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A method is described for the isolation of metabolically active heterocysts from Anabaena cylindrica. These isolated heterocysts accounted for up to 34% of the acetylene-reducing activity of whole filaments and had a specific activity of up to 1,560 nmol of C2H4 formed per mg of heterocyst chlorphyll per min. Activity of glutamine synthetase was coupled to activity of nitrogenase in isolated heterocysts as shown by acetylene-inhibitable formation of [13N]NH3 and of amidelabeled [13N]glutamine form [13N]N2. A method is also described for the production of 6-mCi amounts of [13N]NH3. Isolated heterocysts formed [13N]glutamine from [13N]NH3 and glutamate, and [14C]glutamine from NH3 and [14C]glutamate, in the presence of magnesium adenosine 5'-triphosphate. Methionine sulfoximine strongly inhibited these syntheses. Glutamate synthase is, after nitrogenase and glutamine synthetase, the third sequential enzyme involved in the assimilation of N2 by intact filaments. However, the kinetics of solubilization of the activity of glutamate synthase during cavitation of suspensions of A. cylindrica indicated that very little, if any, of the activity of that enzyme was located in heterocysts. Concordantly, isolated heterocysts failed to form substantial amounts of radioactive glutamate from either [13N]glutamine or alph-[14C]ketoglutarate in the presence of other substrates and cofactors of the glutamate synthase reaction. However, they formed [14C]glutamate rapidly from alpha-[14C]ketoglutarate by aminotransferase reactions, with various amino acids as the nitrogen donor. The implication of these findings with regard to the identities of the substances moving between heterocysts and vegetative cells are discussed.

Cats have higher Se concentrations in plasma, compared with values for other species. However, Se status alone does not appear to affect the incidence of hyperthyroidism in cats. High Se concentrations may have implications for health of cats if such concentrations are influenced by the amount of that micronutrient included in diets.

Transport of Na+ and its relationship with membrane potential (ΔΨm) was examined in Anabaena L‐31 (a fresh water cyanobacterium) and Anabaena torulosa (a brackish water cyanobacterium) which require Na+ for diazotrophic growth. The data on the effect of N,N′‐dicyclohexylcarbodiimide indicated that ΔΨm was generated by electrogenic proton extrusion predominantly mediated by ATPase(s). In addition, operation of a plasmalemmabound, non‐ATP‐requiring, H+ ‐pumping terminal oxidase was suggested by the sensitivity of ΔΨm to anaerobiosis, cyanide and azide, all of which inhibit aerobic respiration. The response of ΔΨm to external pH and external Na+ or K+ concentrations indicated that a diffusion potential of Na+ or K+ may not contribute significantly to ΔΨm. Kinetic studies showed that Na+ influx was unlikely to be a result of Na+/Na+ exchange but was a carrier‐mediated secondary active transport insensitive to low concentrations (< 10 mM) of external K+. There was a close correspondence between changes in ΔΨm and Na+ influx; all the treatments which caused depolarisation (such as low temperature, dark, cyanide, azide, anaerobiosis, ATPase inhibitors) lowered Na+ influx whereas treatments which caused hyperpolarisation (such as 2,4‐dinitrophenol, nigericin) enhanced Na+ influx. Remarkably low intracellular Na+ concentrations were maintained by these cyanobacteria by means of active efflux of the cation. The basic mechanism of Na+ transport in the fresh water and the brackish water cyanobacterium was similar but the latter demonstrated less influx, more efficient efflux, more affinity of carriers for Na+ and less accumulation of Na+, all attributes favouring salt tolerance.

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.

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Presence of certain nitrogenous compounds in the growth medium significantly enhanced the salt tolerance of the freshwater cyanobacterium Anabaena sp. strain L-31 as well as the brackish water cyanobacterium Anabaena torulosa. Among these, nitrate, ammonium, and glutamine were most effective followed by glutamate and aspartate. These nitrogenous compounds also inhibited Na influx in both Anabaena spp. with the same order of effectiveness as that observed for protection against salt stress. The inhibition of Na+ influx on addition of the nitrogenous substances was rapid; nitrate and ammonium inhibited Na+ influx competitively. Proline and glycine did not affect Na+ influx and also had no influence on the salt tolerance of either Anabaena sp. The observed protection was not consequent to a stimulatory effect of combined nitrogen on growth per se. Uptake of N03-and NH4+ increased during salt stress but was not correlated with growth. Intracellular levels of N03-and NH4+ were found to be inadequate to constitute a major component of the intemal osmoticum. The results suggest that inhibition of Na+ influx by combined nitrogen is a major mechanism for protection of cyanobacteria against salt stress.