Optimal supply of nutrients, mainly carbon, nitrogen, and phosphorous, along with various other macro — and micronutrients required for algal growth, is a prerequisite for high growth rates. Deficiencies in any nutrient cause disturbances in metabo­lism, physiological changes, and decreased productivity (Pulz, 2001). The supply of nutrients to the culture is relatively simple, but the supply of nutrients to individual cells depends on efficient mass transfer, which is related to mixing and gas sparging (Grobbelaar, 2009). Nutrients are also a significant cost in microalgal cultivation; therefore, design of the reactor system to allow for efficient recycling of culture medium is essential (Greenwell et al., 2010).

Nutrients, with the exception of light and carbon, are generally provided in the liquid growth medium. Carbon is a major constituent of algal cells (often com­prising 50% of the dry weight), usually obtained from carbon dioxide (CO2) gas (Chisti, 2007). The concentration of CO2 in air (0.04%) is suboptimal for plant growth; therefore, for optimal productivity, CO2-enriched air must be supplied (Pulz, 2001). CO2 may be available from flue gas or other waste gas streams, but the cost of gas compression and extensive sparging systems for arrays of PBRs is significant. The location of large algal plants sufficiently near the source, along with the safety concerns of large-scale distribution of flue gas (low in O2 and high in CO2, NOX, and SOX) at ground level, could present its own challenges (Scott et al., 2010).

In cases of carbon limitation, the efficiency of mass transfer of CO2 from gas to liquid form in the culture medium becomes critical to productivity. Certain algal species can grow heterotrophically or mixotrophically, in which case all or some of the carbon and energy requirements can be supplied from an organic carbon source such as glucose or acetate (Lee, 2001). The use of heterotrophy can reduce the dependence of productivity on light and CO2 supply, which releases some of the key constraints on reactor design (Pulz, 2001). Heterotrophic cultivation of micro­algae in sterilizable fermenters has achieved some commercial success, although biomass productivity has yet to match that of yeast and other heterotrophic organisms (Lee, 2001).

Mixing and nutrient concentrations are also linked to pH control. Mixing promotes reactions of CO2 with H+, OH-, H2O, and NH3 in the medium, which affect the pH and hence CO2 uptake rates (Kumar et al., 2010). The pH increases along the length of tubular reactors due to consumption of CO2. In long reactors, CO2 injection points may be necessary to prevent a rise in pH above optimal levels (Chisti, 2007). CO2 addition is commonly controlled by feedback from a pH meter (Carvalho et al., 2006).

The removal of toxic metabolites is also critical to the efficiency of growth and photosynthesis. Under high irradiance, oxygen generation in closed PBRs can be up to 10 g O2.m-3min-1. Maximum dissolved oxygen levels should not exceed 400% saturation (with respect to air-saturated culture) (Chisti, 2007). A build-up of O2 in the reactor can cause the key carbon-fixing enzyme RuBisCO to bind oxygen instead of carbon dioxide, leading to photorespiration instead of photosynthesis (Dennis et al., 1998). High oxygen concentrations, in addition to intense light, lead to the formation of oxygen radicals that have toxic effects on cells due to membrane dam­age (Molina Grima et al., 2001; Pulz, 2001). Many algal strains cannot survive in O2 over-saturated conditions for more than 2 to 3 hours. High temperatures and light intensify the damage (Pulz, 2001). Oxygen build-up limits the maximum length of a closed tubular reactor. Typically, a continuous tube should not exceed 80 m (Molina Grima et al., 2001), although the exact length depends on biomass concentration, light intensity, liquid velocity, and initial O2 concentration. In a closed reactor, culture must continuously return to a degassing zone, where it is bubbled with air to strip the O2. The degassing zone is typically optically deep compared with the solar collector, and hence poorly illuminated; thus its volume should be small relative to the solar collector (Chisti, 2007).

In high-density algal cultures, the key challenges in nutrient provision are in mass transfer of CO2 to cells and O2 away from cells. Efficient mixing and aeration, with­out inducing shear stress and requiring excessive energy input, are important param­eters. Bubbling of gas through cultures can be used to simultaneously introduce CO2, strip O2, and mix the culture broth (e. g., bubble columns and airlift reactors). The overall mass transfer coefficient (kLa) of the reactor is an important parameter in determining the carbon supply. The kLa depends on reactor geometry, agitation rate, sparger type, temperature, mixing time, liquid velocity, gas bubble velocity, and gas holdup (Ugwu et al., 2008).