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

Abstract

Laboratory studies were conducted to examine the sorption of selected radionuclides ( 234 Th, 233 Pa, 210 Po, 210 Pb, and 7 Be) 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 234 Th, 5.23-6.71 for 233 Pa, 4.44-5.86 for 210 Pb, 4.47-4.92 for 210 Po, and 4.93-7.23 for 7 Be, 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 7 Be (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.

Influence of organic matter on the adsorption of 210Pb, 210Po and 7Be and their fractionation on nanoparticles in seawater

“…The log K c value on SiO 2 in the presence of MOCs was slightly lower than the value obtained from the diatom particles (Chuang et al, 2014). Since biominerals (e.g., calcite and biogenic silica) in natural seawater are usually biogenic and contain protein and acid polysaccharide matrixes (Chuang et al, 2014), and/or coated by MOCs, even after the death of the organisms, these results provide direct evidence of organic matter influence on 210 Po distribution observed in sinking particles (Stewart et al, 2007b) and suspended particles (Yang et al, 2011b).…”

“…The increase introduced by APS and BSA is statistically significant (Table 1). The log K c values of 210 Pb on SiO 2 with MOCs were similar to the value obtained from diatom particles (5.27 ± 0.08), including both biogenic silica and embedded MOCs (Chuang et al, 2014). Also, the average log K c values of 210 Pb in the presence of MOCs (5.33) fell in the range of reported 210 Pb partition coefficients compiled in Yang et al (2013).…”

“…Statistically, the K c value after the addition of MOCs was around three times higher than that for inorganic nanoparticles. This relationship along with other recent findings (Chuang et al, , 2014 may allow one to roughly estimate the partitioning of 210 Po in seawater, although the role of biota needs also to be considered.…”

“…Some studies reported that lithogenic particles were the strongest sorbent for Be , while others conclude that biogenic silica was the strongest sorbent (Chase et al, 2002). Rigorous sorption experiments by Chuang et al (2014) recently demonstrated the role of specific diatom associated biopolymers in diatoms as the responsible carrier phase for 7 Be. Regardless of particle types, the role of organic matter and its interactions with particle surfaces cannot be ignored (Li, 2005;Lin et al, 2015).…”

“…Since inorganic particles always co-exist with organic matter in the ocean, the role of natural organic matter in the scavenging and fractionation of particle-reactive radionuclides can be maximized, especially through organic coatings on particles, or specific organic compounds formed on surfaces/inner structure during the formation of cell exoskeletons, such as silica and carbonate (Li, 2005;Chuang et al, 2014). Thus, understanding the interactions between radionuclides and particles in the presence of natural organic matter is likely at the heart of a better understanding of particle adsorption and fractionation of these nuclides.…”