Investigating the physiological basis for covariations in light-limited and light-saturated photosynthesis

生物海洋学的一个中心目标是确定海洋初级生产力的速率。随着卫星技术的出现，科学家现在能够在适当的时间和距离尺度上估计藻类的叶绿素生物量，但从叶绿素估计产量需要理解和应用生理机制，通过这些机制我们可以将叶绿素丰度与碳产生速率联系起来。长期以来，光合作用与辐照度(PE)的关系被证明是描述藻类生产及其对环境胁迫的响应的一个有用的概念。在这里，研究人员将开展一个项目，该项目源于Behrenfeld等人最近的一项分析。其中，作者询问了光合作用的光限光斜率(Ab)和光饱和速率(Pbmax)之间共同变化的普遍现象的基础。这一现象对PE关系的影响超过了光适应反应的影响，但ab和Pbmax的协变性相对较少受到关注，其生理基础仍未确定。Ab和Pbmax的共变对光合作用的光饱和辐照度(Ek)影响很小或没有影响。存在与Ek无关的变异性。如果如通常假设的那样，ab和Pmax受到光合作用的不同方面的限制，并且ab相对恒定，那么ab和Pbmax中与Ek无关的变化是完全意想不到的。在这个项目中，研究人员概述了与Ek无关的变异性现象，并对这种独特的行为提出了新的解释。我们的假设是，这种不依赖于Ek的现象是细胞内对还原剂需求相对于与特定代谢途径相关的ATP的变化的表达，这些代谢途径在单个昼夜周期的时间尺度上将优势转移到营养受限生长的季节性变化。主要代谢途径的这些转变推动了线性光合作用电子转移(PSII到碳还原)和替代途径之间的电子流动的变化，这些替代途径将电子从PSII迅速转移回O2，并在此过程中产生ATP。更广泛的影响包括促进我们对光合作用基本过程的理解，并将导致改进对全球海洋初级生产力及其在生物中介的碳固存中的作用的估计。它还将支持研究生的教育和实践培训。

A central goal of biological oceanography has been to determine the rates of primary production in the ocean. With the advent of satellite technology, scientists are now able to estimate algal chlorophyll biomass on appropriate time and distance scales, but the estimation of production from chlorophyll requires an understanding and application of the physiological mechanisms by which we can relate chlorophyll abundance to a carbon production rate. The photosynthesis vs. irradiance (PE) relationship has long proven a useful construct for describing variability in algal production and its response to environmental forcings. Here researchers will undertake a project born from a recent analysis by Behrenfeld et al. in which the authors interrogate the basis of the common phenomenon of co-variations in the light-limited slope (ab) and light-saturated rate (Pb max) of photosynthesis. The implications of this phenomenon on PE relationships exceed those of the photoacclimatory responses, yet covariability in ab and Pb max has received relatively little attention and the physiological basis has remained undetermined. Covaritation in ab and Pb max results in little or no influence on the light saturation irradiance for photosynthesis (Ek). There is Ek-independent variability. Ek-independent changes in ab and Pb max are wholly unexpected if, as generally assumed, ab and Pmax are limited by separate aspects of photosynthesis and ab is relatively constant. In this project, the researchers present an overview of the phenomenon of Ek-independent variability and propose a new explanation for this unique behavior. Our hypothesize is that this Ek-independent phenomenon is an expression of intracellular changes in demands for reductant versus ATP associated with specific metabolic pathways that shift in dominance at time scales of single diurnal periods to seasonal changes in nutrient-limited growth. These shifts in dominant metabolic pathways drive changes in electron flow between linear photosynthetic electron transfer (PSII to carbon reduction) and alternative pathways that rapidly transfer electrons from PSII back to O2 and in the process generate ATP. The broader impacts include advancing our understanding of the fundamental processes of photosynthesis and will lead to improved estimates of global ocean primary production and its role in biologically-mediated carbon sequestration. It will also support the education and practical training of a graduate student.