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.