Table of Contents

• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Project Information
• Funding

Laboratory studies were conducted to examine the sorption of selected radionuclides (234Th, 233Pa, 210Po,
210Pb, and 7Be) onto inorganic (pure silica and acid-cleaned diatom frustules) and organic (diatom cells with or
without silica frustules) particles in natural seawater and the role of templating biomolecules and exopolymeric
substances (EPS) extracted from the same species of diatom, Phaeodactylum tricornutum, in the sorption process.
The range of partition coefficients (Kd, reported as logKd) of radionuclides between water and the different
particle types was 4.78–6.69 for 234Th, 5.23–6.71 for 233Pa, 4.44–5.86 for 210Pb, 4.47–4.92 for 210Po, and 4.93–7.23 for 7Be, similar to values reported for lab and field determinations. The sorption of all radionuclides was
significantly enhanced in the presence of organic matter associated with particles, resulting in Kd one to two
orders of magnitude higher than for inorganic particles only, with highest values for 7Be (logKd of 7.2). Results
further indicate that EPS and frustule-embedded biomolecules in diatom cells are responsible for the sorption
enhancement rather than the silica shell itself. By separating radiolabeled EPS via isoelectric focusing, we found
that isoelectric points are radionuclide specific, suggesting that each radionuclide binds to specific biopolymeric
functional groups, with the most efficient binding sites likely occurring in acid polysaccharides, iron hydroxides,
and proteins. Further progress in evaluating the effects of diatom frustule–related biopolymers on binding,
scavenging, and fractionation of radionuclides would require the application of molecular-level characterization
techniques.

Partition coefficient of 234Th, 233Pa, 210Pb, 210Po, and 7Be
Activity concentrations of 234Th, 233Pa, 210Pb, 210Po, and 7Be were measured by gamma counting the 63.5, 312, 46.5, and 477.6 keV lines, respectively, on a Canberra ultra-highpurity germanium well-type detector. The 210Po activity was analyzed by liquid scintillation counting (Beckman Model 8100 Liquid Scintillation Counter). The filter samples (> 0.2 um particulate phase) were soaked with 1 mol L-1 HCl for 20 min in a counting vial, and the filtrate samples (< 0.2 um dissolved phase) were transferred into counting vials directly. Both filter and filtrate samples were then counted for activities of each radionuclide. All reported activities were decay and geometry corrected. 233Pa was added in equilibrium with 237Np. Under a wide range of environmental conditions, aqueous neptunium speciation is believed to be dominated by the pentavalent cation NpO2+. Np(V) is relative soluble and tends to remain in the water phase, unlike other actinides, such as Pu and Am, which are readily adsorbed by particles in the environment (Atwood 2013). In our absorption experiments, 237Np activities could be found only in the dissolved phase for all samples, supporting the assumption that 237Np would not adsorb onto particles during the time for which decay and in-growth corrections of 233Pa were applied. Traditional partition coefficients (Kd) between dissolved and particulate phases were used to quantify the sorption of radionuclides onto different particles in different experimental systems. Kd was defined here as

Kd = Ap x (Ad x Cp)^-1           (1)

where Ap and Ad represent particulate and dissolved activities (Bq L-1) of radionuclides and Cp is the particle concentration (kg L-1; Honeyman and Santschi 1989; Guo and Santschi 1997).

BCO-DMO Processing Notes:
- added conventional header with dataset name, PI name, version date
- modified parameter names to conform with BCO-DMO naming conventions
- combined the two submitted tables together on the pH column, indicating percent activity and percent composition in the parameter names.

NSF Award Abstract:

Particle-associated natural radioisotopes are transported to the ocean floor mostly via silica and carbonate ballasted particles, allowing their use as tracers for particle transport. Th(IV), Pa (IV,V), Po(IV), Pb(II) and Be(II) radionuclides are important proxies in oceanographic investigations, used for tracing particle and colloid cycling, estimating export fluxes of particulate organic carbon, tracing air-sea exchange, paleoproductivity, and/or ocean circulation in paleoceanographic studies. Even though tracer approaches are considered routine, there are cases where data interpretation or validity has become controversial, largely due to uncertainties about inorganic proxies and organic carrier molecules. Recent studies showed that cleaned diatom frustules and pure silica particles, sorb natural radionuclides to a much lower extent (by 1-2 orders of magnitude) than whole diatom cells (with or without shells). Phytoplankton that build siliceous or calcareous shells, such as the diatoms and coccolithophores, are assembled via bio-mineralization processes using biopolymers as nanoscale templates. These templates could serve as possible carriers for radionuclides and stable metals.

In this project, a research team at the Texas A & M University at Galveston hypothesize that radionuclide sorption is controlled by selective biopolymers that are associated with biogenic opal (diatoms), CaCO3 (coccolithophores) and the attached exopolymeric substances (EPS), rather than to pure mineral phase. To pursue this idea, the major objectives of their research will include separation, identification and molecular-level characterization of the individual biopolymers (e.g., polysaccharides, uronic acids, proteins, hydroquinones, hydroxamate siderophores, etc.) that are responsible for binding different radionuclides (Th, Pa, Pb, Po and Be) attached to cells or in the matrix of biogenic opal or CaCO3 as well as attached EPS mixture, in laboratory grown diatom and coccolithophore cultures. Laboratory-scale radiolabeling experiments will be conducted, and different separation techniques and characterization techniques will be applied.

Intellectual Merit : It is expected that this study will help elucidate the molecular basis of the templated growth of diatoms and coccoliths, EPS and their role in scavenging natural radionuclides in the ocean, and help resolve debates on the oceanographic tracer applications of different natural radioisotopes (230,234Th, 231Pa, 210Po, 210Pb and 7,10Be). The proposed interdisciplinary research project will require instrumental approaches for molecular-level characterization of these radionuclides associated carrier molecules.

Broader Impacts: The results of this study will be relevant for understanding biologically mediated ocean scavenging of radionuclides by diatoms and coccoliths which is important for carbon cycling in the ocean, and will contribute to improved interpretation of data obtained by field studies especially through the GEOTRACES program. This new program will enhance training programs at TAMUG for postdocs, graduate and undergraduate students. Lastly, results will be integrated in college courses and out-reach activities at Texas A&M University, including NSF-REU, Sea Camp, Elder Hostel and exhibits at the local science fair and interaction with its after-school program engaging Grade 9-12 students from groups traditionally underrepresented.