Modelling phosphate transport and assimilation in microalgae; how much complexity is warranted?

The operation of a three compartment phosphate model of microalgal growth (phosphate interaction model, PIM), containing internal P pools for polyphosphate (PolyP), soluble inorganic P (SIP) and structural and other organic P (SOP), is compared with that of a conventional quota model which contains only a single internal nutrient pool. PIM enables a simulation of the decoupling of phosphate transport and assimilation and is thus more suitable for studies of short-term transient events and of transport kinetics. For general use, however, the quota model appears satisfactory provided that it has been parameterised using a full data set including cells grown under P-replete conditions in which PolyP can accumulate. This may not be the case for studies undertaken using steady-state P-limited chemostats, which will thus underestimate the value of the maximum value of the P:C quota. From the simulations it appears unwarranted to use a more complex model of P than the three pool PIM. Simulations using PIM indicate that, in addition to the well documented advantage in accumulating P over that required for immediate use, the accumulation of PolyP enables a better decoupling of transport from assimilation and hence the maintenance of high transport rates for longer. This would be of importance to organisms living in a transient nutrient regime and it may be important to simulate competition for P using such a model in ecosystem simulations.