Current ecosystem models suggest that an increase in the 

 global mean temperature will lead to a larger increase in 

 respiration than in photosynthesis, primarily because of microbial 

 metabolism. If so, the predicted effect would be to further 

 increase atmospheric CO- levels, potentially leading to even higher 

 temperatures: a positive feedback. The effect would be most 

 dramatic at high latitudes in the northern hemisphere, where 

 climatic models predict the greatest temperature change, and where 

 significant amounts of organic carbon are deposited. The ecosystem 

 models are poorly parameterized to analyze this response. 



Molecular techniques are potentially available to examine how 

 temperature will differentially affect photosynthesis and 

 respiration. For example, if photosynthesis is not temperature 

 limited, but limited by CO^ availability, then increased 

 temperatures would potentially lead to increased transpiration but 

 decreased efficiency in water- use; e.g., effectively, a drought- 

 stress response. The maximum rate of photosynthesis is internally 

 limited at light saturation by the ratio of dark-carboxylation 

 capacity to electron-transport components. Using protein 

 immunoblotting methods in conjunction with non-invasive biophysical 

 probes of fluorescence, the inherent limitation on photosynthesis 

 in both higher plants and microalgae in nature can be studied. 

 Such studies would provide a basis for understanding how 

 temperature affects carbon fixation, providing that an excess of 

 other potential external limiting factors is present. An increase 

 in temperature would be expected to shift microbial communities 

 slowly from psycrophyllic (cold-loving) to mesophyllic (moderate 

 temperatures) . This community shift might be tracked with 

 molecular taxonomic techniques (see next section on Community 

 Structure) . 



The following are examples of key questions that were 

 identified with respect to inherent biological limitations related 

 to the fluxes of materials and energy: 



• What inherent processes determine the efficiency with 

 which organisms oxidize or reduce carbon? 



• How do various metabolic groups fractionate carbon, 

 nitrogen, sulfur, oxygen, hydrogen, and phosphorus? 



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