On Saturating Response Curves from the Dual Perspectives of Photosynthesis and Nitrogen Metabolism
Saturation kinetic curves are one of the most well-known relationships in phytoplankton ecology. Classic examples are those of the photosynthesis–irradiance relationship and of nutrient uptake kinetics. With saturation curves, the independent factor is controlling at low concentrations and noncontrolling at high concentrations. This simple, nontrivial, and profound concept is at the heart of nearly all biological and ecological systems models and is commonly the basis for evaluating “limitation” by environmental factors, whether it is at the level of cell physiology or ecosystem response. Here, using examples from both photosynthesis and nutrient uptake, we describe how and why a cell does more than “ride up and down” the response curve; there is strong biological regulation between the demand side of the curve (getting what is needed when supplies are low) and the assimilation of the resource (the maximal rate always set by the biochemical reactions and their constants). We discuss how the biological regulation of the limiting and saturating portion of the curve should be the focus—rather than the curve itself—for understanding biological responses and we describe the importance of energy and amino acid ratios as signals for photosynthetic pigment regulation and nitrogen uptake regulation, respectively. Whereas the use of classic, descriptive curves for building phytoplankton productivity models is traditional, we advocate the development of dynamic, regulatory models that better describe the range of variability in resource acquisition in complex environments.
KeywordsPhotosynthesis Nitrogen nutrients Modelling Saturation curves Phytoplankton Kinetics Synechococcus Dynamic balance Photoacclimation
This is contribution number 5127 from the University of Maryland Center for Environmental Science.
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