ironlimiting

Iron limitation is a condition in which the growth and metabolism of organisms are constrained by insufficient bioavailable iron, a micronutrient essential for processes such as photosynthesis, respiration, and nitrogen fixation. In many aquatic environments, particularly open oceans and high-pH soils, iron is scarce relative to biological demand, making it a limiting factor for productivity.

Iron availability is controlled by chemical form and ligands. In oxidizing waters, iron exists mainly as insoluble Fe(III) oxides; only a small fraction is soluble as inorganic Fe' or bound to organic ligands. Microorganisms and plants secrete siderophores and reductants to access iron. External sources such as aeolian dust deposition, upwelling, or hydrothermal vent flux provide iron, so its distribution strongly influences ecosystem structure.

Ecological and physiological consequences: In iron-limited oceans, phytoplankton growth is suppressed, carbon fixation is reduced, and community composition shifts toward species with efficient iron use. Iron limitation also affects diazotrophs and nitrogen cycling; in soils, it can limit microbial activity and plant iron uptake, leading to chlorosis in crops at high pH.

Evidence and management: Field and laboratory iron fertilization experiments in iron-poor waters often induce phytoplankton blooms, demonstrating limitation. In agriculture, iron fertilization or soil amendments can alleviate deficiency. Natural iron inputs from dust, rivers, and mixing events modulate limitation, while iron speciation and ligand binding govern true bioavailability more than total iron.