Rubidium transport in the cyanobacterium Synechococcus R-2 (Anacystis nidulans, S. leopoliensis) PCC 7942
Plant Cell and Environment 20(7): 907-918
Synechococcus R-2 is a unicellular blue-green alga. The cells will grow on Rb+ as a substitute for K+ but at a slower rate (t2 approximately 15 h versus 12 h). Potassium is not, strictly speaking, an essential element for Synechococcus. Rubidium fluxes (using 86Rb+) are much slower than those of potassium, about 1 nmol m-2 s-1 in the light (0.35 mol m-3 Rb+). 86Rb+ fluxes in the dark are about 0-1 nmol m-2 s-1. These fluxes are very slow compared to those of Na+ and other ions. Isotopic influx of Rb+ can supply sufficient Rb+ to keep up with the demands for growth, but the net flux needed to keep up with growth in the light is a large proportion of the total observed flux. Kinetic studies of Rb+ uptake versus [Rb+] show two uptake phases consistent with a high-affinity and a low-affinity system. Both systems appear to be light-activated. Transport of Rb+ appears to be passive at pHo 10 in the light and dark. There is no case for active transport of Rb+ at pHo 7.5 in the light, but a marginal case for active uptake in the dark (about 3 kJ mol-1). There is only a small effect of Na+ upon Rb+ transport. 86Rb+ should not be used in place of 42K+ in K+ nutrition studies as the details of Rb+ transport are different to those of K+ transport.