Award: OCE-1756884

Iron Bioavailability in High-CO2 Oceans: New Perspectives on Iron Acquisition Mechanisms in Diatoms Intellectual Merit: Coastal upwelling regions are among the most biologically productive ecosystems in the ocean but may be threatened by amplified ocean acidification from rising atmospheric CO2. Also, it has become increasingly clear that iron availability plays a major role in regulating the fate of upwelled nitrate (NO3-) and determining the size structure and community composition of phytoplankton assemblages in open-ocean and coastal upwelling regions. However, the molecular mechanisms that govern inorganic iron uptake in ocean phytoplankton are believed to be contingent on carbonate ion concentrations. Therefore, increased acidification is hypothesized to reduce iron bioavailability for marine phytoplankton thereby expanding iron limitation and impacting primary production. Field and laboratory studies conducted through this project showed, from community to molecular levels, that iron-stressed phytoplankton in an upwelling region exhibit resistance to short-term acidification. Molecular-level responses, from studies conducted at sea, showed that, although variable. resistance to acidification-driven changes in iron bioavailability is facilitated by iron uptake pathways that are less hindered by acidification and other cellular strategies that reduce cellular iron demand. These mechanisms, however, may only confer resistance over short time periods, and chronic long-term exposure may result in further iron stress. These field studies confirm that, as ocean acidification reduces carbonate concentrations, inorganic iron uptake may be discouraged in favor of carbonate-independent uptake. Findings from laboratory studies uncovered the biochemical function and evolution of the proteins responsible for diatom acquisition of iron from bacterial siderophores; a mechanism that does not have a carbonate requirement but requires a bacteria-diatom interaction. We demonstrated that the diatom siderophore acquisition system is composed of a hydroxamate siderophore receptor protein of bacterial origin and a NADPH oxidase type ferric reductase of eukaryotic origin. Additionally, using an optimized protocol for subcellular proteomics we further characterized the proteins and processes that occur downstream of diatom iron binding at the cell surface. Based on these results, coupled to additional in vivo and biochemical experiments and evolutionary analyses, we derived a new view of key endosomal processes and biochemical transformations that mediate subsequent intracellular allocation of internalized Fe(III). Finally, using all of our data, we obtained a new comprehensive conceptual overview for iron-trafficking, from the cell surface to the chloroplast. Broader Impacts: A curricular module related to marine microbes and the ocean carbon cycle in the marine environment was developed through the San Diego- based League of Extraordinary Scientists and Engineers http://science-ing.org (LXS). The LXS mission is engaging young people from underserved communities to inspire them to become part of the next generation of scientific and environmental leaders. During the award period, LXS brought science to over 4,500 public elementary school classrooms. Last Modified: 02/08/2022 Submitted by: Andrew E Allen