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，在地球上扮演着重要的角色。美国生态系统、元素循环和气候。这些生物生活在海洋表层，需要阳光和营养来生长和繁殖。在南极洲周围的海洋中，作为氮(N)的营养来源的硝酸盐（NO3-）通常是丰富的，而营养铁往往是稀疏的。光的可用性也从完全的黑暗转变为24小时不间断的阳光，从水柱深处的弱光转变为海洋表面的强光。因此，南大洋的浮游植物经常生活在一个次优环境中，生长条件经常发生变化。科学家们了解到，营养供应和光照的可用性会影响这些生物，而这些生物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出现，包括较轻（14N）和较重（15N）形式的NO3-，这取决于分子中存在哪种稳定的N同位素。浮游植物在吸收和融入生物量的过程中更倾向于使用较轻的同位素，尽管浮游植物使用的15N/14N的比例已被证明因环境条件而异。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。这个项目将测试一个假设，即增强的光和/或铁应力会以特定的方式改变水柱硝酸盐的同位素比例。将进行实验室培养和田间实验相结合的方法。南大洋重要浮游植物物种的培养将在与环境相关的光和铁条件下进行，在此条件下，浮游植物利用15N/14N的比例、生理变化、铁和光胁迫和硝酸盐同化的分子标记将被测量。类似的测量结果将在与南非同事在南大洋巡航的船上实验中进行。这些数据将增加我们对南大洋过去和现在的生产力的理解，以及浮游植物如何改变海水的化学成分。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道明大学的研究生以及来自南非的学生将在这个项目上合作。对N同位素效应的改进过程理解不仅将在国家和国际科学会议上展示，还将在当地K12学校的地方推广活动中展示。对GEOTRACES和其他巡航产生的现代水柱硝酸盐（NO3-）同位素比率（d15N）的解释，以及对古营养物利用指标的解释，都取决于对浮游植物对NO3-同化区别于较重同位素15NO3- （NO3-同化epsilon）的程度的机制理解。我们目前缺乏预测铁和光胁迫如何影响NO3-同化的能力。提出的工作将验证增强光和/或铁胁迫会提高no3同化的epsilon的假设。这一假设将通过实验室培养工作和在南大洋的机会巡航上的实地工作相结合来验证。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-同化效应的响应。在巡航中收集水柱样品，分析溶解和分级颗粒N浓度和d15N，以及浮游植物群落组成，铁和光胁迫和NO3-同化的光生理和基因表达标记。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的田间估算的相对贡献。这些野外测量的结果，连同实验室培养研究，将用于限制在与环境相关的光和铁条件下NO3-同化的epsilon的范围，包括铁胁迫的潜在缓解，这是在末次冰期极大期发生的假设（又名马丁假设）。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.