For optimal microalgal growth, several environmental parameters (e. g., temperature, light intensity, pH, and nutrient concentrations) must be kept within narrow physi­ological limits. The reactor system is critical in the provision and maintenance of a favorable growth environment (Pulz, 2001). Hence, reactor design requires knowl­edge of aspects of algal physiology, such as the morphology, nutrient requirements, and stress tolerance of the species to be grown. Some of the requirements for micro­algal growth are listed in Table 5.1, along with the consequences of under — or over­provision, and the relevant reactor design features.

5.2.1 Light

Light is the principal limiting factor in the culture of photosynthetic organisms (Pulz, 2001); therefore, the intensity and utilization efficiency of the light supply are criti­cal in reactor design (Kumar et al., 2010). The photosynthetic activity of microalgae changes in response to light intensity in three distinct regions. At low light intensities, cells are light limited and the photosynthetic rate increases with increasing irradiance. Once cells become light saturated, the rate of photon absorption exceeds the rate of electron turnover in Photosystem II (PS II), and there is no further increase in the photosynthetic rate with increasing light intensity. Once irradiance increases above

a certain point, cells become photo-inhibited due to damage to the photosynthetic apparatus, and the photosynthetic rate declines with further increases in irradiance (Chisti, 2007; Grobbelaar, 2009). In most algae, photosynthesis is saturated at about 1,700 to 2,000 pmol m-2s-1, while some plankton are photo-inhibited at much lower levels (130 pmol m-2s-1). Photo-inhibition occurs rapidly; irreversible destruction can occur in a few minutes, exceeding 50% damage after 10 to 20 minutes (Pulz, 2001).

In dense algal cultures under high irradiance (e. g., mid-day sunlight), it is likely that the illumination at the culture surface will be sufficient to induce photo­inhibition, while that a few centimeters below the surface will be insufficient for growth. The light conditions experienced by an individual cell within a reactor are constantly changing as a function of

• Culture depth or optical cross-section (the deeper or wider the culture vessel, the longer cells spend in low light conditions)

• Biomass concentration or areal density

• Turbulence induced by mixing (influences light-dark cycling as cells move in and out of the photic volume) (Grobbelaar, 2009).

The requirement for optimal light provision to all cells places unique constraints on the geometry of reactors. As light enters a culture surface, it is absorbed and scattered by the cells, particulate matter, colored or chemical substances, as well as the water itself (Grobbelaar, 2009). As cells at the surface absorb light, they shade those below them. Due to this mutual shading effect, light intensity decreases with culture depth. Light does not penetrate more than a few centimeters into a dense algal culture; therefore, optical depth must be minimized. Reactor scale-up is based on reactor surface area rather than volume, as in the case of heterotrophic fermenta­tions, and the surface-area-to-volume ratio is a critical parameter (Scott et al., 2010). Reactor design is a trade-off between maintaining a shallow depth, or thin optical cross-section, and the increased cost of reactor materials, decreased efficiency of mixing, and greater land area involved.

The density of the culture determines the attenuation of light with distance from the reactor surface. Given a certain reactor path length and light intensity, there will be a corresponding optimal cell density. Below the optimal areal density, all cells are exposed to excess light, and above optimal density, a significant proportion of the culture is in the dark (Grobbelaar, 2009). At the optimal density, given sufficient mix­ing, all cells are subject to equal light-dark fluctuations. Maximum photosynthetic efficiency occurs in relatively dilute cultures. The increase in productivity achieved by maintaining the optimal cell density for light provision must be balanced against the costs of the increased reactor volume and harvesting capacity required to process large volumes of dilute cell suspensions. In addition, a high volumetric yield does not necessarily mean that incident light is being most efficiently used. This is measured by areal yield, not volumetric yield, and for this there is an optimal areal cell density as well as cell concentration (Richmond, 2000).

Microalgal cells can become acclimated to high or low light conditions. In an effort to balance the activity of the light and dark photosynthetic reactions, cells modulate their light-harvesting capacity (e. g., through adjusting the number of PS II reaction centers and the pigment concentration), depending on the ambient light intensity. The process of photo-adaptation takes 10 to 40 minutes (Pulz, 2001). Due to the fact that a culture may become acclimated to prevailing light conditions, the optimal biomass concentration is different at high and low irradiance. It is therefore impossible to operate at a single optimum cell concentration when a range of irradi — ance occurs over the course of the day (Lee, 2001).

It has been postulated that there is a phenomenon known as the flashing light effect that leads to increased productivity at certain frequencies of light-dark cycling. Exposing cells to very short cyclic periods of light and darkness could counterbal­ance the two extremes of light over-saturation and inhibition. However, the effect of flashing light is very difficult to separate experimentally from the effects of the increased turbulence required to generate faster light-dark cycling (Grobbelaar, 2009). It is clear that enhanced mixing, up to a point at which cell damage begins to occur, is beneficial to optimal light provision by creating an average light intensity across the reactor volume by rotating cells between the light and dark phases of the reactor.

In addition to individual reactor design, the configuration of multiple reactor units can be designed for optimal light distribution. For example, placing plate reactors very close together dilutes strong light, which leads to an increase in photosynthetic efficiency. Overlapping tubular systems can also be used to dilute strong sunlight. However, the benefits of high photosynthetic efficiency may be offset by the increased cost of reactor hardware (Richmond, 2000).

The use of internal illumination can remove some of the surface-area-to-volume constraint on bioreactor design (Ugwu et al., 2008). Both natural and artificial light sources could be utilized, either using optic fibers to distribute solar energy inside the PBR or placing waterproof artificial illumination internally. Artificial lighting has the advantage that it can be used to supplement the light supply at night or dur­ing cloudy days. However, it adds to the operational cost and energy input; therefore, higher biomass yields are crucial (Ugwu et al., 2008; Kumar et al., 2010).