Evidence for an ammonium transport system in free-living and symbiotic cyanobacteria

The free-living cyanobacterium Anabaena variabilis showed a biphasic pattern of ¹⁴CH₃NH ₃ ⁺ uptake. Initial accumulation (up to 60 s) was independent of CH₃NH ₃ ⁺ metabolism, but long-term uptake was dependent on its metabolism via glutamine synthetase (GS). The CH₃NH ₃ ⁺ was converted into methylglutamine which was not further metabolised. The addition of l-methionine-dl-sulphoximine (MSX), to inhibit GS, inhibited CH₃NH ₃ ⁺ metabolism, but did not affect the CH₃NH ₃ ⁺ transport system.NH ₄ ⁺ , when added after the addition of ¹⁴CH₃NH ₃ ⁺ , caused the efflux of free CH₃NH ₃ ⁺ ; when added before ¹⁴CH₃NH ₃ ⁺ , NH ₄ ⁺ inhibited its uptake indicating that both NH ₄ ⁺ and CH₃NH ₃ ⁺ share a common transport system. Carbonylcyanide m-chlorophenylhydrazone and triphenyl-methylphosphonium both inhibited CH₃NH ₃ ⁺ accumulation indicating that the transport system was -dependent. At pH 7 and at an external CH₃NH ₃ ⁺ concentration of 30 mol dm^-3, A. variabilis showed a 40-fold intracellular accumulation of CH₃NH ₃ ⁺ (internal concentration 1.4 mmol dm^-3). Packets of the symbiotic cyanobacterium Anabaena azollae, directly isolated from the water fern Azolla caroliniana, also showed a -dependent NH ₄ ⁺ transport system suggesting that the reduced inhibitory effect of NH ₄ ⁺ on nitrogenase cannot be attributed to the absence of an NH ₄ ⁺ transport system but is probably related to the reduced GS activity of the cyanobiont.Abbreviations CCCP carbonylcyanide m-chlorophenylhydrazone - GS glutamine synthetase - HEPES 4-(2-hydroxyethyl)-1-piperazine ethanesulphonic acid - MSX l-methionine-dl-sulphoximine - membrane potential - pH transmembrane pH difference - TPMP⁺ triphenylmethylphosphonium