Iron limitation of phytoplankton growth is now widely accepted, but we know little about the use of iron (Fe) by heterotrophic bacteria. Limited published and preliminary data indicate that bacteria can account for a large but variable fraction of biological Fe uptake. Because so few studies have been conducted, however, we do not understand what controls the relative amount of Fe uptake by bacteria and phytoplankton. Recent reports also suggest that Fe may limit bacterial growth and affect the efficiency of dissolved organic matter (DOM) utilization. If so, then Fe availability could control not only carbon fixation by primary producers, but also the mineralization of DOM by heterotrophic microbes in some oceanic regimes. This project describes research using an interdisciplinary approach to answer the question: What is the relative importance of bacteria and phytoplankton in Fe uptake, and what biological and chemical factors control Fe uptake by each group? We hypothesize that the important biological factors determining the partitioning of Fe include biomass, production and Iron:Carbon (Fe:C) ratios of phytoplankton and bacteria. The critical chemical factor is the degree and nature of organic complexation of dissolved Fe. Field work will be carried out in three contrasting regimes: Delaware Bay (high Fe, nutrients and biomass), the oligotrophic Sargasso Sea (moderate Fe levels, low nutrients and biomass) and a recently discovered Fe limited region off the coast of California (a low Fe, high-nutrient, low-chlorophyll regime). Trace metal clean radiotracer experiments will measure Fe uptake by bacteria and phytoplankton to give us a complete picture of Fe uptake in a broad range of oceanographic regimes. In order to explain the expected. variation in relative Fe uptake, we will also measure several biological and chemical parameters. The biological parameters include Fe:C ratios, bacterial and phytoplankton abundance, and primary and bacterial production. Simultaneous electrochemical measurements will examine the relationship between organic complexation kinetics and biological Fe availability. We hypothesize that slow dissociation kinetics of Fe organic ligand complexes will result in lower phytoplankton Fe uptake but high relative Fe uptake by siderophore utilizing prokaryotes. This research is a logical and necessary first step towards unraveling the unique roles of heterotrophic bacteria and phytoplankton in marine Fe biogeochemistry, and will contribute to our understanding of the processes which ultimately control the cycling of major nutrients and carbon in marine ecosystems.
