Mechanisms controlling the production and fate of DOM during diatom blooms

智力价值：DOM在海洋系统表层水中的停留时间是决定其对生物碳泵的效率和大小的贡献的主要因素。越来越多的证据表明，从表层海洋输出DOM是在某些海洋区域的深海固存有机碳的一条高效途径。目前的估计是，全球通过生物泵向深处输送的碳有20%是通过出口DOC来实现的。DOM的有效出口要求浮游植物产生的DOM持续存在于表层水中，直到垂直交换过程将物质输送到深处。控制DOM在地表水中积累和持久存在的时间尺度的机制是由生物过程主导的，这些生物过程影响DOM产生的数量和化学性质以及微生物对DOM的消耗和分解。该项目研究了这两个相互耦合的生物过程，以研究在温带硅藻水华期间对DOM积累的控制。众所周知，硅藻水华在进入营养胁迫时会产生大量的DOM，其化学成分随营养限制的类型(硅或氮)而变化。这种可变的组成可能会影响DOM对微生物的营养价值，推动物种向能够最好地代谢特定形式的DOM的异养原核生物的功能群进化。迄今为止，对这一生产/消费耦合系统的每一方都进行了独立审查。一些研究考察了不同限制营养物质的限制如何影响浮游植物产生的DOM的化学性质，而另一些研究则侧重于DOM的命运，而没有详细了解影响其初始化学组成的机制。这项研究项目将同时研究这一耦合过程的两个方面，以了解不同形式的营养限制如何驱动DOM的化学性质和随后的微生物反应，这些反应共同决定了硅藻水华期间产生的DOM的命运。研究人员将采用实验室和现场相结合的方法：1)调查氮或硅的限制如何影响硅藻水华释放的DOM的化学成分。2)通过改变浮游细菌的生产力、生长效率和群落结构，确定经历不同营养胁迫的硅藻产生的DOM组成的差异如何影响其对异养微生物处理的敏感性。这项研究将集中在硅藻水华上，原因有两个。硅藻水华是已知DOC出口显著的海洋区域的常规特征，例如北大西洋，这使得水华期间产生的DOM的命运成为C出口的潜在重要机制。此外，从浮游植物直接释放DOM是众多DOM生产过程中研究得最好的，这为提出假设提供了背景，即DOM生产和组成的变化如何影响推动其消费的细菌反应。初步数据表明，加利福尼亚州圣巴巴拉海峡的水域是进行这项研究的理想模式系统，因为春季硅藻水华具有足够的可预测性，并符合这些研究所需的操作类型，而且沿海水域环境中的DOM浓度较低，使得DOM浓度的微小变化可以在实验室和野外实验中解决。广泛的影响：这项研究将有助于实现更大的科学目标，即了解上层海洋食物网在碳循环中的作用。这些发现将提高我们对DOM动力学的基本理解，有助于建模者改进生态系统模型中关键过程的表示。该项目将通过培训研究生来教育下一代生物海洋学家，并将利用该项目将海洋学引入当地K-12学生的课程，使他们认识到海洋对他们生活的重要性以及将海洋学作为职业选择的可能性。

Intellectual Merit: The residence time of DOM in surface waters of marine systems is the main factor determining its contribution to the efficiency and magnitude of the biological carbon pump. There is growing evidence that the export of DOM from the surface ocean represents a highly efficient pathway for the sequestration of organic carbon in the deep sea in some ocean regions. Current estimates are that 20% of the carbon transported to depth by the biological pump globally occurs via the export of DOC. Effective export of DOM requires that the DOM produced by phytoplankton persist in surface waters until vertical exchange processes transport the material to depth. The mechanisms controlling the time scale for the accumulation and persistence of DOM in surface waters are dominated by biological processes that influence the amount and chemical character of the DOM produced and its consumption and decomposition by microbes. This project addresses these two coupled biological processes to examine controls on the accumulation of DOM during temperate diatom blooms. Diatom blooms are known to produce prodigious quantities of DOM upon entering nutrient stress with a chemical composition that varies with the type of nutrient limitation (Si or N). This variable composition likely influences the nutritional value of DOM to microbes driving species successions towards functional groups of heterotrophic prokaryotes that are best able to metabolize particular forms of DOM. To date each side of this coupled system of production/consumption has been examined independently. A few studies have examined how limitation by different limiting nutrients affects the chemical character of the DOM produced by phytoplankton, while others have focused on the fate of DOM without detailed understanding of the mechanisms influencing its initial chemical composition. This research project will examine both sides of this coupled process simultaneously to see how different forms of nutrient limitation drive the chemical character of DOM and the subsequent microbial response which together determine the fate of DOM produced during diatom blooms. The investigators will employ a combination of laboratory and field based approaches to: 1) Investigate how limitation by either N or Si impacts the chemical composition of the DOM released by diatom blooms. 2) Determine how differences in the composition of DOM produced by diatoms experiencing different nutrient stresses affects it susceptibility to heterotrophic microbial processing through changes in the productivity, growth efficiency and community structure of bacterioplankton. The research will focus on diatom blooms for two reasons. Diatom blooms are a regular feature in regions of the ocean where DOC export is known to be significant, i.e. such as the North Atlantic, making the fate of the DOM produced during blooms a potentially significant mechanism of C export. In addition, the direct release of DOM from phytoplankton is the best studied of numerous DOM production process providing the background for formulating hypotheses on how changes in DOM production and composition affect the bacterial response that drives its consumption. Preliminary data indicates that waters of the Santa Barbara Channel, California are an ideal model system for conducting this research because the spring diatom bloom is sufficiently predictable and amenable to the types of manipulations required for these studies, and ambient DOM concentrations are low for coastal waters allowing small changes in DOM concentrations to be resolved in both laboratory and field experiments.Broader Impacts: This research will contribute to the greater scientific goal of understanding the role of upper ocean food webs in carbon cycling. The findings will improve our basic understanding of DOM dynamics aiding modelers in the development of improved representations of key processes in ecosystem models. The project will enable the education of the next generation of biological oceanographers by training graduate students, and it will be leveraged to introduce oceanography into the curriculum of local K- 12 students to make them aware of the importance of the oceans to their lives and of the possibility of oceanography as a career choice.