Role of alkali cations (K+ and Na+) in cyanobacterial nitrogen fixation and adaptation to salinity and osmotic stress.

Cyanobacteria occupy almost every possible ecological niche on earth, being tolerant to a large number of environmental stresses, including salinity and drought. Many of them also fix atmospheric nitrogen. They are responsible for a significant share of biosolar energy conversions on this planet and make substantial contributions to the carbon and nitrogen status of both oceans and soils. Sodium and potassium are two of the most prevalent cations on this planet. While K+ is an essential macronutrient in most life-forms, Na+ is strongly discriminated by means of highly selective alkali cation transport systems, favouring K+ over Na+. Although a nutritional requirement for K+ has not been specifically investigated, rapid accumulation of K+ during salt/osmotic stress has been observed in several cyanobacteria. Genes and proteins constituting a membrane-bound, turgor- and osmo-inducible, Kdp-ATPase-like system in Anabaena strains that may help in their early K+ responses to salt/osmotic stress have been identified. An unusual, specific and absolute requirement for trace quantities of sodium has been documented in cyanobacteria. Work done in our laboratory, and elsewhere, has elucidated the mechanisms underlying such a unique requirement. It has long been believed that cyanobacteria scavenge and immobilise sodium. We have, however, shown that sodium exclusion brought about by curtailment of influx and active efflux of Na+ forms the basis of salt tolerance in these microbes and that the inherent salt tolerance can be modified by factors that modulate Na+ fluxes in cyanobacteria. Identification of genes affecting the cation relationships in nitrogen-fixing cyanobacteria is currently in progress.