Increasing the Photoautotrophic Growth Rate of Synechocystis sp. PCC 6803 by Identifying the Limitations of Its Cultivation

Many conditions have to be optimized in order to be able to grow the cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis) for an extended period of time under physiologically well‐defined and constant conditions. It is still poorly understood what limits growth of this organism in batch and continuous cultures in BG‐11, the standard medium used to grow Synechocystis. Through a series of batch experiments in flasks and continuous mode experiments in advanced photobioreactors, it is shown that the limiting nutrient during batch cultivation is sulfate, the depletion of which leads to ROS formation and rapid bleaching of pigments after entry into stationary phase. In continuous mode, however, the limiting nutrient is iron. Optimizing these growth conditions resulted in a so far highest growth rate of 0.16 h−1 (4.3 h doubling time), which is significantly higher than the textbook value of 0.09 h−1 (8 h doubling time). An improved medium, BG‐11 for prolonged cultivation (BG‐11‐PC) is introduced, that allows for controlled, extended cultivation of Synechocystis, under well‐defined physiological conditions. The data present here have implications for mass‐culturing of cyanobacteria. Properly controlling cultivation conditions for Synechocystis is required for optimal use in lab and industrial applications alike. The traditional growth medium, BG‐11, for continuous cultivation is optimized and the growth‐limiting nutrient of the medium in batch cultivation is identified. Additionally, the maximum growth rate of Synechocystis is approximately doubled, which drastically changes how this model organism for biotechnological applications should be viewed.