Activity concentrations of 234Th, 233Pa, 210Pb, and 7Be were measured by counting the gamma decay energies at 63.5 keV, 312 keV, 46.5 keV, and 477.6 keV, respectively, on a Canberra ultrahigh purity germanium well detector. The 210Po activity was analyzed by liquid scintillation counting (Beckman Model 8100 Liquid Scintillation Counter).

Concentrations of total carbohydrate (TCHO) were determined by the TPTZ (2, 4, 6-tripyridyl-s-triazine) method using glucose as the standard and [Hung and Santschi, 2001]. Protein content was determined using a modified Lowry protein assay, using bovine serum albumin as the standard (Pierce, Thermo Scientific).

Elemental contents of carbon (C) and nitrogen (N), were determined by a Perkin Elmer CHN 2400 analyzer, using cysteine (29.99% C, 11.67% N) as a standard.

In order to examine the specific binding ligands to five different radionuclides in the marine colloids and diatom culture-derived EPS, radiolabeled biopolymers (E1, E3, C9 and C11; suitable due to their high OC content and available sample amount) were subjected to Isoelectric Focusing (IEF) separation with a Gel Electrophoresis apparatus (Amersham Biosciences, Multiphor Electrophoresis System). Briefly, radiolabeled biopolymers and a 140 μL of rehydration solution were loaded onto an IPG strip (GE Healthcare Immobiline™ Drystrip, pH 3–10, 11 cm) and were re-swelled overnight. Afterwards, the strip was loaded into the device for isoelectric focusing for 17.5 h. The strip was then cut into eleven 1 cm-pieces and followed by 1% SDS extraction overnight. Activity concentrations of the five radionuclides in each fraction were subsequently analyzed. Due to the limited amount of each strip fraction, only selected chemical components (TCHO, Proteins and Fe) in individual fractions were characterized by methods described above. To avoid any interference from any background contamination, SDS extractants underwent diafiltration (desalting) using 1 kDa cutoff Microsep™ centrifugal devices (Pall Life Sciences) using ultra pure Milli-Q water for at least three times.