consumption and most of the carbon remineralization in marine waters and sediment 

 Assessments of the role of microbes in transforming and remineralizing organic matter have 

 become a necessary and integral part of any system-level study of the fate of organic matter. 



The measurement of bacterial production is a powerful approach for estimating the 

 contributions of heterotrophic bacteria to overall metabolism in ecosystems. The supply of 

 bacterial biomass potentially available to grazers can be determined from rates of bacterial 

 production. Bacterial production, almost by definition, represents a net conversion rate of 

 DOC to POC. Combined with estimates of the average growth efficiency of the bacterial 

 community, production rates can be used to calculate the total utilization of DOC and 

 consumption of oxygen by bacteria. The validity of these estimates is dependent on how well 

 bacterial growth efficiency is known, and can be checked by comparing bacterial production to 

 direct measurements of respiration (thereby estimating growth efficiency). In addition, rates of 

 bacterial production can be used as a sensitive indicator of the response of bacteria to spatial 

 and temporal fluctuations in environmental conditions. 



At present, most bacterial processes are measured as community averages, with 

 essentially no characterization of the internal structure of bacterial communities or the 

 dynamics of individual species. Because of these limitations, it is possible to measure the 

 average process rate but difficult to gain a mechanistic understanding of the factors which 

 control bacterial processes. Unmeasured changes in community composition can invalidate 

 predictions of the response of bacteria to a particular stimulus, unless the response is uniform 

 across the community. Differential responses of species within the community are masked 

 when all measurements are community averages. 



New methodologies developed in the OMP are directed toward providing assessments of 

 the response of specific bacteria, for example to variations in the concentration or form of 

 DOC derived firom phytoplankton. Based on standard molecular biology techniques, these 

 methods target individual taxa, focus on single-cell measurements, are incubation-free, and are 

 relatively economical in terms of sampling and processing time. 



D. Geochemical Tracers of Organic Matter Transformations 



A geochemical approach can be used to quantify the transformation rates between 

 dissolved, colloidal and particulate forms of organic carbon occurring near Cape Hatteras. 

 One approach is to examine the cycling of dissolved, colloidal and particulate forms of organic 

 carbon by analyzing size-fractionated samples for isotopes of thorium and carbon. Knowledge 

 of the time-scales of transformation and the sources of organic carbon is important to establish 

 the degree to which organic carbon is buried in shelf sediments or exported by advection to 

 the open ocean. 



It is important to note that interactions between colloids, smaU particles and sinking 

 aggregates are complex and mediated by a combination of biological (primary production, 

 respiration, bacterial production, zooplankton grazing and production) and abiological 

 (adsorption, aggregation, disaggregation) processes. The proposed geochemical work will 

 provide information on the abiologically controlled transformation rates, which are, in turn, 

 intimately related to physical and biological processes. Together with direct information on 

 the biologically controlled transformation rates, the Th isotopic results will provide a 

 comprehensive understanding of the time scales of organic carbon transformation. By using 



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