Acclimation of Synechocystis sp. PCC 6803 to Alkaline pH Under Ambient Air

Cyanobacteria can thrive at a wide pH range from neutral up to pH 11 depending on the species. Even though cyanobacteria are alkaliphilic, only limited information on the metabolic acclimation to alkaline pH is available. In this study, we conducted a mass-spectrometry-based comparative proteomic analysis of Synechocystis sp. PCC 6803 grown in BG-11 medium buffered at pH 7.5, pH 8.2, and pH 9.2 under ambient air. When comparing cells grown at pH 8.2 to those at pH 7.5, only minor changes in the proteome were observed, with most of the altered proteins originating from the plasma membrane. However, when comparing cells grown under pH 9.2 to those at pH 7.5, a total of 235 proteins were differentially expressed. Synechocystis cells growing at pH 9.2 seem to display a different composition of bicarbonate transporters. Subunits of the ATP-dependent bicarbonate transporter BCT1 increased in abundance under pH 9.2 compared to pH 7.5 or pH 8.2, while the abundance of the sodium-dependent bicarbonate transporter SbtA was diminished. The increased contribution of the BCT1 complex to total bicarbonate transport at high pH was confirmed by evaluating the activity of different bicarbonate transporters. Furthermore, several changes in the abundance of ion transporters were observed, including upregulation of copper, potassium, and zinc efflux proteins. In contrast, the levels of several proteins involved in nitrogen assimilation and amino acid biosynthesis were reduced at pH 9.2. To conclude, the induction of several transporters indicates the cells' ability to regulate the internal ion and pH homeostasis as well as the carbon: nitrogen ratios under alkaline conditions.