IRON UPTAKE AND GROWTH LIMITATION IN OCEANIC AND COASTAL PHYTOPLANKTON

Iron concentrations in open ocean are orders of magnitude lower than levels in coastal waters. Experiments with coastal and oceanic phytoplankton clones representing different algal groups and cell sizes indicate that cellular iron uptake rates are similar among the species when rates are normalized to cell surface area. This similarity in rates apparently is explained by evolutionary pressures that have pushed iron uptake in all species toward the maximum limits imposed by diffusion and ligand exchange kinetics. Because of these physical/chemical limits on uptake, oceanic species have been forced to decrease their cell size and/or to reduce their growth requirements for cellular iron by up to 8-fold. The biochemical mechanisms responsible for this reduction in metabolic requirements are unknown.