The PSI Photobioreactors are used for precise phototrophic cultivation of algae and cyanobacteria. They feature a unique combination of the cultivator and monitoring device. Light power and spectral composition as well as the temperature and aeration gas composition can be set with a high accuracy. In addition, cultivation conditions can be dynamically varied according to a user defined protocol. Light, temperature, and gas composition can oscillate with various amplitudes and frequencies. The growth of the cultures is monitored by the integrated densitometer (OD 735) that measures light scattering at 735 nm. Chlorophyll content of the culture can also be monitored continuously by the differences of optical densities at 680 and 735 nm (NDVI). The instantaneous physiological state of the culture is measured by the Photosystem II quantum yield (Fm’-Fs)/Fm’.