Is denitrification by ammonia oxidising bacteria a response to nitrite toxicity?

氧化亚氮（N2O）的生产是一个环境问题，因为它涉及全球变暖和平流层臭氧的破坏。由于土壤占大气N2O负荷的约70%，迫切需要采取管理策略来降低排放。这样的策略需要围绕我们对土壤中微生物n2o产生过程的调节的理解来制定。该项目将确定氨氧化细菌（AOB）在将亚硝酸盐（NO2-）还原为氧化亚氮的功能中表现出灵活性的条件，从而促进全球变暖和破坏平流层臭氧。具体来说，该项目将研究这种灵活性是否是对硝化过程中亚硝酸盐毒性的反应，从而在好氧条件下释放N2O。将测试假设:(i)的氨氧化细菌脱氮能力主要是NO2 -毒性反应,而不是一个兼性反应降低氧化还原条件下维持呼吸(2)AOB切换到使用NO2 -作为氧源来维持功能感觉阈值NO2 -的浓度时,减少一氧化二氮,和(3)在土壤环境中,国内企业的必要性降低NO2——将取决于耦合AOB与亚硝酸盐氧化细菌(头)。学生将进行培养实验，测试AOB和NOB菌株对培养物中NO2-添加速率的响应，确定AOB还原亚硝酸盐的阈值浓度，AOB最大N2O产量的最佳NH4+:NO2-比，以及NOB对NO2-浓度和AOB N2O产量的调节作用。采用稳定同位素方法（15n - 18o富集NO2-和N2O）来确定NO2-被AOB用作高于NO2-浓度阈值的氧源进行反硝化。通过减少AOB nirK基因的表达，在接种了目标1的AOB和NOB菌株的消毒（γ辐照）土壤中，以及在自然土壤群落中，采用15n -同位素体和实时分子方法的组合，研究NOB调节NO2-浓度的潜力，从而减少AOB对NO2-的还原。该项目将为学生提供广泛的分析技术培训，包括气相色谱和质谱分析以及微生物技术，以及广泛的土壤物理和化学分析。学生将从SCRI获得分子技术、样品制备和分析方面的培训。

Production of nitrous oxide (N2O) is of environmental concern due to its involvement in global warming and destruction of stratospheric ozone. As soils account for ~70% of the atmospheric loading of N2O management strategies are urgently required to lower emissions. Such strategies need to be formulated around our understanding of the regulation of microbial N2O-producing processes in soils. This project will determine the conditions under which ammonia oxidising bacteria (AOB) demonstrate flexibility in function to reduce nitrite (NO2-) to nitrous oxide, thereby contributing to global warming and destruction of stratospheric ozone. Specifically this project will examine whether this flexibility is a response to nitrite toxicity during nitrification thus releasing N2O under aerobic conditions. It will test the hypotheses: (i) the ability of ammonia oxidising bacteria to denitrify is primarily a NO2- toxicity response, rather than a facultative response to maintain respiration under reduced Redox conditions (ii) AOB switch to using NO2- as an oxygen source to maintain function when a threshold concentration of NO2- is sensed, reducing it to N2O, and (iii) in the soil environment, the necessity of AOB to reduce NO2- will depend on coupling of AOB with nitrite oxidising bacteria (NOB). The student will undertake culture experiments to test the response of strains of AOB and NOB to NO2- addition rates in culture and ascertain a threshold concentration for nitrite reduction by AOB, the optimum NH4+:NO2- ratio for maximum N2O production by AOB, and the influence of NOB in regulating the NO2- concentration and AOB N2O production. A stable isotope approach (15N-18O-enrichment of NO2- and N2O) will be adopted to ascertain that NO2- is used by AOB as an oxygen source above the threshold NO2- concentration for denitrification. The potential for NOB to regulate NO2- concentrations and therefore reduction of NO2- by AOB will be examined through reduced expression of the AOB nirK gene, in sterilised (gamma-irradiated) soil inoculated with AOB and NOB strains from objective 1, and in a natural soil community adopting a combination of 15N-isotopomer and real time molecular approaches. This project will provide the student with training in a wide range of analytical techniques, including analysis by gas chromatography and mass spectrometry and microbiological techniques, as well as a wide range of soil physical and chemical analyses. The student will obtain training in molecular techniques, sample preparation and analysis from SCRI.