Involvement of particle-reactive radionuclides in organic carbon cycling in the Ocean.
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) radioisotopes are important proxies in oceanographic investigations, used for tracing particle and colloid cycling, estimating export fluxes of POC, tracing air-sea exchange, paleoproductivity, and/or ocean circulation in paleo-oceanographic 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. Our 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 nano-scale templates. These templates could serve as possible carriers for radionuclides and stable metals. 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. Thus, we propose here to identify the individual biopolymeric carrier molecules that strongly bind specific radionuclides.
The intellectual merit of the proposed research 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). Our proposed interdisciplinary research project will require instrumental approaches for molecular-level characterization of these radionuclides associated carrier molecules.