Cell-quota theory

The second level, or cell-quota theory, allows organisms to vary their content of nutrient Q (and hence their yield of biomass from assimilated nutrient). It is increasingly referred to as the DROOP model after one of its authors (Droop, 1968, 1983). In principle, the quota should be defined as the ratio of nutrient to biomass (Droop, 1979) and will here be understood as the population (atomic) ratio of the nutrient element to carbon. A simplified version of the theory and some deductions from it, is given in Box 1 (refer page 348). The key equation (ignoring physical transports) is ... 

Fig. 9.1 Cell-quota theory for control of photo-autotroph growth by internal nitrogen or phosphorus. Q is the cell quota for the nutrient, in atoms of the element per atom of organic carbon. kfj is the minimum value, or subsistence quota. /////1IU1X gives growth as a proportion of maximum rate. The function 10) multiplies nutrient uptake (which is also a function of ambient concentration) and brings it towards zero as Q tends towards (i llax. The third part of the diagram compares typical ranges of values of cellular N and P content and show how these contribute to variation in the cell N P ratio.

Finally, cell-quota theory treats the entire cellular content of a nutrient as being the pool controlling growth rate (under limiting conditions), and deals with multiple nutrient interactions empirically. At the third level of description, MECHANISTIC models aim to embody realistic accounts of the main biochemical processes and pools within cells. A recent example (Flynn Hipkin, 1999 Flynn, 2001) deals with nitrogen, phosphorus, silicon and iron as well as the carbon content of cells, photosynthesis, and the uptake competition between ammonium and nitrate. However, the model embodies many parameters and there is currently insufficient information to use it to distinguish between groups or species of phytoplankters. Its characteristic nutrient quota parameters are included in Table 9.3 except for those for silicon, which are cell-based. 

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