Sodium transport in filamentous nitrogen fixing cyanobacteria

Abstract

Two filamentous, nitrogen fixing cyanobacteria were examined for their salt tolerance and sodium (Na⁺) transport. Anabaena torulosa, a saline form, grew efficiently and fixed nitrogen even at 150 mM salt (NaCl) concentration while, Anabaena L-31, a fresh water cyanobacterium, failed to grow beyond 35 mM NaCl. Anabaena torulosa showed a rapidly saturating kinetics of Na⁺ transport with a high affinity for Na⁺ (K_m, 0.3 mM). Anabaena L-31 had a much lower affinity for Na⁺ (K_m, 2.8 mM) than Anabaena torulosa and the pattern of uptake was somewhat different. Both Anabaena spp. exhibited an active Na⁺ extrusion which seems to be mediated by a Na⁺-K⁺ ATPase and aided by oxidative phosphorylation. Anabaena L-31 was found to retain much more intracellular Na⁺ than Anabaena torulosa. The results suggest that the saline form tolerates high Na⁺ concentrations by curtailing its influx and also by an efficient Na⁺ extrusion, although these alone may not entirely account for its success in saline environment.