Nitrogen Fixation in Deep-Sea Sediments

生命需要氮来生长。大气氮（N2）是地球表面最丰富的氮，但大多数生物不能直接吸收N2。因此，植物可能是氮有限的，这意味着对“生物可利用”氮的需求超过了供应，其可用性控制了群落的整体增长和生产力。一小部分微生物，称为固氮生物，在一个称为N2固定的过程中将N2转化为生物可利用的氮形式，包括铵和含氮有机物。固氮生物是地球上最大的生物可利用氮的天然来源，它们固定N2的速率可以控制其他重要微生物过程发生的速率，例如温室气体的产生和消耗。因此，了解环境中的固氮生物-它们的身份，分布，活动水平和生物地球化学控制-对于了解整体微生物群落活动和生物地球化学循环至关重要。该项目的目标是描述深海沉积物中的N2固定，这是一个普遍研究不足但广阔的栖息地，覆盖了我们地球的近三分之二。该项目将通过教育推广、支持和培训早期职业科学家以及科学影响产生更广泛的影响：由于海洋甲烷、二氧化碳和一氧化二氮循环的速率受到氮供应的影响，因此研究结果将有助于我们了解海洋环境中的温室气体循环，从而有助于了解气候稳定性，N2固定是海洋透光层中一个重要的代谢过程，也是研究的热点。对深海沉积物中的N2固定知之甚少，但它可能是底栖生物生产力和海洋尺度元素循环的一个重要因素。一些观测表明或直接探测到在海底生产力提高的局部地区（例如，甲烷渗漏和热液喷口），增加了深海固氮广泛存在的可能性。然而，在这些异常区域之外，对N2固定的测量很少，因此对绝大多数深海海底的N2固定知之甚少。初步数据表明，N2固定确实发生在典型的深海沉积物中，并由一组不同的尚未鉴定的微生物介导。该项目将联合收割机技术与分子生物学和地球化学相结合，系统地研究加州近海沿着深度剖面（水深500至4 500米）收集的代表性深海沉积物中的N2固定情况。该项目将确定（1）固氮速率和分布;（2）与固氮（nif）相关的基因和转录本的丰度、多样性和分布;（3）生物介质（固氮生物）的系统发育特性;（4）固氮生物群落结构和活动的理化控制。对于上下文，还将表征非固氮细菌群落的活性。这些结果可能导致对海底新氮产量的估计值向上修正，从而改变我们对海洋氮循环目前平衡的理解。总之，这一假设驱动的表征N2固定在深海沉积物中将揭示一个广阔的，气候上重要的，传统上研究不足的栖息地，并促进更准确的外推率和分布的N2固定在整个海底，以及海底群落的代谢反应，以环境变化。

Life requires nitrogen for growth. Atmospheric nitrogen (N2) is the most abundant form of nitrogen on the surface of the planet, but most organisms cannot assimilate N2 directly. Habitats can therefore be nitrogen limited, meaning the demand for "bioavailable" nitrogen exceeds the supply, and its availability controls the overall growth and productivity of the community. A small subset of microorganisms, termed diazotrophs, convert N2 to bioavailable forms of nitrogen, including ammonium and nitrogenous organic matter, in a process known as N2 fixation. Diazotrophs are the largest natural source of bioavailable nitrogen on the planet, and the rate at which they fix N2 can control the rates at which other important microbial processes occur, such as the production and consumption of greenhouse gases. Understanding diazotrophs in the environment - their identity, distribution, activity levels, and biogeochemical controls - is therefore essential to understanding overall microbial community activity and biogeochemical cycling. The goal of this project is to characterize N2 fixation in deep-sea sediments, a generally understudied but expansive habitat, covering nearly two thirds of our planet. The project will have broader impacts via educational outreach, support and training of early career scientists, and scientific impact: since rates of marine methane, carbon dioxide, and nitrous oxide cycling are affected by nitrogen availability, the results will inform our understanding of greenhouse gas cycling in the marine environment, and therefore climate stability, a topic central to global security.N2 fixation is a critical and intensely studied metabolism in the marine photic zone. Much less is known about N2 fixation in deep-sea sediments, but it could be an important factor in both benthic productivity and ocean-scale elemental cycling. Several observations have suggested or directly detected N2 fixation at localized areas of enhanced productivity on the seafloor (e.g., methane seeps and hydrothermal vents), raising the possibility that deep-sea N2 fixation is widespread. However, few measurements of N2 fixation have been made outside of these anomalous areas, and thus little is known about N2 fixation in the vast majority of the deep ocean floor. Preliminary data suggest N2 fixation does occur in typical deep marine sediment, and is mediated by a diverse set of yet unidentified microorganisms. This project will combine techniques from molecular biology and geochemistry to systematically investigate N2 fixation in representative deep-sea sediments collected along a depth profile (500 to 4500 m water depth) offshore California. The project will determine the (1) rates and distribution of N2 fixation (2) abundance, diversity, and distribution of genes and transcripts associated with N2 fixation (nif) (3) phylogenetic identity of the biological mediators (diazotrophs) and (4) physiochemical controls on diazotrophic community structure and activity. For context, the activity of the non-diazotrophic bacterial community will also be characterized. The results may lead to upward revisions of the estimates of new nitrogen production in the seafloor, and therefore change our understanding of the current balance of the marine nitrogen cycle. Together, this hypothesis-driven characterization of N2 fixation in deep-sea sediments will shed light on an expansive, climatically important, and traditionally understudied habitat, and facilitate more accurate extrapolation of the rates and distribution of N2 fixation on the whole seafloor as well as the metabolic response of the seafloor community to environmental change.