Organic Carbon Remineralization Rates in Polar Sediments: Relationships Between Initial and Terminal Steps, and Microbial Community Composition, Distribution, and Activity

长期低温是深海和极地海洋的特征，因此也是大多数海洋沉积物的特征，但我们对这些环境的了解仍然有限。关于这些环境中的微生物过程必然受到低温或温度和底物共同作用的阻碍的说法(Reichardt，1987；Pomeroy等人，1990；Wiebe等人，1992)最近被一些研究反驳，这些研究发现，微生物群落在极地和温带环境中同样有效地发挥作用(Thingstad和Martinussen，1991；Nedwell等人，1993；Wheeler等人，1996)。然而，极地微生物群落在抑制其温带微生物群落的条件下保持其活动的具体手段尚未得到详细的调查。该项目的目标是详细研究永久寒冷的北极沉积物中微生物群落对有机碳周转的调节和效率。重点将放在有机碳再矿化的初始步骤和结束步骤的速率、微生物群落的组成和大小、群落中单个成员的活动和分布以及孤立生物和净微生物群落的温度响应之间的关系。这项工作将由马克斯·普朗克海洋微生物研究所(德国不来梅)和北卡罗来纳大学教堂山分校的科学家联合开展。该项目将使用新开发和成熟的方法来表征极地微生物群落的生理、活动、分布和温度敏感性，并测量有机物再矿化的初始和最终过程的速率。实地研究将主要在北冰洋斯瓦尔巴群岛西海岸附近的峡湾进行。细胞外酶解大分子是有机碳再矿化的第一步，将通过使用一种新开发的涉及荧光标记多糖的技术来测量(Arnosti，1996)。多糖将是目标大分子，因为它们占总有机质的很大比例，最近的研究(Amon和Benner，1994)强调了它们在全球碳循环中的重要性。缺氧海洋沉积物中有机碳再矿化的最终步骤--硫酸盐还原，将使用成熟的技术进行测量(Jorgensen，1978)。有机碳再矿化的初始步骤和最终步骤都将在原地温度下在完整的沉积物岩心中进行测量。此外，有机碳再矿化的初始步骤和最终步骤都将作为温度的函数进行测量，以确定微生物群落的净响应。硫酸盐还原细菌和胞外产酶细菌将在低温下分离和鉴定，并同样测定它们的温度敏感性和生长反应。拟议研究的一个主要目标是开发分子探针，以确定沉积微生物群落的种群组成和沉积物中特定生物的分布。此外，还将在细菌富集培养中研究一系列多糖的水解、转化和再矿化，以提供从完整沉积物中无法获得的特定水解机制和碳降解途径的信息。对缺氧极地沉积物中有机碳再矿化初始步骤和最终步骤的速率和温度响应的详细研究将有效地提供发生大部分有机物再矿化的‘书挡’，并将有助于划定微生物群落对环境温度的反应边界。多糖在最终促进硫酸盐还原的有机大分子中所占比例很高；确定它们对最终再矿化过程的贡献将更接近定量了解沉积碳循环及其所有复杂性的目标。通过确定一些负责碳再矿化初始步骤和最终步骤的生物体的生理、活动和种群分布，该研究项目将阐明这些重要但尚未被充分了解的微生物群落。这项研究应该会对控制永久寒冷沉积物中微生物活动和碳循环的因素有新的见解，这些因素构成了全球沉积物环境的主要部分。

9906054ArnostiPerrmanently cold temperatures are characteristic of the deep and polar oceans and therefore of the majority of ocean sediment yet our understanding of these environments is still limited. The suggestion that microbial processes in these environments are necessarily hampered by lowtemperatures or combined effects of temperature and substrate (Reichardt,1987; Pomeroy et al., 1990; Wiebe et al., 1992) has recently been countered by a number of studies which have found evidence that microbial communities function as efficiently in polar as in temperate environments (Thingstad and Martinussen, 1991; Nedwell et al., 1993; Wheeler et al., 1996). The specific means by which polar microbial communities maintain their activities under conditions which inhibit their temperate counterparts, however, have yet to be investigated in detail. The objective of this project is to examine in detail the regulation and efficiency of organic carbon turnover by microbial communities in permanently cold Arctic sediments. The focus will be on the relationships between rates of the initial and terminal steps of organic carbon remineralization, the composition and size of the microbial community, the activities and distributions of individual members of the community, and the temperature responses of isolated organisms as well as of the net microbial community. The work will be carried out jointly by scientists from the Max-Planck Institute for Marine Microbiology (Bremen, Germany) and the University of North Carolina-Chapel Hill. The project will use both newly-developed and well-established methods to characterize the physiology, activity, distributions, and temperature sensitivity of polar microbial communities, and to measure the rates of initial and terminal processes in organic matter remineralization. Field studies will be carried out principally in fjords near the west coast of Svalbard, in the Arctic Ocean. Extracellular enzymatic hydrolysis of macromolecules, the initial step of organic carbon remineralization, will be measured through the use of a newly-developed technique involving fluorescently-labeled polysaccharides (Arnosti, 1996). Polysaccharides will be the target macromolecules because they comprise a significant proportion of total organic matter, and recent studies (Amon and Benner, 1994) have highlighted their importance in the global carbon cycle. The terminal step of organic carbon remineralization in anoxic marine sediments, sulfate reduction, will be measured using well-established techniques (Jorgensen, 1978). Both the initial and the terminal steps of organic carbon remineralization will be measured in intact sediment cores at in situ temperatures. In addition, both the initial and the terminal steps of organic carbon remineralization will be measured as functions of temperature in order to determine the net microbial community response. Sulfate-reducing as well as extracellular-enzyme producing bacteria will be isolated at low temperature and characterized, and their temperature sensitivities and growth responses will likewise be determined. A major goal of the proposed research is to develop molecular probes to determine the population composition of the sedimentary microbial community and distributions of specific organisms in sediments. In addition, the hydrolysis, transformation, and remineralization of a suite of polysaccharides will be investigated in bacterial enrichment cultures in order to provide information on specific hydrolysis mechanisms and carbon degradation pathways which cannot be obtained from intact sediments. A detailed study of the rates and temperature responses of the initial and terminal steps of organic carbon remineralization in anoxic polar sediments will effectively provide 'bookends' between which remineralization of a major portion of organic matter occurs, and will help delineate the boundaries of the microbial community response to environmental temperature. Polysaccharides constitute a high percentage of the organic macromolecules which ultimately fuel sulfate reduction; defining their contribution to terminal remineralization processes will bring closer the goal of a quantitative understanding of the sedimentary carbon cycle, with all of its complexities. By determining the physiology, activity, and population distribution of some of the organisms responsible for the initial and terminal steps of carbon remineralization, this research project will shed light on these important but insufficiently understood microbial communities. This study should lead to new insights into the factors controlling microbial activity and carbon cycling in permanently cold sediments, which comprise a major portion of the global sedimentaryenvironment.