Physical limits on iron uptake mediated by siderophores or surface reductases

The excretion of siderophores and the reduction of organic iron-complexes a t the cell surface are common reactions of terrestrial plants, fungi and ba cteria in response to low availability of iron. However, there is much less evidence for the use of these strategies by marine phytoplankton. It has b een argued that siderophore excretion is inefficient in an aquatic environm ent due to rapid diffusion. This study examines how diffusion and chemical reactions in the microenvironment of a phytoplankton cell influence the eff iciency of both strategies to increase the bioavailability of iron and to r educe iron stress. a numerical model of the cell surroundings is presented that calculates the concentration distribution for different iron species a nd allows to study the effect of siderophores or surface reductases. It cal culates the efficiency of these mechanisms, defined as the quotient between the increase in iron uptake rate and the excretion rate of siderophores or electrons. needed to obtain this increase. The dependence of this efficien cy on rates of iron coordination reactions, on diffusivity, and on the kine tics of iron uptake is discussed with the aid of some analytical calculatio ns. (C) 1999 Elsevier Science B.V. All rights reserved.