Coccolithophore Mixotrophy

颗石藻是单细胞藻类，被称为颗石的石灰石（方解石）板覆盖。它们可能构成海洋中浮游植物生物量的大部分。 颗石藻通常被认为是自养生物，这意味着它们利用光合作用将碳固定到软植物组织和硬矿物方解石中，利用阳光作为能源（“自养”）。然而，越来越多的证据表明，颗石藻是“混合营养”的，这意味着它们可以通过光合作用固定碳，并通过吞噬小的有机颗粒以及从海水中吸收其他简单的碳分子来在黑暗中生长。 颗石藻参与混合营养的程度可以影响碳向深海的转移。 这项工作从根本上是针对量化颗石藻混合营养-使用溶解和减少碳化合物作为能源的能力-使用实验室和现场实验，并澄清其与海洋生物学和化学的相关性。 这项工作将在三个方面产生更广泛的影响：1）本科生培训：两名REU本科生将在项目期间接受培训。第二年的学生将参加研究巡航。2)咖啡馆科学计划：这项工作将在毕格罗实验室？科学咖啡馆计划。这些都是免费的公众聚会，邀请公众参加关于海洋科学和技术的最新想法和问题的对话。3)数字电子书：我们建议制作一本数字电子书，专门强调和解释颗石藻内的混合营养。混合营养颗石藻的图像将是这本书的主要视觉元素。电子书将免费提供并分发给我们的教育附属机构，科尔比学院。这本书的目的是进一步传达微生物世界的复杂性，食物网动态，以及它们与全球碳循环的关系，激发对这些惊人的生命形式的兴趣，教育和好奇心。颗石藻可以显著影响大气中二氧化碳的下降，它们可以通过两种碳泵机制将二氧化碳从海洋表面转移到深海中：（1）“碱度泵”（也称为碳酸钙泵），海洋表层的颗石藻吸收溶解的无机碳（DIC;主要是一种称为碳酸氢盐的形式，是海洋碱度的主要成分）。它们将一半转化为二氧化碳，二氧化碳要么作为植物生物量固定下来，要么作为气体释放出来，一半被合成为矿物球石。 因此，颗石藻钙化实际上可以在短时间尺度（即几周）内增加表面CO2。 然而，经过数月到数年的时间，球石会沉入数千米以下，在那里它们会溶解并将碳酸氢盐释放回深水中。因此，下沉的球石实质上是将碳酸氢盐碱度从表层“泵”到深层沃茨，在那里，碳在深海深处被隔离了数千年。(2)“生物泵”，其中致密石灰石球石的压载作用加速了有机软组织碎片（颗粒有机碳或POC）的下沉，基本上将这种软碳组织“泵”到深处。 生物泵最终减少了地表CO2。软组织和碱度泵在维持DIC的垂直梯度（比表面更深）方面相互加强，但它们在CO2的海气交换方面相互对立。因此，颗石藻对大气CO2的净效应取决于其与碱度泵相关的CO2升高效应和与软组织生物泵相关的CO2降低效应的平衡。几乎总是假设颗石颗粒有机碳（PIC）完全来自溶解有机碳（DIC，作为碳酸氢盐），而不是溶解有机碳（DOC）。本提案的目的是描述a）一系列DOC化合物被颗石藻吸收和同化的可能性，B）吸收速率，以及颗石藻将DOC并入PIC颗石藻的可能性，如果这是真的，将代表碱度泵范例的重大转变。 这项工作的基本目的是量化颗石藻混合营养使用实验室和现场实验，并澄清其相关的海洋生物学和化学。 在解决这一问题方面取得了一些技术进步，所有这些都将应用于这项工作。研究者将：（a）筛选球石藻培养物对大量DOC分子的吸收和同化，（B）用特定DOC分子进行示踪实验，以检查在环境现实浓度下的吸收，（c）测量DOC固定到有机组织中，与固定到PIC球石藻中的DOC分离，（d）使用流式细胞术将颗石藻与其他浮游植物和细菌分离，以及（e）区分这些分选的颗石藻细胞中的营养模式。这项工作将从根本上推进在海洋中的颗石藻混合营养的知识状态，并解决碳的平衡，颗石藻来自自养与异养来源。

Coccolithophores are single-cell algae that are covered with limestone (calcite) plates called coccoliths. They may make up most of the phytoplankton biomass in the oceans. Coccolithophores are generally considered to be autotrophs, meaning that they use photosynthesis to fix carbon into both soft plant tissue and hard minerogenic calcite, using sunlight as an energy source ("autotrophic"). However, there is an increasing body of evidence that coccolithophores are "mixotrophic", meaning that they can fix carbon from photosynthesis as well as grow in darkness by engulfing small organic particles plus taking up other simple carbon molecules from seawater. The extent to which Coccolithophores engage in mixotrophic can influence the transfer of carbon into the deep sea. This work is fundamentally directed at quantifying coccolithophore mixotrophy -- the ability to use dissolved and reduce carbon compounds for energy -- using lab and field experiments plus clarifying its relevance to ocean biology and chemistry. This work will generate broader impacts in three areas: 1) Undergraduate training: Two REU undergraduates will be trained during the project. The student in the second year will participate in the research cruise. 2) Café Scientifique program: This work will be presented in Bigelow Laboratory?s Café Scientifique program. These are free public gatherings where the public is invited to join in a conversation about the latest ideas and issues in ocean science and technology. 