Shortcuts in the Oceanic Nitrogen Cycle: Fluxes and Microbial Pathways of Nitrogen Remineralization in the Ocean's Twilight Zone

海洋的黄昏（中层，100- 1000米）区位于阳光照射的海洋表面之下，光合作用的光线太少，但在漆黑的深海之上，大型动物再也看不到它们的猎物。在所有从海面下沉的有机物中，90%以上在中层中降解，只有一小部分逃入深海（> 1 000米）。因此，中层是一个重要的屏障：落入中层的大多数物质都被微矿化作用阻止进一步下沉，微矿化作用是一种降解过程，分解有机物质，将二氧化碳和无机营养物质释放到溶液中。最终，物理混合或海洋环流将营养物质送回表面，以促进浮游植物的生长。因此，中层海洋生物化对控制海洋生物泵至关重要，并可能影响海洋封存大气CO2的能力。氮通常是全球海洋中生物生产的限制性营养素，因此其再矿化将是生物泵效率的关键。然而，N-矿化机制的特点很差，也没有率测量这一过程中的中层。氮在海洋中的再矿化包括氨化作用：有机氮降解为铵（NH 4+），随后是硝化作用：NH 4+氧化为亚硝酸盐，然后是硝酸盐。然而，最近的一项研究表明，一些硝化菌（进行硝化作用的微生物）可以直接利用有机氮，从而在氮循环中呈现出可能的捷径。因为不同的生物对碳有不同的进食方式（固定或生产二氧化碳），这些生物功能群的相对丰度和活动将对二氧化碳平衡产生不同的直接影响，而潜在的捷径的存在可能会导致向自养的转变（CO2固定）。该项目旨在确定究竟如何以及有多少氮在黄昏海洋中被固定，使用最先进的地球化学速率测量和分子生物学分析的组合。特别是，我们将确定上述捷径是否存在于有机氮（N）到硝酸盐（NO3-）的反硝化作用中，并定量评估其对海洋N-循环相对于常规氨化-硝化途径的潜在意义。总之，这些计划的分析将给出最完整的数据集的直接测量N-矿化通量以往任何时候都试图在oceans. State的最先进的15 N-稳定同位素标记实验将进行测量同步N-转换率更准确的评估上层海洋N-预算：氨化，硝化，同化（纳入生物量的N）和溶解有机N的释放。我们将通过追踪15 N（自然界中罕见的N的重稳定同位素，与常见的14 N相反）同时从各种改良的有机基质进入不同的N库，以确定有机N是否直接引导硝化或通过氨化作用。与此同时，将通过阐明在基因转录物和蛋白质水平上对这些N-转化起关键作用的生物标志物酶的表达来确定主要的再矿化途径，作为相应过程的可量化活性指标。计划沿沿着大西洋经向断面（AMT）从北（英国）到南大西洋进行采样（福克兰群岛/智利），以研究N-矿化在不同的营养制度，而时间的变化，探讨通过季节性采样在百慕大大西洋时间序列（BATS）网站。这种时空覆盖和互补的跨学科数据集将产生一个高度代表性的描述海洋中的中层N-同化，以及迄今为止最全面的评估在海洋N-循环中的曙暮光区的意义。

The Ocean's twilight (mesopelagic, 100-1000m) zone lies beneath the sunlit surface ocean, with too little light for photosynthesis but above the pitch-black deep ocean, where large animals can no longer see their prey. Of all organic matter that sinks out from the surface, >90% is degraded in the mesopelagic with only a small fraction escaping into the deep ocean (>1000 m). The mesopelagic thus represents an important barrier: most material falling into it is prevented from sinking further by remineralisation - the degradation process that breaks down organic matter and releases CO2 and inorganic nutrients to solution. Eventually, physical mixing or ocean circulation deliver the nutrients back to the surface to fuel phytoplankton growth. Hence, remineralisation in the mesopelagic is critical to controlling the oceanic biological pump, and can affect the ocean's ability to sequester atmospheric CO2. Nitrogen is often the limiting nutrient for biological production in global oceans, its remineralisation would thus be key to biological pump efficiency. However, the mechanisms of N-remineralisation are poorly characterized, and there are no rate measurements of this process in the mesopelagic. The remineralisation of nitrogen (N) in the oceans encompasses ammonification: the degradation of organic N to ammonium (NH4+), and subsequently nitrification: the oxidation of NH4+ to nitrite and then nitrate. However, a recent study suggests that some nitrifiers (microorganisms conducting nitrification) can utilise organic N directly, thus presenting a possible shortcut in the N-cycle. Because the respective organisms have different feeding styles regarding carbon (CO2- fixing or producing), the relative abundance and activities of these functional groups of organisms will have different direct impacts on the CO2 balance, and the existence of the potential shortcut will likely cause a shift towards autotrophy (CO2 fixation).This project aims to determine exactly how and how much nitrogen is remineralised in the twilight ocean, using a combination of state-of-the-art geochemical rate measurements and molecular biological analyses. In particular, we will determine whether the above-mentioned shortcut exists in the remineralisation of organic nitrogen (N) to nitrate (NO3-), and quantitatively assess its potential significance to the oceanic N-cycle relative to the conventional ammonification-nitrification pathway. Together, these planned analyses will give the most complete dataset of directly measured N-remineralisation fluxes ever attempted in the oceans.State-of-the-art 15N-stable-isotope-labeling experiments will be conducted to measure rates of concurrent N-conversions for a more accurate assessment of upper ocean N-budget: ammonification, nitrification, assimilation (incorporation of N into biomass) and release of dissolved organic N. We will do this by tracing 15N (the heavy stable isotope of N that is rare in nature, as opposed to the common 14N) from various amended organic substrates into different N-pools at the same time, to determine whether organic-N is directly channeled to nitrification or via ammonification. In parallel, major remineralisation pathways will be identified by elucidating the expression of biomarker enzymes key to these N-conversions at both gene transcript and protein levels, as quantifiable activity indicators for the respective processes.Sampling is planned along the Atlantic Meridional Transect (AMT) from the north (UK) to south Atlantic (Falklands/Chile) to examine N-remineralisation in diverse nutrient regimes, while temporal variability is explored via seasonal sampling at the Bermuda Atlantic Time Series (BATS) site. Such spatiotemporal coverage and complementary, interdisciplinary dataset would yield a highly representative depiction of mesopelagic N-remineralisation in the oceans, and the most comprehensive assessment to date on the significance of the twilight zone in oceanic N-cycle.