COLLABORATIVE RESEARCH: Quantifying the effects of variable light and iron on the nitrate assimilation isotope effect of phytoplankton

合作研究：量化可变光和铁对浮游植物硝酸盐同化同位素效应的影响

• 批准号: 1851113
• 负责人: Sven Kranz
• 金额: $ 76.44万
• 依托单位: Florida State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-02-15 至 2023-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1851113&HistoricalAwards=false
• 关键词: COLLABORATIVE; RESEARCH; Quantifying; effects; variable

Phytoplankton are microscopic, single-celled organisms that play an important role in the Earth?s ecosystems, elemental cycles, and climate. These organisms, which live in surface ocean waters, require sunlight and nutrients to grow and reproduce. In the oceans around Antarctica, nitrate (NO3-) as a nutrient source of nitrogen (N) is usually abundant while the nutrient iron is often sparse. Light availability also changes from complete darkness to 24 hours of constant sunlight, as well as from low light deeper in the water column to high, stressful light at the ocean surface. As a consequence, the phytoplankton in the Southern Ocean often live in a suboptimal environment in which conditions for growth are frequently changing. Scientists understand that nutrient supply and light availability affect these organisms and that these organisms, in turn, can alter the chemical composition of the seawater. For example, nitrate can occur in different forms, including a lighter (14N) and heavier (15N) form of NO3-, depending on which stable isotope of N is present in the molecule. Phytoplankton prefer to use the lighter isotope during uptake and incorporation into biomass, though the ratio of 15N/14N used by phytoplankton has been shown to vary depending on environmental conditions. Notably, the isotope ratio used by phytoplankton is recorded in sediments and can be used to determine both the historic composition of ocean waters and the productivity of phytoplankton. This project will test the hypothesis that enhanced light and/or iron stress change the isotopic ratios of water column nitrate- in specific ways. A combination of laboratory culture and field experiments will be conducted. Cultures of important Southern Ocean phytoplankton species will be grown under environmentally-relevant light and iron conditions where ratio of 15N/14N used by phytoplankton, physiological changes, and molecular markers of iron and light stress and nitrate assimilation will be measured. Similar measurements will be done in shipboard experiments on a cruise in the Southern Ocean with South African colleagues. These data will increase our understanding of past and present productivity in the Southern Ocean, and how phytoplankton changed the chemical composition of the seawater. Undergraduates from underrepresented groups in the STEM field and graduate students from Florida State University and Old Dominion University as well as students from South Africa will collaborate on this project. The improved process understanding of the N isotope effect will be presented not only at scientific national and international conferences but also during local outreach events at local K12 schools.Interpretation of both modern water column nitrate (NO3-) isotopic ratio (d15N) measurements generated by GEOTRACES and other cruises, as well as metrics of paleo-nutrient utilization, depend upon a mechanistic understanding of the degree to which NO3- assimilation by phytoplankton discriminates against the heavier isotope, 15NO3- (NO3- assimilation epsilon). We currently lack the ability to predict how iron and light stress impacts the NO3- assimilation epsilon. The proposed work will test the hypothesis that enhanced light and/or iron stress elevates the epsilon for NO3-assimilation. This hypothesis will be tested by a combination of laboratory culture work and field work on a cruise of opportunity in the Southern Ocean. Mesocosm experiments will include both increasing and alleviating light and/or iron stress on monoclonal phytoplankton cultures and in natural phytoplankton communities while measuring the response of the NO3- assimilation epsilon. Water column samples will be collected on the cruise for analysis of dissolved and size-fractionated particulate N concentration and d15N, as well as phytoplankton community composition, photophysiology and gene expression markers of iron and light stress and NO3- assimilation. In particular, the expression of iron and light stress markers will be used to evaluate the relative contribution of iron and light stress to field-based estimates of the NO3-- assimilation epsilon. The results from these field measurements, together with lab-based culture studies, will be used to constrain the range of the epsilon for NO3- assimilation under environmentally-relevant light and iron conditions, including the potential alleviation of iron stress as has been hypothesized to have occurred during the last glacial maximum (a.k.a. the Martin hypothesis).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

