Rubidium transport in the cyanobacterium Synechococcus R‐2 (Anacystis nidulans, S. leopoliensis) PCC 7942

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 ( t 2 ∼ 15 h versus 12 h). Potassium is not, strictly speaking, an essential element for Synechococcus. Rubidium duxes (using 86 Rb + ) are much slower than those of potassium, about 1 nmol m −2 s −1 in the light (0.35 mol m −3 Rb + ). 86 Rb + 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 dux needed to keep up with growth in the light is a large proportion of the total observed dux. 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 pH o 10 in the light and dark. There is no case for active transport of Rb + at pH o 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. 86 Rb + should not be used in place of 42 K + in K + nutrition studies as the details of Rb + transport are different to those of K + transport.