Variable fluorescence techniques are increasingly used to assess phytoplankton photosynthesis. All fluorescence techniques and models for photosynthetic electron transport rates (ETRs) are amplitude-based and are subject to errors, especially when phytoplankton growth is nutrient-limited. Here we develop a new, kinetic-based approach to measure, directly and in absolute units, ETRs and to estimate growth rates in phytoplankton. We applied this approach to investigate the effects of nitrogen limitation on phytoplankton photophysiology and growth rates. Nutrient stress leads to a decrease in the quantum yield of photochemistry in Photosystem II (Fv/Fm); however, the relationship between Fv/Fm and growth rates is highly nonlinear, which makes it impossible to quantify the reduction in phytoplankton growth rates from Fv/Fm alone. In contrast, the decline in growth rates under nitrogen stress was proportional to the decrease in kinetic-based photosynthetic rates. Our analysis suggests the kinetic fluorescence measurements markedly improve the accuracy of ETR measurements, as compared to classical amplitude-based measurements. Fluorescence-based methods for primary production rely on measurements of ETRs and then conversion to carbon fixation rates by using the electron yields of carbon fixation. The electron yields exhibit 10-fold variability in natural phytoplankton communities and are strongly affected by nutrient limitation. Our results reveal that a decrease in the growth rates and the electron yields of carbon fixation is driven by, and can be quantified from, a decrease in photosynthetic turnover rates. We propose an algorithm to deduce the electron yields of carbon fixation, which greatly improve fluorescence-based measurements of primary production and growth rates. In contrast to terrestrial ecosystems, oceanic primary production and growth are fundamentally limited by the availability of nutrients, such as nitrogen and iron, and in some regions colimited by phosphorus. The paucity of nutrients in the global ocean leads to a substantial decrease in the efficiency of oceanic photosynthesis. On the global scale, oceanic phytoplankton operates at only 50% of its potential maximum (Lin et al. 2016). At first approximation, the distributions of nutrient stress in the global ocean show a marked meridional pattern, with most of tropical and subtropical gyres being limited by the paucity of nitrogen and polar and subpolar regions by iron (Moore et al. 2013). The physiological effects of nutrient stress on phytoplankton growth are determined by the concentrations and fluxes of nutrients in the upper water column and these fluxes are highly dynamic and variable in space and time. Also, susceptibility to nutrient stress varies significantly among phytoplankton species, taxa, and size groups (Sunda and Huntsman 1997; Litchman and Klausmeier 2008). Understanding and quantifying the effects of nutrient stress on primary production is a fundamental task of biological oceanography and addressing this challenging task requires development of technologies for rapid identification and quantitative analysis of nutrient stress in the ocean. Chlorophyll a (Chl a) fluorescence techniques are commonly used to measure biomass and physiological status of phytoplankton and benthic organisms in marine ecosystems (Falkowski et al. 2004). Assessment of the photosynthetic efficiency in these organisms relies on the measurement and analysis of Chl a “variable fluorescence,” a property unique to the photosynthetic machinery (Falkowski et al. 2004). Variable Chl a fluorescence is the most sensitive, nondestructive signal detectable in the upper ocean that reflects instantaneous phytoplankton photophysiology and photosynthetic rates (Falkowski and Kolber 1995; Kolber et al. 1998). Variable fluorescence techniques rely on the relationship between fluorescence yields and the efficiency of photosynthetic processes and provide a comprehensive suite of characteristics of energy transfer in lightharvesting complexes, photochemical reactions in Photosystem *Correspondence: gorbunov@marine.rutgers.edu This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

中文翻译：

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使用叶绿素荧光动力学确定水生生态系统中的光合作用

可变荧光技术越来越多地用于评估浮游植物的光合作用。光合电子传输速率 (ETR) 的所有荧光技术和模型都是基于振幅的，容易出错，尤其是当浮游植物生长受到营养限制时。在这里，我们开发了一种新的基于动力学的方法来直接和以绝对单位测量 ETR 并估计浮游植物的增长率。我们应用这种方法来研究氮限制对浮游植物光生理学和生长速率的影响。营养压力导致光系统 II (Fv/Fm) 中光化学量子产率的降低；然而，Fv/Fm 和增长率之间的关系是高度非线性的，这使得无法量化仅 Fv/Fm 对浮游植物增长率的降低。相比之下，氮胁迫下生长速率的下降与基于动力学的光合速率的下降成正比。我们的分析表明，与经典的基于振幅的测量相比，动态荧光测量显着提高了 ETR 测量的准确性。基于荧光的初级生产方法依赖于 ETR 的测量，然后通过使用碳固定的电子产率转换为碳固定率。电子产量在天然浮游植物群落中表现出 10 倍的可变性，并且受到营养限制的强烈影响。我们的结果表明，碳固定的增长率和电子产量的下降是由光合周转率的下降驱动的，并且可以从光合周转率的下降中量化。我们提出了一种推导碳固定电子产率的算法，这大大改善了初级生产和增长率的基于荧光的测量。与陆地生态系统相比，海洋初级生产和增长从根本上受到氮和铁等养分供应的限制，并且在一些受磷限制的地区。全球海洋中营养物质的缺乏导致海洋光合作用效率大幅下降。在全球范围内，海洋浮游植物仅以其潜在最大值的 50% 运作（Lin 等人，2016 年）。粗略估计，全球海洋中养分胁迫的分布显示出明显的经向模式，大多数热带和亚热带环流受氮缺乏限制，极地和亚极区受铁限制（Moore 等人，2013 年）。养分胁迫对浮游植物生长的生理影响取决于上层水体中养分的浓度和通量，这些通量在空间和时间上具有高度动态性和可变性。此外，浮游植物物种、分类群和体型组对营养胁迫的敏感性差异很大（Sunda 和 Huntsman 1997；Litchman 和 Klausmeier 2008）。了解和量化养分胁迫对初级生产的影响是生物海洋学的一项基本任务，解决这一具有挑战性的任务需要开发快速识别和定量分析海洋养分胁迫的技术。叶绿素 a (Chl a) 荧光技术通常用于测量海洋生态系统中浮游植物和底栖生物的生物量和生理状态（Falkowski 等人，2004 年）。对这些生物体光合效率的评估依赖于对“可变荧光”叶绿素的测量和分析，这是光合机制独有的特性（Falkowski 等人，2004 年）。可变叶绿素 a 荧光是在上层海洋中可检测到的最灵敏、无损的信号，它反映了浮游植物的瞬时光生理学和光合速率（Falkowski 和 Kolber 1995；Kolber 等人 1998）。