Identification of growth limiting factors in Synechocystis sp. PCC 6803

Abstract

In this study we showed that Synechocystis sp. PCC 6803 can achieve fast division rates using urea, a low-cost nitrogen source that does not require energy for uptake and hydrolysis, and releases CO2 into to the cell during its assimilation. However, even using urea, we still observed a gradual decrease of growth rates in batch cultures. This was not explained by physiological or metabolic limitations. Further analyses also excluded the occurrence of quorum sensing or nutrients limitations and only self-shading (light limitation) had a moderate effect under moderate light intensity. The results indicate that growth rates are negatively affected by increases in cell density and Synechocystis likely present a cell-cell interaction mechanism that enables it to sense the increase of culture density and adjust its division rates to enter in stationary phase before facing light and nutrient limitations. This mechanism could not be bypassed with modifications of growth conditions, although the length of the lag phase was varying with the light period in a temperature dependent way. The variations observed in the length of the lag phase are not explained by variations in the DNA replication rate or differences in the evolution of cell volume, and, thus, possibly might not be linked with photosynthesis and metabolism. Hence, it is possible that the internal signal triggered by cell-cell interaction is present when cells are transferred from high to low densities and the initiation of cell division may demand the depletion of this signal, which probably happens faster when Synechocystis is cultured in higher temperatures. Finally, molecular analysis showed that pili structures are not related with sensing cell density and, therefore, further efforts are necessary to identify the cell wall components and signalling pathway involved in the adjustment of growth rates by cell-cell interaction.