Optimal phytoplankton cell size in an allometric model

The competitive ability of phytoplankton cells is simulated in a model chemostat in which cell size is considered to be an adaptive trait. Parameters describing nutrient uptake kinetics are constrained by empirically derived allometric rules. With a steady input of a single nutrient, the evolutionarily stable cell size is selected through competition among phytoplankton. We find that large cells may be favored when (1) phytoplankton growth is limited by the rate at which internally stored inorganic nutrients can be converted into biomass, and (2) maximum quotas increase with size faster than minimum quotas. Increased internal quotas then accelerate the rate of biomass production in large cells, despite their enhanced requirements for resources. The evolutionarily stable strategy is set by the allometric relationships for nutrient uptake kinetics and by metabolism.