechanically (by pumping or stirring) or by aeration via a variety of gas transfer systems (e. g., bubble diffusers, pipes, blades, propellers, jet aerators, or aspirators). Stirring is efficient but incurs high mechanical stress. Mixing by gas injection is relatively gentle and efficient, but may require energy intensive gas pressurization. Gas introduced into reactors can serve a number of purposes, including supply of nutrients, control of pH, stripping of O2, and mixing. Bubbling of CO2 into the bottom of reactors is generally favored, although it achieves only moderate transfer efficiencies (13% to 20%) due to loss of CO2 to the atmosphere, fouling of diffusers, and poor mass transfer (Kumar et al., 2010).

Mixing must be almost continuous to prevent settling of biomass (Molina Grima, 1999) and can represent the major energy input into reactor maintenance. High rates of mixing can also impose shear stress on microalgal cells, particularly in filamen­tous species or those with delicate morphology (Greenwell et al., 2010). Mixing rates are therefore a trade-off among enhanced growth rate, cell damage, and energy requirement.