Comparative Osmoregulatory Studies of K and Rb in the Cyanobacterium Nostoc muscorum

Rubidium (Rb ) is an analog of potassium (K ) and has been used as a tracer of potassium. In the + + present study we have analyzed its role as a nutrient as well as in osmoregulation in the cyanobacterium Nostoc muscorum. The filaments of N. muscorum will grow well in the medium in which K is replaced by Rb . Salt and + + osmotic sensitivity of the Rb grown cells increased in terms of percent survival, photosynthetic O evolution + 2 and nitrogenase activity. Like K grown cells exogenous proline provides no protection to the Rb grown cells + + under salt and osmotic stresses. This suggested that intracellular accumulation of K is a prerequisite for uptake + and accumulation of proline. These results suggested that Rb like that of K is nutrient compatible, but could + + not act as an osmolyte.