Rhodococcus sp RB1 grows in the presence of high nitrate and nitrite concentrations and assimilates nitrate in moderately saline environments

Rhodococcus sp. RB1 was able to thrive in media with up to 0.9 M NaCl or KC l and in the presence of high concentrations of nitrate (up to 0.9 M) and n itrite (up to 60 mM), but only under oxic conditions. An adaptation period was not required for salt tolerance, but a rapid extrusion of K+ and intake of Na+ was observed after addition of 0.5 M NaCl. Nitrate assimilation was limited by the carbon supply, but nitrite was not accumulated in the cultu re medium, even at nitrate concentrations as high as 0.8 M, thus suggesting that nitrite reduction does not limit nitrate assimilation. The presence o f NaCl or KCl did not affect nitrate or nitrite uptake, which were complete ly inhibited by ammonium or glutamine. Rhudococcus sp. RB1 nitrate reductas e had an apparent molecular mass of 142 kDa and used NADH and reduced bromo phenol blue or viologens as electron donors, independently of the presence of salt. The enzyme was associated with an NADH-diaphorase activity and was induced by nitrate and repressed by ammonium or glutamine, thus showing ty pical biochemical and regulatory properties of bacterial assimilatory NADH- nitrate reductases. The enzyme was active in vitro in the presence of 3 M N aCl or KCl. but the maximal activity was observed at 0.5 M salt. Addition o f 2 M NaCl increased the optimal temperature of the enzyme from 12 to 32 de greesC, but the optimal pH (10.3) was unaffected.