Involvement of potassium transport systems in the response of synechocystis PCC 6803 cyanobacteria to external pH change, high-intensity light stress and heavy metal stress

The unicellular photosynthetic cyanobacterium, able to sur- vive in varying environments, is the only prokaryote that directly converts solar energy and CO2 into organic material and is thus relevant for primary production in many ecosys- tems. To maintain the intracellular and intrathylakoid ion homeostasis upon different environmental challenges, the concentration of potassium as a major intracellular cation has to be optimized by various K+ uptake-mediated trans- port systems. We reveal here the specific and concerted physiological function of three K+ transporters of the plasma and thylakoid membranes, namely of SynK (K+ chan- nel), KtrB (Ktr/Trk/HKT) and KdpA (Kdp) in Synechocystis sp. strain PCC 6803, under specific stress conditions. The behavior of the wild type, single, double and triple mutants was compared, revealing that only Synk contributes to heavy metal-induced stress, while only Ktr/Kdp is involved in os- motic and salt stress adaptation. With regards to pH shifts in the external medium, the Kdp/Ktr uptake systems play an important role in the adaptation to acidic pH. Ktr, by affect- ing the CO2 concentration mechanism via its action on the bicarbonate transporter SbtA, might also be responsible for the observed effects concerning high-light stress and calcification. In the case of illumination with high-intensity light, a synergistic action of Kdr/Ktp and SynK is required in order to avoid oxidative stress and ensure cell viability. In summary, this study dissects, using growth tests, measurement of photosynthetic activity and analysis of ultrastructure, the physiological role of three K+ transporters in adaptation of the cyanobacteria to various environmental changes.