The physiological requirements of microalgae determine the basic design of a photobioreactor. Phototrophic microorganisms capture sunlight or artificial light and transform light energy into chemical energy in the form of ATP and reduced NADPH which are essential for carbon fixation. A photobioreactor needs to supply cells with sufficient light and at the same time with enough carbon dioxide to build up carbon hydrates for anabolism and storage purpose. The generation of oxygen is stoichiometrically linked with carbon dioxide consumption. Accordingly, excess oxygen needs to be removed from the system. Furthermore, microalgae need inor­ganic nutrients and trace elements for growth. The stoichiometric demand of nitro­gen and phosphorous, for example, can be deduced from the elemental composition of the microalgae.

In terms of hydrodynamics, a photobioreactor represents a three phase system with the liquid system providing the inorganic nutrients which are dissolved in the broth. The gaseous phase supplies carbon dioxide and excess oxygen is removed from the system via gas bubbles. Eventually, the solid phase consists of cells. A fourth interacting component is the superimposed light radiation field.

The major challenge, and the function of a bioreactor, is to provide favorable conditions which allow for high productivities and avoidance of inhibiting or limiting effects. However, considerable gradients of CO2 and O2 can affect growth. Light that impinges on the reactor surface (I0) is absorbed and scattered and thus not all cells in the reactor receive light with the same intensity. Even local differences in concen­trations of inorganic nutrients can occur (Fig. 1). No photobioreactor concept provides optimal mass transfer and light distribution in a manner that the occur­rence of gradients is completely avoided.

Improvements of photobioreactor designs mainly focus on three key areas, namely light transfer, reaction (related to aeration), and hydrodynamics. It is not sufficient to address these aspects all separated from each other but there are significant interferences which mainly influence physiology, growth, and productivity in photobioreactors (Fig. 2). Yet in order to reduce complexity, experimental scale — down approaches try to separate interdependent influences to unravel underlying mechanisms influencing productivity [37].

These key aspects and their interdependency are further addressed in the follow­ing sections.