Award: OCE-1658181

The major goal of this project was understanding factors controlling incorporation of iron, zinc and other metals into the silica shells of marine organisms called diatoms. Diatoms are abundant in the Southern Ocean that surrounds Antarctica. Similar to land plants, diatoms make their own food by using light from the sun in a process called photosynthesis. Most diatoms are tiny with a diameter much smaller than that of a human hair. Diatoms and many other organisms in the ocean that use photosynthesis are termed phytoplankton. Phytoplankton form the base of the marine food chain. All higher marine organisms, like fish, ultimately depend on the growth of phytoplankton either directly or indirectly for food. Phytoplankton are also very important in removing carbon dioxide from the atmosphere, which keeps the earth livable for organisms including humans. The growth of phytoplankton is controlled by the availability of iron, zinc and other metals. However, the processes controlling the amount of iron and zinc available to phytoplankton at the surface of the ocean remain unclear. This research showed that diatoms remove substantial amounts of iron and zinc from the surface waters of the Southern Ocean. To investigate how and where diatoms incorporate metals like iron and zinc and ultimately remove them from the ocean surface, a unique approach was applied using a facility at Argonne National Laboratory called a synchrotron. A synchrotron produces a powerful beam of x-rays that can be used to: 1) measure concentration of metals in tiny things like individual diatoms; 2) show where in a diatom the metals are located; 3) identify the mineral or chemical form of the iron or zinc. The use of the synchrotron is a major advance in how metals are studied in phytoplankton compared to previous approaches. The findings of this research were based on analyses of hundreds of individual diatom specimens collected in the Southern Ocean. These specimens were collected and preserved as part of a previous oceanographic research program. Results showed that both zinc and iron reside deep within diatoms shells in chemical compounds very resistant to being dissolved as the diatoms sink through the water column. Calculations indicate that iron and zinc associated with the sinking dense silica shells of diatoms represents a substantial loss of these metals from ocean surface waters. Incorporation of metals into diatoms may shift phytoplankton population structure towards organisms with relatively lower metal requirements and reduce ecosystem productivity and associated removal of carbon dioxide. The synchrotron technology, developed for this project was also ideally suited to understanding processes affecting the delivery of soluble iron to the ocean associated with atmospheric aerosol particles. As such, aerosol samples collected by other researchers were also examined. This research found that the level of acid called the pH, in the waters associated with atmospheric aerosol particles had a strong influence on iron solubility in aerosols. When aerosol samples were close to neutral pH, iron solubility was on average 3.4%, and when samples were more acidic, below a pH of 4, the solubility of iron increased up to 35%. This observed aerosol iron solubility profile is largely consistent with thermodynamic predictions for the solubility of iron oxides, a chemical compound found in rust.  Research also showed that despite the notable variations in the minerals present in aerosol samples from different source regions, the surface area of aerosol particles is also an important factor affecting total iron solubility. Aerosol iron solubilization through acidic surface reactions would be most effective in samples with the highest surface area which nicely connects the two findings. Beyond the scientific research, the principal investigator has spent approximately 100 contact hours working with about 50 Elementary School teachers from disadvantaged Title 1 schools around Atlanta to enrich science education. Activities included 5 full day sessions on weekends throughout the school year as well as intensive summer activities (10 full day sessions during the summer of 2018). Numerous engaging, hands on exercises as well as field trips have been developed for this course with the aim of giving teachers better resources to more effectively teach science in Georgia. Last Modified: 06/30/2022 Submitted by: Ellery D Ingall