Department of Earth and Environmental Engineering, Columbia University, New York, NY 10027, USA.
BMC Microbiol. 2010 Mar 4;10:70. doi: 10.1186/1471-2180-10-70.
Nitrosomonas europaea is a widely studied chemolithoautotrophic ammonia oxidizing bacterium. While significant work exists on the ammonia oxidation pathway of N. europaea, its responses to factors such as dissolved oxygen limitation or sufficiency or exposure to high nitrite concentrations, particularly at the functional gene transcription level are relatively sparse. The principal goal of this study was to investigate responses at the whole-cell activity and gene transcript levels in N. europaea 19718 batch cultures, which were cultivated at different dissolved oxygen and nitrite concentrations. Transcription of genes coding for principal metabolic pathways including ammonia oxidation (amoA), hydroxylamine oxidation (hao), nitrite reduction (nirK) and nitric oxide reduction (norB) were quantitatively measured during batch growth, at a range of DO concentrations (0.5, 1.5 and 3.0 mg O2/L). Measurements were also conducted during growth at 1.5 mg O2/L in the presence of 280 mg-N/L of externally added nitrite.
Several wide ranging responses to DO limitation and nitrite toxicity were observed in N. europaea batch cultures. In contrast to our initial hypothesis, exponential phase mRNA concentrations of both amoA and hao increased with decreasing DO concentrations, suggesting a mechanism to metabolize ammonia and hydroxylamine more effectively under DO limitation. Batch growth in the presence of 280 mg nitrite-N/L resulted in elevated exponential phase nirK and norB mRNA concentrations, potentially to promote utilization of nitrite as an electron acceptor and to detoxify nitrite. This response was in keeping with our initial hypothesis and congruent with similar responses in heterotrophic denitrifying bacteria. Stationary phase responses were distinct from exponential phase responses in most cases, suggesting a strong impact of ammonia availability and metabolism on responses to DO limitation and nitrite toxicity. In general, whole-cell responses to DO limitation or nitrite toxicity, such as sOUR or nitrite reduction to nitric oxide (NO) did not parallel the corresponding mRNA (nirK) profiles, suggesting differences between the gene transcription and enzyme translation or activity levels.
The results of this study show that N. europaea possesses specific mechanisms to cope with growth under low DO concentrations and high nitrite concentrations. These mechanisms are additionally influenced by the physiological growth state of N. europaea cultures and are possibly geared to enable more efficient substrate utilization or nitrite detoxification.
硝化单胞菌是一种广泛研究的化能自养氨氧化细菌。虽然关于硝化单胞菌的氨氧化途径已经有了大量的研究，但它对溶解氧限制或充足或暴露于高亚硝酸盐浓度等因素的反应，特别是在功能基因转录水平上相对较少。本研究的主要目的是研究在不同溶解氧和亚硝酸盐浓度下培养的硝化单胞菌 19718 批培养物中的全细胞活性和基因转录水平的反应。在批生长过程中，定量测量了编码主要代谢途径的基因（氨氧化（amoA）、羟胺氧化（hao）、亚硝酸盐还原（nirK）和一氧化氮还原（norB））的转录，在不同的 DO 浓度（0.5、1.5 和 3.0 mg O2/L）下进行测量。还在 1.5 mg O2/L 下生长时进行了测量，同时存在 280 mg-N/L 的外加亚硝酸盐。
在硝化单胞菌批培养物中观察到几种对 DO 限制和亚硝酸盐毒性的广泛反应。与我们最初的假设相反，amoA 和 hao 的指数期 mRNA 浓度随着 DO 浓度的降低而增加，这表明在 DO 限制下更有效地代谢氨和羟胺的机制。在 280 mg 亚硝酸盐-N/L 的存在下进行批生长导致指数期 nirK 和 norB mRNA 浓度升高，可能是为了促进将亚硝酸盐用作电子受体并解毒亚硝酸盐。这种反应符合我们最初的假设，与异养反硝化细菌的类似反应一致。与大多数情况下的指数期反应不同，静止期反应表明氨可用性和代谢对 DO 限制和亚硝酸盐毒性的反应有强烈影响。一般来说，全细胞对 DO 限制或亚硝酸盐毒性的反应，例如 sOUR 或亚硝酸盐还原为一氧化氮（NO），与相应的 mRNA（nirK）图谱不平行，这表明基因转录和酶翻译或活性水平之间存在差异。
本研究的结果表明，硝化单胞菌具有特定的机制来应对低 DO 浓度和高亚硝酸盐浓度下的生长。这些机制还受到硝化单胞菌培养物生理生长状态的影响，可能旨在实现更有效的底物利用或亚硝酸盐解毒。
