Appendix I 

 Molecular Approaches to Understanding Limiting Factors 



Understanding the factors limiting biogeochemical cycles is 

 crucial to predicting how changes in the physical and chemical 

 environment will affect ecosystem responses. Although many 

 regulatory processes have been well characterized at the organismal 

 and biochemical levels, we know much less about molecular 

 mechanisms in nature and how they are regulated by the present 

 global environment. The carbon and nitrogen cycles have been 

 singled out as examples, although cycling of other elements and 

 their interactions also are important. 



The concept of limitation has been broadly defined. An 

 environmental factor can be limiting in a true ecological sense, 

 i.e., a factor limiting the growth or standing crop below its 

 potential maximum growth rate or biomass. Mechanisms of 

 physiological acclimation that optimize growth rates under limiting 

 conditions can be used to develop diagnostic tools. We use the 

 term "diagnostics" to denote a signal or procedure which provides 

 specific information to symptomatically identify an environmental 

 constraint. The aim of diagnostics is to interrogate natural 

 populations without experimentally manipulating them. 



Limitation of growth by an environmental factor can become so 

 severe (stress) as to go beyond the tolerance limits of an 

 organism, so causing the replacement or disappearance of a species 

 within an ecosystem. We can stretch the concept of limiting 

 factors to include specific ecological adaptations to a particular 

 environment. These adaptations can be indicative of which factors 

 dominate the dynamics of an ecosystem or a particular niche within 

 an ecosystem. This ecological adaptation can be reflected as a 

 physiological response to a stress (either reversible or 

 irreversible; e.g., adaptation to high pressure; facultative vs. 

 obligate anaerobes) . 



Selected specific examples of ecologically relevant, rate- 

 limiting processes that can be, or have been, approached with 

 molecular biological techniques are presented below (see also 

 Falkowski and LaRoche, 1991) . The potential limiting factors 

 discussed are COp/water, nitrogen, iron, trace metals, temperature, 



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