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

海洋系统中的大部分有机碳来自浮游植物，浮游植物从二氧化碳中生物合成蛋白质、脂类复合体和多糖等细胞大分子。随着碳通过海洋食物网，这些浮游植物大分子被顺序转化，并在大部分情况下重新矿化回二氧化碳。微生物在这些再矿化过程中发挥关键作用，约占海洋初级生产力的50%(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度和方法在很大程度上还没有被探索。因此，微生物的活动及其催化的碳转化往往被归入黑匣子。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，通过确定碳启动细胞外酶活动循环的速度和方式来阐明黑匣子的工作原理。有人断言，胞外酶解是最重要的。碳循环中的限速步骤(例如Arrieta和Herndl 2002)以前的工作(Arnosti等人。然而，(1994年)表明，这一断言并非先验正确：需要更有区别的观点。在某些情况下，给定的大分子底物在水柱中的水解速度极快，而同一底物在其他地方的海水中不能被检测到的水解(Arnosti等人。已呈交)。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比率存在系统性差异(Arnosti，2000)。是什么因素控制了这些差异？这项拟议的工作旨在系统地审查可能控制海水和沉积物微生物群落中胞外酶的模式和活性的因素。以努力识别和限制一些减速带。在碳循环中，酶解也将与缺氧沉积物中随后的碳转化步骤定量地联系在一起，那里对碳流的时空联系的了解尤其稀少。为了实现这些目标，将结合对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来实现这些目标。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的限制，该项目还将扩大具有良好特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。这些新的底物将用于野外实验和实验室调查，以通过缺氧沉积物中微生物群落的转化来定量测量碳流。

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.