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

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.