Abstract: The availability of iron exerts an important influence on the biological productivity of marine systems. Dissolved iron(III) speciation in seawater is dominated by complexation with strong organic ligands which mediate the biological availability and geochemical cycling of iron. Some of these chelators are thought to be derived from intracellular material released via grazing or cell lysis. Tetrapyrroles, for example, derived from chlorophyll pigments and cytochrome enzyme systems, have been hypothesized to contribute significantly to the ligands that bind iron in seawater, and to participate actively in biological iron recycling. We propose to test this hypothesis and evaluate the role of tetrapyrroles as iron complexing agents in seawater by looking at two important aspects of this problem: (1) the potential for chlorophyll degradation products to bind iron in seawater; and (2) the biological availability of iron-porphyrins. This work will employ a combination of techniques (radiotracers, electrochemistry, chromatography, spectroscopy, mass spectrometry) to elucidate the iron-binding characteristics of model tetrapyrroles, the structural nature of iron-tetrapyrrole association, and the biological availability of iron-tetrapyrrole complexes. It is anticipated that the results of this work will be immediately relevant to researchers studying the marine biogeochemistry of iron, by providing information about the chemical speciation and reactivity of dissolved iron in seawater. In addition this research is likely to elucidate some under-studied aspects of tetrapryrrole diagenesis, with perhaps significant implications for the use of metalloporphyrins as geochemical biomarkers.