3) Digital E-Book: We propose to make a digital E-book to specifically highlight and explain mixotrophy within coccolithophores. Images of mixotrophic coccolithophores would be the primary visual elements of the book. The E-book will be publically available and distributed to our educational affiliate, Colby College. The goal of the book is to further communicate the intricacies of the microbial world, food web dynamics, plus their relationship to the global carbon cycle, to inspire interest, education, and curiosity about these amazing life forms.Coccolithophores can significantly affect the draw-down of atmospheric CO2 and they can transfer CO2 from the surface ocean and sequester it in the deep sea via two carbon pump mechanisms: (1) The "alkalinity pump" (also known as the calcium carbonate pump), where coccolithophores in the surface ocean take up dissolved inorganic carbon (DIC; primarily a form called bicarbonate, a major constituent of ocean alkalinity). They convert half to CO2, which is either fixed as plant biomass or released as the gas, and half is synthesized into their mineral coccoliths. Thus, coccolithophore calcification can actually increase surface CO2 on short time scales (i.e. weeks). However, over months to years, coccoliths sink below thousands of meters, where they dissolve and release bicarbonate back into deep water. Thus, sinking coccoliths essentially "pump" bicarbonate alkalinity from surface to deep waters, where that carbon remains isolated in the abyssal depths for thousands of years. (2) The "biological pump", where the ballasting effect of the dense limestone coccoliths speeds the sinking of organic, soft-tissue debris (particulate organic carbon or POC), essentially "pumping" this soft carbon tissue to depth. The biological pump ultimately decreases surface CO2. The soft-tissue and alkalinity pumps reinforce each other in maintaining a vertical gradient in DIC (more down deep than at the surface) but they oppose each other in terms of the air-sea exchange of CO2. Thus, the net effect of coccolithophores on atmospheric CO2 depends on the balance of their CO2-raising effect associated with the alkalinity pump and their CO2-lowering effect associated with the soft-tissue biological pump. It is virtually always assumed that coccolith particulate organic carbon (PIC) originates exclusively from dissolved organic carbon (DIC, as bicarbonate), not dissolved organic carbon (DOC). The goal of this proposal is to describe a) the potential uptake and assimilation of an array of DOC compounds by coccolithophores, b) the rates of uptake, and potential incorporation of DOC by coccolithophores into PIC coccoliths, which, if true, would represent a major shift in the alkalinity pump paradigm. This work is fundamentally directed at quantifying coccolithophore mixotrophy using lab and field experiments plus clarifying its relevance to ocean biology and chemistry. There have been a number of technological advances to address this issue, all of which will be applied in this work. The investigators will: (a) screen coccolithophore cultures for the uptake and assimilation of a large array of DOC molecules, (b) perform tracer experiments with specific DOC molecules in order to examine uptake at environmentally-realistic concentrations, (c) measure fixation of DOC into organic tissue, separate from that fixed into PIC coccoliths, (d) separate coccolithophores from other phytoplankton and bacteria using flow cytometry and e) distinguish the modes of nutrition in these sorted coccolithophore cells. This work will fundamentally advance the state of knowledge of coccolithophore mixotrophy in the sea and address the balance of carbon that coccolithophores derived from autotrophic versus heterotrophic sources.