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（原位）和微尺度（船载瓶）铁富集实验的结果强烈表明，浮游植物生长和生物量积累的速度，至少在一定程度上，受到铁的可用性的限制。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，将在南大洋进行一项原位中尺度铁富集实验。南大洋是地球上最大的高通量低通量区域，可以说是调节全球气候的最重要区域。由Moss Landing海洋实验室领导的SOFeX项目将在今年南方夏季（12月至1月）沿着170′w子午线进行两次铁富集实验。在一次为期45天的巡游中，他们将在极锋以南富含硝酸盐和硅酸盐的水域以及极锋以北缺乏硅酸盐但富含硝酸盐的水域施肥。本子项目将研究天然浮游生物群落对铁富集的氮动态响应。15n稳定同位素技术将用于量化富铁点内外的硝酸盐、亚硝酸盐、铵和尿素的吸收，从而提供一种直接而明确的氮吸收测量，而不依赖于这些水域环境氮浓度的任何可观察到的变化。由于细胞大小被认为是控制物质从表层流向深水（即出口生产）的一个主要因素，因此将通过使用不同孔径的过滤器收集短期15n培养后的颗粒来确定氮利用的大小光谱（微型、纳米和微型浮游生物）。为了评估控制该HNLC区域氮偏好（或抑制）的因素，将评估硝酸盐还原酶（NR）和亚硝酸盐还原酶（NiR）的活性，并结合氮吸收速率测量，以确定铁限制是否直接调节浮游植物（大型和小型）同化铵与硝酸盐的能力，并测试铁限制浮游植物是否铁氮共同限制。甲板实验旨在确定辐照度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）生长及其对氮和硅酸盐的利用的影响。为了确保氮营养速率是新产量和再生产量的准确测量，实验设计包括对同位素稀释效应的校正，溶解有机氮（DON）和二氧化氮中15n释放的损失，以及异养吸收的竞争。这些氮吸收实验是SOFeX研究的关键组成部分，其结果对于全面了解南大洋生态系统对铁富集的响应至关重要。适度的浮游植物生长和缺乏大量营养物质的消耗是HNLC区域共同的两个基本观测结果，但在南大洋仍有待解释。这项研究将直接回答以下问题：铁是否刺激硝酸盐的吸收和同化？如果是，刺激哪一组？所有这些都是在原地生态系统对铁富集的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.