Iron Recycling

Assessing the chemical speciation and bioavailability of iron regenerated by marine zooplankton

Iron is required for numerous biological processes in the upper ocean and is recognized as a limiting nutrient in many marine ecosystems. Iron limitation impacts phytoplankton community structure, as well as C fixation and export from surface waters. While external Fe inputs constrain the export of this element over longer timescales, Fe is rapidly cycled in surface waters through biological uptake and regeneration. Indeed, by far the largest direct source of Fe for phytoplankton growth in the open ocean is regeneration via zooplankton grazing. Digestion of phytoplankton biomass in protistan vacuoles and metazoan guts is expected to result in chemical redox and speciation changes, but these alterations remain largely unstudied and untested. Only one study to date has quantified Fe-binding ligands produced during grazing, and the results are somewhat equivocal and complicated by complex trophic interactions during the experiment. The bioavailability of regenerated Fe has been studied largely with model ligands, and the few studies to examine availability of grazer-produced ligands reach conflicting conclusions. Here we propose a comprehensive laboratory and field investigation into the mechanisms of zooplankton regeneration of Fe and the consequences for chemical speciation and bioavailability. We have four science objectives: (a) determine pH and redox state in the digestive vacuole/guts of protistan and metazoan zooplankton; (b) determine the effect of prey – grazer pairing on organic and redox speciation of regenerated Fe; (c) determine the bioavailability of regenerated Fe to Fe-limited cultures of model phytoplankton; and (d) determine Fe regeneration, speciation and bioavailability in field experiments using natural prey- grazer assemblages from a temperate coastal system and an open ocean system. These will be accomplished through grazing experiments with a suite of ecologically-representative and biogeochemically-important model organisms (bacterial, diatom and coccolithophore prey; heterotrophic ciliate and dinoflagellate protozoa grazers; small and large copepod grazers), as well as with natural coastal (Gulf of Maine) and open ocean (Southern Ocean) plankton communities. Regenerated Fe and ligands will be chemically characterized (Fe(II), soluble/colloidal size fractionation, ligand concentration and conditional stability constant), and bioavailability directly assayed via uptake by Fe-limited phytoplankton cultures (diatom, coccolithophore and cyanobacterial strains). This will enable broad application of these results to existing datasets. Further, chemical conditions (pH, redox) within grazer vacuoles/guts will be directly assayed with fluorescent dyes and microelectrode measurements, with the goal of providing a mechanistic understanding of taxonomic variations in regeneration. The proposed experiments will significantly advance our understanding of zooplankton Fe regeneration processes, a critical and understudied component of the ocean Fe cycle.