Important role of biomolecules from diatoms in the scavenging of particle-reactive radionuclides of thorium, protactinium, lead, polonium, and beryllium in the ocean: A case study with Phaeodactylum tricornutum

Laboratory studies were conducted to examine the sorption of selected radionuclides (Th-234, Pa-233, Po-210, Pb-210, and Be-7) 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 (K-d, reported as logK(d)) of radionuclides between water and the different particle types was 4.78-6.69 for Th-234, 5.23-6.71 for Pa-233, 4.44-5.86 for Pb-210, 4.47-4.92 for Po-210, and 4.93-7.23 for Be-7, 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 K-d one to two orders of magnitude higher than for inorganic particles only, with highest values for Be-7 (logK(d) 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.