Environmental controls on nitrogen cycling in coastal sediments

人类活动正在以前所未有的程度改变全球元素循环。例如，Haber Bosch工艺的发明，以及从大气氮中制造肥料的能力，大大增加了沿海水域的溶解氮（硝酸盐，亚硝酸盐和铵）的输入。由于氮在水生系统中通常是一种限制性营养物质，因此初级产量的增加和随后的有机质沉积可能导致深度O2的消耗。这种现象被称为富营养化，会导致有害藻华（HABs）或缺氧“死亡区”的形成。如果我们要有效地管理我们的沿海资源，我们必须能够理解和预测这种营养负荷的影响。这样的理解需要能够精确模拟物理、化学和生物过程的数值模型。不幸的是，这些生物地球化学模型的一个主要不确定性来源是缺乏关于催化许多生物地球化学过程的微生物群落如何对环境扰动作出反应的知识。造成这种情况的两个原因是：1)缺乏执行相关过程的微生物群落的明确表示；2)缺乏对控制微生物群落组织的基本原则的理解。我的研究试图通过寻求改进生物地球化学模型中微生物过程的表示来解决这一弱点。这将通过在三个相互关联的主题背景下研究沿海沉积物中的氮循环来进行调查。主题一将研究两种不同海岸系统中氮循环动态对养分和碳负荷变化的响应；1)贝德福德盆地，哈利法克斯，2)新斯科舍省南部海岸鲑鱼养殖场下面的沉积物。水产养殖活动改变了沉积物的碳和养分输入，因此提供了一个理想的“天然实验室”来研究这些影响。主题二将研究最大熵生产理论（MEP）作为生态系统组织的原则。这个来自非平衡热力学的理论指出，生态系统将组织起来，通过允许的最快途径消散自由能的梯度。主题三将致力于改进生物地球化学模型中（元）基因组数据的表示。在过去的10年里，基因组测序在环境系统中的应用为微生物群落的多样性和功能潜力提供了前所未有的视角，然而，生物地球化学模型尚未制定出一种利用这些数据的方式。***通过这三个主题，本研究将加强我们预测沿海养分负荷影响的能力，从而提高我们管理沿海资源的能力。**

Human activity is currently altering global elemental cycles to an unprecedented degree. For example, the invention of the Haber Bosch process, and with it, the ability to make fertilizer from atmospheric nitrogen, has greatly increased dissolved nitrogen (nitrate, nitrite, and ammonium) inputs to coastal waters. Because nitrogen is often a limiting nutrient in aquatic systems, the resulting increase in primary production and subsequent organic matter deposition can lead to O2 depletion at depth. This phenomenon, termed eutrophication, can induce harmful algal blooms (HABs) or the development of anoxic “dead zones”. If we are to effectively manage our costal resources, we must be able to understand and predict the impact of this nutrient loading. Such an understanding requires numerical models that can accurately simulate physical, chemical, and biological processes. Unfortunately, a major source of uncertainty in these biogeochemical models is a lack of knowledge as to how the microbial communities that catalyze many biogeochemical processes will respond to environmental perturbations. Two reasons for this are: 1) the lack of an explicit representation of the microbial communities carrying out the relevant processes, and 2) a lack of understanding of the fundamental principles controlling the organization of microbial communities. ***My research attempts to address this weakness by seeking to improve the representation of microbial processes in biogeochemical models. This will be investigated by studying nitrogen cycling in coastal sediments in the context of three interrelated themes. Theme I will examine nitrogen cycling dynamics in response to the changes in nutrient and carbon loading in two different coastal systems; 1) Bedford Basin, Halifax NS, and 2) sediments underlying salmonid aquaculture farms along the southern coast of Nova Scotia. Aquaculture operations alter carbon and nutrient inputs to sediments and so provide an ideal “natural laboratory”, to study these effects. Theme II will examine the theory of Maximum Entropy Production (MEP) as a principle of ecosystem organization. This theory from non-equilibrium thermodynamics states that ecosystems will organize to dissipate gradients in free energy by the fastest allowable pathway. Theme III will work on improving the representation of (meta)genomic data in biogeochemical models. Over the last 10 years, the application of genomic sequencing to environmental systems has provided an unprecedented view into the diversity and functional potential of microbial communities, however biogeochemical models are not yet formulated in a way to take advantage of these data. ***Through these three themes, this research will strengthen our ability to predict the impacts of coastal nutrient loading, therefore improving our ability to manage our coastal resources. **