A mechanistic microbial underpinning for the size-reactivity continuum of dissolved organic carbon degradation

海洋溶解有机质（DOM）是地球上最大的活跃循环有机碳储存库之一，是全球碳循环的重要组成部分。DOM的高分子量（HMW）部分比低分子量（LMW）部分更年轻，更容易被微生物消耗，但HMW DOM和LMW DOM之间反应性差异的原因尚不清楚。两个因素可能解释了HMW DOM更强的反应性：(i)特定细菌对HMW DOM的靶向吸收，PI和她在马克斯普朗克海洋微生物研究所（MPI）的合作者最近在表层海水中发现了这一过程；（ii） HMW DOM更倾向于聚集并形成凝胶和颗粒，这些凝胶和颗粒可以被有能力分解有机物的细菌定植。来自北卡罗来纳大学教堂山分校的科学家和学生将与MPI海洋微生物研究所（德国不来梅）的研究人员合作，研究海洋微生物群落对HMW DOM的分解。这些调查将包括在北大西洋进行实地考察，在此期间，将比较HMW DOM在不同水团和不同有机物可用性条件下的降解速率和模式。还将评估DOM聚集的潜力和这些聚集的降解率。将使用专门的显微镜来确定HMW DOM的摄取机制和速率。这项工作将由正在进行的对分解HMW DOM、它们的基因和蛋白质的特定细菌的研究来补充。研究生和本科生将作为研究团队的重要成员参与实验室和实地工作的各个方面；该项目的各个方面也将被整合到科学家在北卡罗来纳大学教授的课程中。DOM的尺寸-反应性连续体的存在-观察和测量表明，高分子量的DOM往往比低分子量的DOM更年轻，反应性更强。已经在海洋不同区域的实验室和实地调查中得到证实。然而，对于HMW DOM更强的反应性还缺乏一个机制上的解释。该项目将研究HMW DOM降解的机制和测量速率，重点是识别参与者并确定导致HMW DOM快速循环的因素。UNC和MPI-Bremen的合作研究最近发现了一种在远洋海洋细菌中常见的HMW底物摄取新机制：这些细菌快速结合，部分水解并直接通过外膜运输大片段的HMW底物，然后在质周空间内降解，避免在外部环境中产生LMW DOM。这种处理底物的模式被称为自私，因为有针对性的HMW底物摄取将资源从微生物群落的其他成员中隔离出去。因此，测量和模型必须考虑海洋中底物利用的三种模式：自私，共享（外部水解，导致低分子量产物）和清除（摄取低分子量水解产物而不产生细胞外酶）。通过实地研究和中观实验，研究小组将调查不同底物吸收模式占主导地位的情况和地点。该项目的第二个重点是确定聚合的HMW DOM的聚集潜力和微生物降解。初步研究表明，颗粒相关的微生物群落比自由生活的微生物群落利用更广泛的酶促能力。这些能力使颗粒相关群落能够有效地针对广泛的复杂基质。因此，该项目将重点关注HMW DOM的两个关键方面——特殊细菌选择性隔离HMW底物的能力，以及HMW底物聚集的更大潜力。并将在海洋的不同位置和深度量化这些因素。因此，该项目将为观察DOC大小-反应性连续体提供一个机制基础，这是发展对海洋有机物降解的整体机制理解的重要组成部分。

Marine dissolved organic matter (DOM) is one of the largest actively-cycling reservoirs of organic carbon on the planet, and thus a major component of the global carbon cycle. The high molecular weight (HMW) fraction of DOM is younger in age and more readily consumed by microbes than lower molecular weight (LMW) fractions of DOM, but the reasons for this difference in reactivity between HMW DOM and LMW DOM are unknown. Two factors may account for the greater reactivity of HMW DOM: (i) targeted uptake of HMW DOM by specific bacteria, a process the PI and her collaborators at the Max Planck Institute for Marine Microbiology (MPI) recently identified in surface ocean waters; and (ii) a greater tendency of HMW DOM to aggregate and form gels and particles, which can be colonized by bacteria that are well-equipped to breakdown organic matter. Scientists and students from the University of North Carolina (UNC) - Chapel Hill will collaborate with researchers at the MPI for Marine Microbiology (Bremen, Germany) to investigate this breakdown of HMW DOM by marine microbial communities. These investigations will include a field expedition in the North Atlantic, during which HMW DOM degradation rates and patterns will be compared in different water masses and under differing conditions of organic matter availability. DOM aggregation potential, and degradation rates of these aggregates, will also be assessed. Specialized microscopy will be used in order to pinpoint HMW DOM uptake mechanisms and rates. The work will be complemented by ongoing studies of specific bacteria that breakdown HMW DOM, their genes, and their proteins. Graduate as well as undergraduate students will participate as integral members of the research team in all aspects of the laboratory and field work; aspects of the project will also be integrated into classes the scientist teaches at UNC. The existence of a size-reactivity continuum of DOM - observations and measurements showing that HMW DOM tends to be younger and more reactive than lower MW DOM ? has been demonstrated in laboratory and field investigations in different parts of the ocean. A mechanistic explanation for the greater reactivity of HMW DOM has been lacking, however. This project will investigate the mechanisms and measure rates of HMW DOM degradation, focusing on identifying the actors and determining the factors that contribute to rapid cycling of HMW DOM. Collaborative work at UNC and MPI-Bremen recently identified a new mechanism of HMW substrate uptake common among pelagic marine bacteria: these bacteria rapidly bind, partially hydrolyze, and transport directly across the outer membrane large fragments of HMW substrates that can then be degraded within the periplasmic space, avoiding production of LMW DOM in the external environment. This mode of substrate processing has been termed selfish, since targeted HMW substrate uptake sequesters resources away from other members of microbial communities. Measurements and models thus must account for three modes of substrate utilization in the ocean: selfish, sharing (external hydrolysis, leading to low molecular weight products), and scavenging (uptake of low molecular weight hydrolysis products without production of extracellular enzymes). Using field studies as well as mesocosm experiments, the research team will investigate the circumstances and locations at which different modes of substrate uptake predominate. A second focal point of the project is to determine the aggregation potential and microbial degradation of aggregated HMW DOM. Preliminary studies have demonstrated that particle-associated microbial communities utilize a broader range of enzymatic capabilities than their free-living counterparts. These capabilities equip particle-associated communities to effectively target a broad range of complex substrates. The project will thus focus on two key aspects of HMW DOM - the abilities of specialized bacteria to selectively sequester HMW substrates, as well as the greater potential of HMW substrates to aggregate ? and will quantify these factors at different locations and depths in the ocean. The project will thereby provide a mechanistic underpinning for observations of the DOC size-reactivity continuum, an essential part of developing an overall mechanistic understanding of organic matter degradation in the ocean.