The global cycle of iron (Fe), a key element to life and one of the most abundant elements in the earth’s crust, is hindered by its low solubility in seawater. This results in phytoplankton (microscopic algae) in large swaths of the global ocean being limited by the scarcity of this important micronutrient. With the weight of its influence on global carbon cycling, the last 30 years of this paradigm has resulted in a field of research dedicated to the accurate quantification of Fe in the marine environment through trace metal clean sampling and handling procedures. The development and standardization of these methods has culminated in an international effort to sample large and representative portions of the global ocean for dissolved trace elements, isotopes, and properties that influence their cycling through the GEOTRACES program . The last 10 years of GEOTRACES efforts has resulted in a 100-fold increase in high quality measurements of trace elements, with Fe at the forefront, providing a wealth of new information to decipher the cycling processes of this micronutrient in the ocean.
One of the primary goals of these data sets is to understand how the distribution of Fe limits primary productivity of phytoplankton and the drawdown of CO2 from the atmosphere into the ocean interior, termed the marine biological carbon pump. However, this task is complicated by the fact that not all chemical forms of Fe in the ocean are available for uptake by phytoplankton. This was demonstrated early in culture studies using organic molecules that bind Fe and change the rate of Fe internalization into phytoplankton cells. Measurements of organic forms of Fe in the natural marine environment has revealed >99% of the dissolved Fe exists in these forms, an observation that has held throughout the water column and across ocean basins sampled in the GEOTRACES program. Understanding the bioavailability of Fe in natural marine systems therefore requires an understanding of the stability and concentration of these organic complexes and how they influence the Fe available for uptake by marine phytoplankton. Drawing a connection between uptake rates and measurements of organic forms of Fe is not trivial, as evidence suggests that these forms are not a simple mixture of one or two compounds that can be chemically characterized, but rather a complex spectrum of compounds whose shared influence contribute to an integrated measurement of stability and concentration observed in the natural marine environment.
This study represents an important advancement in building a framework for connecting these two independent measurements in the marine Fe cycle. We have taken samples from distinct regions across the global ocean, measured their properties of organic complexation, loaded these complexes with a radioactive Fe isotope, and tracked the internalization rates from these forms to a diverse set of phytoplankton species. The approach established and verified in this study opens a new way for determining Fe bioavailability in samples from across the oceans, and enables modeling of in situ Fe uptake rates by phytoplankton based simply on measured Fe concentrations.