Inorganic and Organic Nitrogen Utilization in the Southern Ocean Mesoscale Iron Enrichment Experiment (SOFeX)

在亚北极太平洋、赤道太平洋和南大洋的高硝酸盐、低叶绿素(HNLC)水域进行的中尺度(现场)和微型(船上瓶装)铁富集性实验的结果有力地表明，浮游植物的生长和生物量积累的速度至少部分地受到铁的可获得性的限制。为了在不受甲板孵化实验限制的情况下直接评估这一假设，将在南大洋进行现场中尺度铁富集化实验--南大洋是地球上最大的HNLC区域，也是调节全球气候最重要的区域。SOFEX项目由莫斯·兰丁海洋实验室领导，将在今年夏天(12月至1月)沿着170‘W子午线进行两次铁浓缩实验。在一次为期45天的航行中，他们将在极地阵线以南富含硝酸盐和硅酸盐的水域以及极地阵线以北富含硅酸盐但富含硝酸盐的水域给水域施肥。这一子项目将调查自然浮游生物群落对铁的富集所产生的氮素动态。15N稳定同位素技术将被用来量化富铁场所内外的硝酸盐、亚硝酸盐、铵和尿素的吸收，从而提供一种不依赖于这些水域内环境氮浓度的可观测变化的氮吸收的直接和明确的衡量标准。由于细胞大小被认为是控制物质从表层流向深海(即出口生产)的主要因素，因此将通过使用不同孔径的过滤器在短期15N孵化后收集颗粒物来确定氮利用的粒度谱(微微、纳米和微型浮游生物)。为了评估该HNLC区域氮素偏好(或抑制)的控制因素，将结合N吸收速率测量来评估硝酸盐还原酶(NR)和亚硝酸盐还原酶(NIR)的活性，以确定铁限制是否直接调节浮游植物(无论是大的还是小的)吸收氨和硝酸盐的能力，并测试铁限制的浮游植物是否是铁氮共限制的。甲板实验的目的是确定辐射对硝酸盐和铵吸收的影响。在与MLML和Mark Brzezinski(UCSB)的合作研究中，这个子项目中的科学家将确定不同浓度的铁和锌对浮游植物(特别是硅藻)的生长以及它们对氮和硅酸盐的利用的影响。为了确保氮营养速率是新的和再生的生产的准确衡量标准，实验设计包括对14NH4的同位素稀释的影响、溶解的有机氮(DON)和N02中15N的释放损失以及来自异养吸收的竞争的影响。这些氮吸收实验是拟议的SOFEX研究的一个关键组成部分，其结果对于全面了解南大洋生态系统对铁的富集化的反应至关重要。浮游植物的适度增长和大量营养素的缺乏是HNLC区域共同的两个基本观察结果，在南大洋仍有待解释。这项研究将直接回答这样一个问题：铁是否促进硝酸盐的吸收和同化？如果是，刺激的是哪一组人？所有这些都是在就地生态系统对铁的富集化做出反应的背景下进行的。

The results of mesoscale (in situ) and microscale (shipboard-bottle) iron enrichment experiments conducted in the high nitrate, low chlorophyll (HNLC) waters of the sub-arctic Pacific, equatorial Pacific and the Southern Ocean strongly suggest that the rate of phytoplankton growth and biomass accumulation are limited, at least in part, by the availability of iron. To directly assess this hypothesis without the restrictions of deck-board incubation experiments, an in situ mesoscale iron enrichment experiment will be conducted in the Southern Ocean - the largest HNLC region on Earth and arguably the most important area in regulating global climate. The SOFeX project, led by Moss Landing Marine Laboratories, will involve two iron enrichment experiments along the 170'W meridian during this austral summer (December-January). During one 45-day cruise they will fertilize waters in the nitrate-rich and silicate-rich waters south of the Polar Front, and in the silicate-poor, but nitrate-rich waters north of the Polar Front. This sub-project will investigate the nitrogen dynamics of the natural planktonic community in response to the iron enrichment. The 15 N stable isotope technique will be used to quantify nitrate, nitrite, ammonium and urea uptake within and outside of the iron-enriched sites, thus providing a direct and unambiguous measure of nitrogen uptake independent of anv observable changes in the ambient nitrogen concentrations within these waters. Since cell size is thought to be a major factor controlling the flux of material from the surface to deep waters (i.e. export production), the size- spectrum of nitrogen utilization (pico-, nano- and microplankton) will be determined by using different pore-sized filters to collect particulates after short-term 15 N incubations. To evaluate the factors controlling nitrogen preference (or inhibition) in this HNLC region, the activities of nitrate reductase (NR) and nitrite reductase (NiR) will be assessed, in concert with N uptake rate measurements, to determine if iron limitation directly regulates the ability of phytoplankton (both large and small) to assimilate ammonium versus nitrate and test whether iron-limited phytoplankton are Fe-nitrogen co-limited. Deck experiments are designed to determine the effects of irradiance on nitrate and ammonium uptake. During collaborative studies with MLML and Mark Brzezinski (UCSB), the scientists in this sub-project will determine the effects of various concentrations of iron and zinc on the growth of phytoplankton (in particular diatoms) and their use of nitrogen and silicate.To ensure that the rates of nitrogenous nutrition are an accurate measure of new and regenerated production, the experimental design includes corrections for the effects of isotopic dilution from 14 NH4, losses from release of 15 N in dissolved organic nitrogen (DON) and N02, and competition from heterotrophic uptake. These N uptake experiments are a key component in the proposed SOFeX study and their results are critical to a comprehensive understanding of the ecosystem response to iron enrichment in the Southern Ocean. Modest phytoplankton growth and the lack of significant depletion of macronutrients are two fundamental observations common to HNLC regions and still remain to be explained in the Southern Ocean. This study will directly answer the question 'does iron stimulate the uptake and assimilation of nitrate, and if so, what group is stimulated?' all within the context of an in situ ecosystem response to iron enrichment.