The Isotope Exchange Hypothesis: Nitrification and the Oxygen Isotopic Signatures in Nitrate, Nitrite, and Nitrous Oxide

硝酸盐（NO3-）是海洋中固定氮的主要形式。目前海洋硝酸盐的质量和同位素平衡没有得到很好的约束，估计汇比源多出两倍。解决硝酸盐源汇失衡问题对于理解海洋初级生产力的营养驱动变化和气候变化具有重要意义。对NO3-氮氧同位素组成的耦合分析可以为这些预算提供独特的约束，许多实验室正在获得进行测量的能力，但在做出准确的解释之前，我们必须首先了解产生（硝化）和消耗（同化，反硝化）硝酸盐的过程如何影响其氧同位素特征。例如，硝化作用，即微生物将氨氧化为硝酸盐，是产生硝酸盐的主要机制，然而，最近的硝酸盐氧同位素数据与硝化过程中氧原子掺入的传统观点不一致，表明我们对这一重要过程的理解存在差距。伍兹霍尔海洋研究所的研究人员将测试一种假设，即在硝化细菌的催化下，亚硝酸盐和水之间的氧同位素交换会去除硝化途径第一步中从O2中获得的大部分氧同位素特征。将进行各种实验室和现场实验，以量化海洋氨氧化和亚硝酸盐氧化细菌催化亚硝酸盐与水之间氧同位素交换的程度。在这些实验中产生的亚硝酸盐、硝酸盐和氧化亚氮中的氧同位素将被测量并纳入一个概念模型，以评估主要假设。接受或拒绝这一假设将促进我们对硝酸盐中氧同位素特征的理解及其对潜在过程的反映，为海洋氮循环提供强有力的约束。这项工作的另一个好处是它与理解硝化作用在氧化亚氮生产中的作用直接相关，氧化亚氮是一种重要的气候微量气体。就更广泛的影响而言，本研究的结果将对研究海洋氮循环、N2O生产、气候变化以及硝化菌生理和生物化学的广泛社区感兴趣。分子生物地球化学方面的学生培训将是拟议活动的一个重要组成部分，并为分子生物学合成和稳定同位素地球化学方面的研究生培训提供资金。参加WHOI夏季本科和少数民族研究项目的本科生也将有机会接受分子生物学和/或同位素分析方面的培训，重点是将这些方法结合起来回答具有全球重要性的问题。

ABSTRACTOCE-0526277Nitrate (NO3-) is the primary form of fixed nitrogen in the ocean. Current mass and isotope balances of oceanic nitrate are poorly constrained, with estimated sinks exceeding sources by a factor of two. Resolving the imbalance of nitrate sources and sinks is important for understanding nutrient driven changes in oceanic primary productivity and climate change. Coupled analysis of the nitrogen and oxygen isotopic composition of NO3- could provide unique constraints on these budgets and many laboratories are gaining the capacity to make the measurements, but before accurate interpretations can be made we must first understand how processes that produce (nitrification) and consume (assimilation, denitrification) nitrate affect its oxygen isotopic signature. For instance, nitrification, the microbial oxidation of ammonia to nitrate, is the primary mechanism for generating nitrate, however, recent nitrate oxygen isotope data are inconsistent with the traditional view of oxygen atom incorporation during nitrification, indicating a gap in our understanding of this important process. Investigators from the Woods Hole Oceanographic Institution will test the hypothesis that oxygen isotope exchange between nitrite and water, catalyzed by nitrifying bacteria, removes much of the oxygen isotopic signature obtained from O2 in the first step of the nitrification pathway. A variety of laboratory and field experiments will be conducted to quantify the extent of oxygen isotope exchange between nitrite and water catalyzed by marine ammonia-oxidizing and nitrite-oxidizing bacteria. Oxygen isotopes in nitrite, nitrate, and nitrous oxide produced in these experiments will be measured and incorporated into a conceptual model to evaluate the main hypothesis. The acceptance or rejection of this hypothesis will advance our understanding of oxygen isotope signatures in nitrate and their reflection of underlying processes to provide a strong constraint on the oceanic nitrogen cycle. An additional benefit of this work is its direct relevance to understanding the role of nitrification in the production of nitrous oxide, a climatically important trace gas.In terms of broader impacts, results of this research will be of interest to a broad community studying oceanic nitrogen cycling, N2O production, climate change, as well as nitrifier physiology and biochemistry. Student training in molecular biogeochemistry will be an important component of the proposed activity, with funds provided for graduate student training in the synthesis of molecular biology and stable isotope geochemistry. Undergraduate students participating in the WHOI summer undergraduate and minority research programs will also have the opportunity to receive training in molecular biology and/or isotopic analysis, with an emphasis on combining these approaches to answer questions of global importance.