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Environmental and Stress Physiology

Enhancement of Cyanobacterial Salt Tolerance by Combined Nitrogen ¹

Molecular Biology and Agriculture Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India

Presence of certain nitrogenous compounds in the growth medium significantly enhanced the salt tolerance of the fresh-water 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 NO₃^– and NH₄⁺ increased during salt stress but was not correlated with growth. Intracellular levels of NO₃^– and NH₄⁺ were found to be inadequate to constitute a major component of the internal osmoticum. The results suggest that inhibition of Na⁺ influx by combined nitrogen is a major mechanism for protection of cyanobacteria against salt stress.

¹ Supported, in part, by the Indian Council of Agricultural Research and the Department of Science and Technology, New Delhi, under the Indo-U.S. Science and Technology Programme.