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

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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.