Evidence for the nitrate assimilation-dependent nitrite excretion in cyanobacterium Nostoc MAC

Nitrate uptake and nitrite efflux patterns in Nostoc MAC showed a rapid phase followed by their saturation. Nitrite efflux was maximum in nitrate medium whereas the cells incubated in N2 and NH4+media exhibited a decreased nitrite efflux activity. The simultaneous presence of NH4+and nitrate significantly decreased nitrite efflux. L-Methionine-Dl-sulphoximine (MSX) prevented inhibition of nitrite efflux by NH4+. In the dark there was negligible nitrite efflux, whereas illumination increased the rate of nitrite efflux significantly. The nitrite efflux system was maximally operative at pH 8.0, 30°C and a photon fluence rate of 50 μmol m-2. s-1. These results confirm that (i) the nitrite efflux system in Nostoc MAC is dependent upon nitrate uptake and assimilation and is repressible by NH4+; (ii) NH4+itself is not the actual repressor of nitrite efflux; a product of NH4+assimilation via glutamine synthetase (GS) is required for repression to occur; (iii) the catalytic function of GS does not appear to be involved in nitrate assimilation-dependent nitrite efflux, and (iv) the optimum pH, temperature and illumination for maximum nitrite efflux were found to be 8.0, 30°C and 50μmol m-2. s respectively.

念珠藻（Nostoc MAC）的硝酸盐吸收和亚硝酸盐外排模式呈现出一个快速阶段，随后达到饱和。在硝酸盐培养基中亚硝酸盐外排最多，而在氮气和铵盐培养基中培养的细胞亚硝酸盐外排活性降低。铵盐和硝酸盐同时存在显著降低亚硝酸盐外排。L - 甲硫氨酸 - D,L - 磺基肟（MSX）阻止了铵盐对亚硝酸盐外排的抑制。在黑暗中，亚硝酸盐外排可忽略不计，而光照显著提高了亚硝酸盐外排速率。亚硝酸盐外排系统在pH值为8.0、30°C以及光子通量密度为50 μmol m⁻²·s⁻¹时活性最高。这些结果证实：（i）念珠藻（Nostoc MAC）的亚硝酸盐外排系统依赖于硝酸盐的吸收和同化，并且可被铵盐抑制；（ii）铵盐本身不是亚硝酸盐外排的实际抑制物；需要通过谷氨酰胺合成酶（GS）同化铵盐的产物才能产生抑制作用；（iii）谷氨酰胺合成酶的催化功能似乎不参与依赖于硝酸盐同化的亚硝酸盐外排；（iv）亚硝酸盐外排达到最大时的最适pH值、温度和光照分别为8.0、30°C和50 μmol m⁻²·s⁻¹。