A wide variety of open and closed reactor systems have been proposed for microal­gal cultivation, possibly reflecting the diversity in the physiology and requirements of different algal species. Ultimately, the overall goal is the continuous maintenance of a desired algal culture under conditions for optimal productivity. High volumet­ric and areal yields reduce cost by minimizing the reactor volume and land area required, respectively. Important factors in achieving this include (Richmond, 2000):

• Provision of sufficient light, despite daily and seasonal variations and dense algal culture

• Optimal mixing and mass transfer, while avoiding damage to cells by shear stress

• Minimization of deviation from optimal temperature (requires cooling in summer and heating in winter)

• Minimization of dissolved oxygen tension

• Simple cleaning and maintenance

• Minimization of energy input requirements

• Minimization of water use (e. g., evaporation from ponds, evaporative cooling use)

• Low capital and operating costs per unit of harvested product