Speed Bumps in the Carbon Cycle: Enzymatic Hydrolysis and Carbon Flow in Marine Systems

海洋系统中大部分有机碳来源于浮游植物，浮游植物利用二氧化碳合成细胞大分子，如蛋白质、脂质复合物和多糖。当碳通过海洋食物网时，这些浮游植物大分子依次转化，大部分再矿化回二氧化碳。微生物在这些再矿化过程中发挥关键作用，循环约50%的海洋初级生产力（Azam 1998）。尽管它们的重要性是显而易见的，但微生物将浮游植物衍生的碳转化为二氧化碳的具体速率和方式在很大程度上尚未被探索。因此，微生物活动及其催化的碳转化常常被归入黑箱。碳循环，其内部工作原理是未知的。在这个项目中，北卡罗来纳大学教堂山分校的研究人员将进行调查，通过确定碳通过细胞外酶的活动启动循环的速率和方式，来阐明黑盒子的工作原理。有人断言，胞外酶解是。然而，之前的工作（Arnosti et al. 1994）表明，这一断言并非先验的正确：需要一个更有区别的视角。在某些情况下，给定的大分子底物在水柱中的水解速度非常快，而相同的底物在海水中的其他位置无法被检测到水解（Arnosti等人提交）。也有证据表明，海水和沉积异养微生物的胞外酶活性的性质和速率存在系统性差异（Arnosti, 2000）。是什么因素控制了这些差异？提出的工作旨在系统地研究可能控制海水和沉积微生物群落中细胞外酶的模式和活动的因素。为了识别和限制一些减速带。在碳循环中，酶水解也将定量地与缺氧沉积物中随后的碳转化步骤联系起来，其中碳流的时空联系的知识尤其稀少。为了实现这些目标，酶水解速率和模式的实地测量将与可能影响异养微生物群落中酶表达的特定因素的实验室研究相结合。由于酶活性的测量受到商业上可用的底物和底物替代品数量有限的限制，该项目还将扩大已被充分表征的浮游植物大分子的光谱，这些大分子的水解和随后的转化可以被详细研究。这些新的基质将用于野外实验和实验室研究，以定量测量缺氧沉积物中微生物群落通过转化产生的碳流。

ABSTRACTOCE-0323975Most organic carbon in marine systems originates from phytoplankton, which biosynthesize cellular macromolecules such as proteins, lipid complexes, and polysaccharides from CO2. These phytoplankton macromolecules are sequentially transformed and for the most part remineralized back to CO2 as carbon passes through the marine food web. Microbes play a key role in these remineralization processes, cycling ca. 50% of marine primary productivity (Azam 1998). Although their importance is clear, the specific rates and means by which microbes transform phytoplankton-derived carbon to CO2 are largely unexplored. Microbial activities and the carbon transformations they catalyze therefore are often relegated to the black box. of carbon cycling, whose inner workings are unknown.On this project, researchers at the University of North Carolina at Chapel Hill will conduct investigations to illuminate the workings of the black box by determining the rates and means by which carbon initiates cycling through the activities of extracellular enzymes. The assertion has been made that extracellular enzymatic hydrolysis is .the. rate-limiting step in carbon cycling (e.g. Arrieta and Herndl 2002) Previous work (Arnosti et al. 1994), however, demonstrates that this assertion is not a priori true: a more differentiated perspective is required. In some cases, hydrolysis of a given macromolecular substrate in the water column is extremely rapid, while the same substrate is not detectably hydrolyzed in seawater at other locations (Arnosti et al. submitted). There is also evidence for systematic differences in the nature and rates of extracellular enzymatic activities of seawater and of sedimentary heterotrophic microbes (Arnosti, 2000). What factors control these differences? The proposed work is intended to examine systematically factors that may control the patterns and activities of extracellular enzymes among seawater and sedimentary microbial communities. In an effort to identify and constrain some of the speed bumps. in the carbon cycle, enzymatic hydrolysis will also be linked quantitatively with subsequent carbon transformation steps in anoxic sediments, where knowledge of spatial and temporal connections of carbon flow is particularly sparse.To accomplish these goals, field measurements of enzymatic hydrolysis rates and patterns will be coupled with laboratory studies of specific factors that may affect enzyme expression in heterotrophic microbial communities. Since measurements of enzyme activities have been restricted by the limited number of commercially available substrates and substrate proxies, this project will also expand the spectrum of well-characterized phytoplankton macromolecules whose hydrolysis and subsequent transformations can be studied in detail. These new substrates will be used in field experiments as well as in laboratory investigations to quantitatively measure carbon flow via transformations carried out by microbial communities in anoxic sediments.