metabolic budget of the cell that is concerned with survival and 

 maintenance rather than growth. Such maintenance functions are 

 usually thought of as, for example, the maintenance of osmotic 

 gradients, repair of cellular damage, and transport of nutrients. 

 Under many conditions, especially those of nutrient limitation or 

 slow growth, maintenance energy is thought to be a major portion of 

 the metabolic budget of cells. Alternatively, during long-term 

 starvation, all metabolism, including maintenance energy, may 

 approach non- functional (or at least non-detectable) levels. This 

 issue may be of central importance to our understanding of the role 

 of the biota in the carbon cycle of many different environments. 

 For example, the importance of the microbial loop in aquatic 

 ecology is assessed directly from estimates of growth rates of 

 bacterial populations. 



One of the tenets of the carbon cycle is that burial has 

 accounted for a major geological sink for carbon on Earth. Such 

 processes occur when refractory carbon is buried before it can be 

 oxidized to CC^, and is, thus, fixed or sequestered as sedimentary 

 carbon. Thus, understanding the processes that lead to the 

 oxidation and burial of carbon are of central importance in 

 understanding the global fluxes of carbon. 



Proceeding vertically downward through virtually any 

 stratified environment, including lakes, fjords, ocean basins, 

 marshes, marine and freshwater sediments, and even many soils and 

 other terrestrial sediments, oxygen usually disappears as a major 

 element, because of aerobic respiration and the resulting 

 consumption of oxygen. Subsequently, other available electron 

 acceptors are sequentially used, usually in a manner consistent 

 with the thermodynamic energy available (e.g., nitrate is used 

 after oxygen and followed by manganese, iron, sulfite, sulfate, and 

 CC^) . If the rates of consumption or production of metabolic 

 products are greater than their rates of diffusion, then redox 

 boundaries are established, leading to boundaries between oxygen 

 and nitrate, nitrate and manganese, manganese and iron. Across 

 these boundaries, it is common to find high accumulations of 

 microorganisms capable of more specific redox reactions. For most 

 carbon- oxidizing anaerobes, there are methods to estimate reaction 

 rates in the laboratory; however, there is a critical need to 

 develop techniques which also can be applied to rate measurements 

 in the field. 



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