Factors governing algal growth in photobioreactors: the “open” versus “closed” debate

Until recently, most large commercial scale microalgal production systems employed open systems. However, several large-scale closed systems have now been built and, for the first time, actual comparisons can be made. There are major operational differences between open and closed photobioreactors and, consequently, the growth physiology of the microalgae is different between the two systems. Several of the factors governing growth can, within certain boundaries, be manipulated while others are specific to the cultivation system. Crucial factors are the optical depth, turbulence, light acclimated state of the organism, nutrient availability and metabolite accumulation. In the final analyses, systems are used for specific purposes and each will determine which system is the most suitable, since there is no universal all-purpose photobioreactor.