A model of phytoplankton acclimation to iron-light colimitation

We developed and calibrated a dynamic model for cellular carbon, chlorophyll (Chl), and iron under iron-light colimitation. The model allows growth rate and two other state variables (Fe-p : C and Chl : C) to be described as functions of light intensity and the free iron concentration (Fe'). The model requires specification of the values of nine parameters. We obtained values for these parameters using published experimental results for Thalassiosira pseudonana using a combination of a random parameter initialization and a golden section search to minimize the cost function. The tuned model explained 95% of the variability in the observations of growth rate, 94% in Chl : C, and 90% in Fe-p : C. Although the model is applicable to both balanced and unbalanced growth conditions, data were only available for balanced growth; thus, the dynamics of state variables during unbalanced growth conditions could not be investigated. A limitation in calibrating the model was in the scarcity of suitable experimental data sets under well-defined environmental forcing. This points to the need for new experimental work on iron-limited cultures, including measurements of photosynthesis-light curves and the dynamic responses to changed Fe' and light intensity. This phytoplankton growth model provides a physiological treatment of iron-light colimitation for implementation within ocean biogeochemical models. By including both growth rate and elemental stoichiometry (e.g., Fe-p : C) as state variables, the model can be applied to assess both rate and yield limitation.