Iron is critically needed for growth of all marine phytoplankton, the microscopic plants at the base of the ocean food chain. Consequently, lack of iron in large regions of the global ocean limits phytoplankton growth and commercial fisheries. Ocean acidification (OA) is the ongoing decrease in seawater pH due to the ocean absorbing carbon dioxide from the atmosphere. OA is predicted to affect seawater chemistry by reducing the concentration of carbonate ions. Carbonate ions are required for phytoplankton to take up iron from their environment, which suggests that OA might inhibit iron nutrition. Further complicating the scenario, pH changes affect iron chemistry in seawater, such that OA is predicted to shift the relative abundance of various forms of iron. But despite these expectations, little is known about how the changes in ocean chemistry due to OA will impact the availability of iron to phytoplankton. Changes in phytoplankton iron uptake and associated growth rates would likely have large effects on how the ocean captures atmospheric carbon dioxide (CO2). This has important consequences for ecosystem productivity and for global cycles of critical chemical elements, such as carbon and nitrogen, and their chemistry. This project aims to help us understand how shifts in seawater pH and the chemistry of dissolved inorganic carbon will affect both iron uptake rates and iron acquisition strategies in the laboratory and in natural communities. This project also includes development of educational outreach activities which target primary school students in the areas of microbiology, biogeochemical cycles and current global change topics. These science outreach activities benefit from collaborations with the following San Diego-based organizations: the League of Extraordinary Scientists and Engineers (LXS), The Birch Aquarium at Scripps (BAS), and The Ocean Discovery Institute (ODI).

This project seeks to understand the differential sensitivity of diatom iron acquisition strategies to changes in seawater pH and carbonate chemistry. Ultimately a more thorough and detailed mechanistic understanding of diatom iron uptake pathways will facilitate a much-improved ability to forecast the impact of anticipated changes in ocean pH and inorganic carbon chemistry on rates of iron uptake by diatoms. This critical biogeochemical issue is addressed through trace metal clean manipulation experiments incorporating state-of-the-art analytical methodology to probe phytoplankton cellular physiology and biogeochemistry in laboratory cultures and natural communities. In the first year, laboratory experiments with a model pennate diatom leverage a collection of targeted knockout transgenic lines to evaluate the substrate specificity and relative importance of distinct iron assimilation pathways under a range of pCO2 and iron availability conditions. Additionally, quantitation of mRNA and proteins for key diatom iron assimilation pathways in natural communities in the Southern California Current further clarify the relative importance and sensitivity of distinct iron assimilation pathways in relation to pCO2 and iron availability. In year two a Lagrangian study of iron uptake rates and associated mRNA and protein abundance is performed on upwelled high pCO2 water over the course of offshore advection. Additionally, the investigators are conducting mesocosm experiments using naturally elevated high pCO2 seawater as well as laboratory experiments on multiplex knockout lines. Year three is dedicated to data analyses and overall project synthesis. Overall aims of the research activities include, 1) development and validation of a refined conceptual model of iron uptake in key marine phytoplankton and subsequent utilization of the model to characterize the sensitivity of distinct iron uptake pathways to the effects of ocean acidification, and 2) determination of the effects of acidification on iron uptake, and quantification of the relative contribution of distinct iron acquisition pathways in high pCO2 phytoplankton communities.

National Science Foundation (NSF)
Division of Ocean Sciences (OCE)
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Michael Sieracki
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J. Craig Venter Institute, Inc.
La Jolla
United States
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