Regenerated Fe is tasty!

Bioavailability of nutrients is an essential factor controlling primary productivity in the ocean. In addition to macronutrients such as nitrogen and phosphorous, availability of the trace element iron unequivocally affects growth rates and community structure of phytoplankton and thereby primary productivity in many ocean regions. External sources of iron such as Aeolian dust, upwelling of Fe-rich waters, and hydrothermal are reduced in high-nutrient low-chlorophyll regions, and most Fe used by phytoplankton has been regenerated by zooplankton. While zooplankton regeneration of Fe was first shown two decades ago, major factors controlling this process such as chemical composition of prey and grazer taxonomy are not well constrained. As pH varies significantly in digestive systems between protozoa and mesozooplankton, we hypothesize that the extent and the bioavailability of regenerated Fe is a function of the digestive physiology. Furthermore, major element components such as silica for diatoms and calcium carbonate for cocolithophores may be able to buffer the pH of digestive systems of different grazer taxa. Such effects may further influence the magnitude and bioavailability of regenerated Fe. In order to constrain the effect of grazer taxonomy and chemical composition of prey on Fe bioavailability, 55Fe-labeled phytoplankton were fed to different grazers and unlabeled phytoplankton were subsequently inoculated to the filtrate of the grazing experiment in the regrowth phase of the experiment, and the uptake of 55Fe into the phytoplankton biomass was monitored over time. A parallel uptake experiment using inorganic 55Fe was used to compare the bioavailability of regenerated and inorganic Fe to the same phytoplankton species. Furthermore, some samples of the inorganic and the regenerated uptake experiments were treated with an oxalate rinse to remove any adsorbed Fe. This allowed us to estimate the adsorption of 55Fe from either source to the cell walls of phytoplankton. In an experiment using the diatom Thalassiosira pseudonana as prey and regrowth organism and the copepod Acartia tonsa as grazer, ~45% of regenerated Fe was taken up in the regrowth phase within 30 minutes. After 24 hours almost all regenerated Fe was taken up by T. pseudonana. In contrast, only ~10% and ~60% of inorganic Fe was associated with T. pseudonana cells after 0.5 and 24 hours, respectively. Furthermore, inorganic Fe adsorbed strongly to the frustule of T. pseudonana. At 0.5 hours almost 60% of cell-associated Fe was adsorbed in the inorganic uptake experiment. In contrast, experiments with regenerated Fe showed that all cell associated Fe was taken up after 30 min. These results indicate that copepod grazing produces readily bioavailable Fe.Results from additional experiments probing the effects of grazer taxonomy and chemical composition of prey on the bioavailability of regenerated Fe will be presented as well.