Relationship between Sodium Influx and Salt Tolerance of Nitrogen-Fixing Cyanobacteria

Abstract

The relationship between sodium uptake and cyanobacterial salt (NaCl) tolerance has been examined in two filamentous, heterocystous, nitrogen-fixing species of Anabaena. During diazotrophic growth at neutral pH of the growth medium, Anabaena sp. strain L-31, a freshwater strain, showed threefold higher uptake of Na + than Anabaena torulosa , a brackish-water strain, and was considerably less salt tolerant (50% lethal dose of NaCl, 55 mM) than the latter (50% lethal dose of NaCl, 170 mM). Alkaline pH or excess K + (>25 mM) in the medium causes membrane depolarization and inhibits Na + influx in both cyanobacteria (S. K. Apte and J. Thomas, Eur. J. Biochem. 154:395-401, 1986). The presence of nitrate or ammonium in the medium caused inhibition of Na + influx accompanied by membrane depolarization. These experimental manipulations affecting Na + uptake demonstrated a good negative correlation between Na + influx and salt tolerance. All treatments which inhibited Na + influx (such as alkaline pH, K + above 25 mM, NO 3 − , and NH 4 + ), enhanced salt tolerance of not only the brackish-water but also the freshwater cyanobacterium. The results indicate that curtailment of Na + influx, whether inherent or effected by certain environmental factors (e.g., combined nitrogen, alkaline pH), is a major mechanism of salt tolerance in cyanobacteria.

Relationship of combined nitrogen sources to salt tolerance in freshwater cyanobacterium Anabaena doliolum

“…Brewer and Goldman (1976) have shown a 1 : 1 ratio between NH: uptake and H + output. The results are in accordance with the observations of enhanced salt tolerance in cyanobacteria (Apte et al 1987;Reddy et al 1989), as well as that showing increased biomass production of salt marshes in the presence of the combined nitrogen sources (Naidoo 1990).…”

“…Sodium uptake is reduced to minimum in the presence of nitrate. These results and those of others (Apte et al 1987;Thomas et al 1988;Reddy et al 1989) are in agreement that combined nitrogen sources, especially nitrate, effectively restrict the entry of sodium into cyanobacterial cells, probably by interacting with the Na' carrier and thus resulting in the inhibition of Na' influx.…”

“…Cyanobacterial cells do not accumulate Na + (Paschinger 1977); however, a transient sodium uptake in the cyanobacterium Synechocystis PCC 6874 in response to hypersaline treatment has been observed . The inhibition of sodium-ion influx appears as a major mechanism for the survival of the cyanobacteria against salt stress (Apte and Thomas 1983;Apte et al 1987).…”

Adaptation of cyanobacteria to environmental stimuli: new steps towards molecular mechanisms

“…The physiological and biochemical aspects of salt acclimation are rather well documented in cyanobacteria [535,536,[538][539][540][541][542][543]. Two salt-acclimation strategies have been elucidated: (i) avoidance of toxic internal amounts of inorganic ions by means of active export systems accompanied by enhanced respiratory enzyme activities and ultrastructural changes [540][541][542][544][545][546][547][548][549][550][551]; (ii) synthesis and accumulation of compatible solutes, low-molecular mass hydrophilic compounds which substitute ions and are less toxic than ions even at high concentrations [535,536,538,543,552]. Three main groups of cyanobacteria are distinguished according to the osmoprotective compounds accumulated: strains with the lowest salt tolerance synthesize sucrose and trehalose, intermediate strains use glucosylglycerol and strains with the highest tolerance accumulate glycine betaine and glutamate betaine.…”

Transcript levels of rbcR1, ntcA, and rbcL/S genes in cyanobacterium Anabaena sp. PCC 7120 are downregulated in response to cold and osmotic stress

“…Although the exact molecular mechanisms governing stress-inducible genes are not yet fully elucidated, speci¢c physiological changes have been identi¢ed that confer osmotic tolerance in cyanobacteria [9]. These changes include, but are not limited to, (i) secretion of sodium ions [10], (ii) accumulation of potassium ions [11], and (iii) accumulation of compatible solutes such as glucosylglycerol, trehalose, sucrose [12], betaines and other osmolytes [8,9,13,14].…”