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

浮游植物是微小的单细胞生物，在地球-S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，需要阳光和营养才能生长和繁殖。在南极洲周围的海洋中，硝酸盐(NO3-)作为氮(N)的营养来源通常是丰富的，而营养铁往往是稀少的。光的可获得性也从完全黑暗转变为24小时持续的阳光，以及从水柱深处的弱光转变为海洋表面的强光。因此，南大洋的浮游植物通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，营养物质的供应和光照的可获得性会影响这些生物，而这些生物反过来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)的NO3-，这取决于分子中存在哪种稳定的N同位素。浮游植物在吸收和并入生物量时更喜欢使用较轻的同位素，尽管浮游植物使用的15N/14N的比率已被证明随环境条件的不同而不同。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，可以用来确定海水的历史组成和浮游植物的生产力。该项目将以特定的方式检验这一假说，即增强的光照和/或铁胁迫会改变水柱硝酸盐的同位素比率。将进行实验室培养和田间试验相结合。南大洋重要浮游植物的培养将在与环境相关的光和铁条件下进行，在那里将测量浮游植物利用15N/14N的比率、生理变化以及铁和光胁迫以及硝酸盐同化的分子标记。类似的测量将在船上与南非同事一起在南大洋的一艘邮轮上进行。这些数据将增加我们对南大洋过去和现在的生产力，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老多米尼恩大学的研究生以及来自南非的学生将在这个项目上进行合作。对N同位素效应的改进过程的理解不仅将在科学国内和国际会议上提出，而且还将在当地K12学校的地方推广活动中提出。对GEOTRACES和其他航次产生的现代水柱硝酸盐(NO3-)同位素比(D15N)测量结果的解释，以及古代营养物质利用的度量，取决于对浮游植物对N3-的同化与较重的同位素15NO3-(N03-同化)的区别程度的机械理解。我们目前缺乏预测铁和光胁迫如何影响硝酸根同化的能力。这项拟议的工作将检验这一假说，即增强的光和/或铁胁迫提高了NO3同化的epsilon。这一假设将通过实验室培养工作和南大洋机遇号邮轮上的实地工作相结合来验证。中游实验将包括增加和减轻对单克隆性浮游植物培养物和自然浮游植物群落的光和/或铁胁迫，同时测量硝酸根同化反应。将在邮轮上收集水柱样本，以分析溶解和粒度分级的颗粒物N浓度和d15N，以及浮游植物群落组成、光生理学和铁、光胁迫和NO3同化的基因表达标记。特别是，铁和光胁迫标记的表达将被用来评估铁和光胁迫对基于野外的NO3-同化epsilon估计的相对贡献。这些现场测量的结果，以及基于实验室的培养研究，将被用来限制在与环境相关的光和铁条件下N3-同化的epsilon范围，包括被假设在上一次冰川盛期(又名)期间发生的铁胁迫的潜在缓解。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.