浮游植物是微观的单细胞生物，在地球生态系统、元素循环和气候中发挥着重要作用。这些生物生活在表层海水中，需要阳光和营养物质才能生长和繁殖。在南极洲周围的海洋中，作为氮（N）营养源的硝酸盐（NO3-）通常很丰富，而营养铁却往往很稀少。光照的可用性也从完全黑暗变为 24 小时持续阳光，以及从水体深处的弱光变为海洋表面的强光。因此，南大洋的浮游植物常常生活在生长条件经常变化的次优环境中。科学家们了解到，养分供应和光的可用性会影响这些生物体，而这些生物体反过来又会改变海水的化学成分。例如，硝酸盐可以以不同的形式存在，包括较轻 (14N) 和较重 (15N) 形式的 NO3-，具体取决于分子中存在哪种稳定的 N 同位素。浮游植物在吸收和融入生物质时更喜欢使用较轻的同位素，尽管浮游植物使用的 15N/14N 比例已被证明会根据环境条件而变化。值得注意的是，浮游植物使用的同位素比率记录在沉积物中，可用于确定海水的历史组成和浮游植物的生产力。该项目将测试以下假设：增强的光和/或铁胁迫会以特定方式改变水柱硝酸盐的同位素比率。将进行实验室培养和现场实验的结合。重要的南大洋浮游植物物种的培养将在与环境相关的光和铁条件下生长，其中将测量浮游植物使用的 15N/14N 比率、生理变化以及铁和光胁迫以及硝酸盐同化的分子标记。类似的测量将与南非同事一起在南大洋巡航的船上实验中进行。这些数据将增进我们对南大洋过去和现在生产力以及浮游植物如何改变海水化学成分的了解。来自 STEM 领域代表性不足群体的本科生、佛罗里达州立大学和奥多明尼恩大学的研究生以及来自南非的学生将合作开展该项目。对 N 同位素效应的改进过程理解不仅会在国家和国际科学会议上展示，还会在当地 K12 学校的当地外展活动中展示。 GEOTRACES 和其他航行以及古养分利用指标取决于对浮游植物对 NO3 同化程度的机械理解反对较重的同位素，15NO3-（NO3-同化ε）。我们目前缺乏预测铁和光胁迫如何影响 NO3-同化 epsilon 的能力。拟议的工作将检验这样的假设：增强的光和/或铁胁迫会提高 NO3 同化的 epsilon。这一假设将通过实验室培养工作和南大洋机会巡航的实地工作相结合来检验。中生态实验将包括增加和减轻单克隆浮游植物培养物和天然浮游植物群落的光和/或铁胁迫，同时测量NO3同化ε的反应。将在航行中收集水柱样本，用于分析溶解和尺寸分级的颗粒氮浓度和 d15N，以及浮游植物群落组成、光生理学以及铁和光胁迫以及 NO3 同化的基因表达标记。特别是，铁和光胁迫标记的表达将用于评估铁和光胁迫对 NO3-同化 epsilon 现场估计的相对贡献。这些现场测量的结果与基于实验室的培养研究一起，将用于限制环境相关的光和铁条件下 NO3 同化的 epsilon 范围，包括铁应力的潜在缓解，正如假设的那样发生在末次盛冰期（又称马丁假说）。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

项目成果

FIELD-BASED NITRATE ASSIMILATION ISOTOPE EFFECT ESTIMATES FROM THE ATLANTIC SECTOR OF THE SOUTHERN OCEAN

南大洋大西洋区基于现场的硝酸盐同位素效应估计

• 发表时间: 2022-01
• 期刊: 2022 Ocean Sciences Meeting
• 作者: Thomas, R.;Fawcett, S.;Forrer, H.;Kranz, S.;Chappell, P.D.;Einarsson, S.;Ryan;Graves, E.;Roychoudhury, A.;Haraguchi, L.;et al
• 通讯作者: et al