Award: OCE-1356453

Intellectual Merit. This project revolves around questions about organic carrier compounds for a number of natural radionuclides that are used for various purposes in oceanography e.g., as tracers and proxies for particle-cycling and fluxes, which help to better understand carbon cycling and fluxes. While many oceanographers consider only inorganic carrier compounds because they observe correlations between different radionuclides and major inorganic particle phases, organic macromolecular carrier compounds have largely been ignored up to now. Organic chelating compounds can originate intracellularly (e.g., as organic templates for phytoplankton CaCO3 or SiO2 shell formation), or extracellularly (as exopolymeric substances that can form marine snow), or from land (e.g., as humic acid compounds). Inorganic carrier compounds that had been suggested include CaCO3, SiO2, MnO2, and Fe2O3. This is important if we want to predict their behavior under changing conditions, and to help better interpreting data from oceanographic surveys such as GEOTRACES. Thus, the goal of this project was to identify and characterize major organic or inorganic radionuclide carrying phases in the ocean, quantify particle-water partition coefficients for these selected radionuclides (e.g., Th(IV), Pa (IV,V), Po(IV), Pb(II) and Be(II) radioisotopes) to important carrying phases, organic as well as inorganic (e.g., SiO2, CaCO3, MnO2, etc), and identify underlying mechanisms. Phytoplankton cells that build siliceous or calcareous shells, such as the diatoms and coccolithophores, are assembled via bio-mineralization processes using biopolymers as nano-scale templates. These templates could serve as possible carriers for radionuclides and stable metals. In one study, we showed that cleaned diatom frustules (from adhering organic phases), as well as pure silica particles, sorb natural radionuclides to a much lesser extent (by 1-2 orders of magnitude) than whole diatom cells (with or without SiO2 shells). Isoelectric-focusing experiments using radionuclide-labeled exopolymeric substances (EPS) from the Phaeodactylum tricornutum diatom showed that a variety of individual biopolymers were indeed responsible for radionuclide binding. Laboratory incubation experiments using CaCO3 containing coccolithophore Emiliania huxleyi showed again much higher uptake to whole cells than to cleaned (by removal of organic phases) coccoliths or inorganic CaCO3 phases. While in diatoms, we identified specific proteins that carry most of the radionuclides, in coccolithophores these organic carrier compounds were uronic acid enriched non-attached exopolymeric substances and various intracellular biopolymers. In marine colloids, collected via ultrafiltration from large volumes of water, we demonstrated the importance of hydroxamate siderophores (strong natural iron chelating agents released by microorganisms) as carrier compounds for Th, Pa and Po radionuclides. These elements have similar ratios of ionic charge to ionic radii as Fe3+. Hydroquinones/Quinone moieties that can facilitate redox and chelation reactions seem to be involved in the binding of Pa. Our combined results should allow one to better interpret radionuclide data in the ocean, and predict their behavior under changing conditions. In another study where radionuclides were sorbed to different MnO2 phases, substantial improvements for capturing 234Th and 7Be from seawater were achieved using nanostructured MnO2. In a study where radionuclides were sorbed to different humic substances from world wide locations with widely varying compositions, novel insight into the binding of six natural radionuclides with different organic functionalities of three size fractions (e.g., particulate, colloidal and dissolved) were gained. Results of this study will help in interpreting and applying radionuclides in humic acid tracing research. Broader Impact. Exopolymeric substances (EPS), exudates from marine microbes, are responsible for enhancing the biological pump in the ocean, and diatoms for providing ballast to sinking aggregates. EPS also play an important role in facilitating biofilms (Biology/Biochemistry/Health Sciences), aggregate formation (marine and freshwater science), flocculation (civil engineering) and gel formation (bioengineering). This project provides, in addition to training in biogeochemical and radiochemical procedures, ample opportunities for the development and education of one graduate student and one postdoctoral research scientist, in addition to 2 research scientists and approximately 6 undergraduate students. In particular it allowed them to widen their research focus and develop skills for extraction, purification, and chemical characterization of exopolymeric substances (EPS) and phytoplankton cells. Undergraduate and graduate student development has also been through group discussions and readings within classes taught by the PI, e.g., graduate courses in Marine and Environmental Isotope Geochemistry and Radiochemistry, and Environmental Colloids (mostly on biophysical, biochemical and organic chemical aspects), and undergraduate courses in Environmental Chemistry and Nuclear Chemistry. The PI is director of the Laboratory for Environmental Research and the Coastal Zone Laboratory, which aims to provide support institutional infrastructure via a common instrument park. Funding of this research allowed us to maintain and renew the instrumentation park of the radiochemistry laboratory as well as the biochemistry/organic chemistry laboratory of Texas A&M University - Galveston. Funding of this project also allowed leveraging other radiochemistry projects, e.g., work on radioactive contaminants in Fukushima in Japan, through the Department of Energy. Last Modified: 03/30/2018 Submitted by: Peter H Santschi