Binding of thorium(IV) to carboxylate, phosphate and sulfate functional groups from marine exopolymeric substances (EPS)
Introduction
Air–sea exchange of CO2, coupled to downwelling and/or the “biological pump”, are the main mechanisms of the ocean to counteract increasing anthropogenic CO2 concentrations in the atmosphere. The “biological pump”, i.e., new production or carbon export flux, consists of the fraction of the primary production of organic matter resulting from CO2 fixation by phytoplankton that is exported as particulate organic matter (POM) to the deep ocean, thus escaping internal recycling. Carbon export fluxes can be assessed by measuring biogenic particle fluxes caught in sediment traps (Buesseler et al., 1992, Buesseler et al., 1995), as well as by the product of measured 234Th flux times measured ratios of particulate organic carbon (POC) to 234Th(IV) in either sinking (caught in sediment traps) or large (filterable) particles (Moran et al., 2003, and references therein). Organic carbon to 234Th(IV) ratios in suspended matter, used to determine new production rates, decrease with increasing depth and particle size, and are often different for settling particles caught in sediment traps than for large (> 53 μm) suspended particles (Buesseler et al., 1995, Buesseler et al., 1998, Buesseler et al., 2006, Murray et al., 1996, Bacon et al., 1996, Moran et al., 1997, Santschi et al., 2003, Santschi et al., 2006, Hung et al., 2004).
Thorium-234 (234Th(IV), t1/2 = 24.1 days) is a naturally occurring highly particle-reactive radionuclide continuously produced from alpha decay of Uranium-238 (t1/2 = 4.47 × 109 years) in seawater. Uranium in seawater exists as the soluble carbonate species [UO2(CO3)34−] that is relatively un-reactive to particles (less than 0.1% of uranium is particulate in seawater). Thus, uranium exhibits a rather conservative distribution in the open ocean (Ku et al., 1977, Edwards et al., 1987), with an average U-concentration of 1.4 × 10− 8 M at a salinity of 35, thus providing a constant source of its daughter product, 234Th. Long-lived 232Th(IV) occurs in seawater at concentrations of about 4.5 × 10− 12 M (Choppin and Wong, 1998) and short-lived 234Th(IV) at about 10− 18 M. This can be compared to natural seawater concentrations of major and minor cations, which are in the millimolar to micromolar range. The Th(IV) concentration is thus well below the solubility limit of ThO2 of about 10− 9 M (Fanghänel and Neck, 2002). While Th(IV) might form a humic acid complex in seawater (Nash and Choppin, 1980), it was also found strongly associated with an acid polysaccharide (APS)-rich compound in both laboratory and field experiments (Quigley et al., 2002, Guo et al., 2002, Santschi et al., 2003). Acidic polysaccharide fibrils have been identified as important biopolymers in different aquatic systems (Alldredge et al., 1993, Leppard, 1995, Santschi et al., 1998, Santschi et al., 2006), and are likely derived from exopolymeric substances (EPS) excreted by micro-organisms. EPS compounds are amphiphilic or amphiphatic (Buffle, 1990, Leppard, 1995, Leppard, 1997) and act as biosurfactants (Ron and Rosenberg, 2001). Because of their surface activity, EPS in the ocean form alcian-blue stainable transparent exopolymeric particles (TEP; Alldredge et al., 1993). APS compounds, although generally only a minor fraction (∼10%) of the polysaccharide pool (Santschi et al., 2003, Hung et al., 2003a), are present in both particulate and colloidal material, and play a critical role in the formation of marine snow flocs, mucilaginous aggregates, and the removal of trace elements and radionuclides from the water.
Using radiolabeled glucose, Stoderegger and Herndl (1998) determined the incorporation of glucose into intracellular and capsular pools (i.e., EPS) to acquire production estimates. Their results indicated that 55% of labeled glucose was incorporated intracellularly and 45% to capsular material. Release rates of the capsular material represented about 25% of bacterial respiration suggesting that a significant portion of the DOC pool is composed of bacterially derived semi-labile EPS (Stoderegger and Herndl, 1998).
It was recently reported that the strongly Th(IV)-binding APS compound has a low pHIEP and pKa of ≤ 3 (Quigley et al., 2002), and a high sticky coefficient in seawater of 0.9 (Quigley et al., 2001). Such a “sticky” macromolecular ligand is likely instrumental in removing Th(IV) from the ocean. The exact functional group composition of this APS compound is, however, not well known, although Santschi et al. (2003) reported that it might also contain phosphate.
In order to determine the functional group composition, isoelectric focusing experiments were conducted with samples of EPS taken from cultured phytoplankton and bacteria, as well as of colloidal macromolecular organic matter from the Gulf of Mexico.
Section snippets
Sampling
Colloidal organic matter (COM) water samples were taken from the chlorophyll a maximum layer inside a Cold Core Ring (CCR, Station 4, 27°38′N, 94°59′W) at a depth of 72 m along a N–S transect in the Gulf of Mexico (GOM) aboard the R/V Gyre during May 17–25, 2001 (Santschi et al., 2003, Hung et al., 2003a, Hung et al., 2003b). Briefly, cross-flow ultrafiltration (CF–UF) was used to extract the colloidal fraction from large volumes of seawater (Benner, 1991, Guo et al., 1994, Guo et al., 1996,
Radioisotope labeling incubation experiment – 14C and 234Th(IV)
The purpose for conducting this radionuclide experiment was to verify mass balance and extraction efficiency. IEF gel electrophoresis was performed on radiolabeled EPS in order to study and compare the isoelectric (pI) profile of 14C incubated material with that non-incubated 234Th-labeled EPS.
Summary and conclusions
The primary objective of our experiments was to test how the 234Th(IV) is bound to oceanic particles, which could be important to our understanding of the variability of POC/234Th ratios in the ocean. This was accomplished through controlled laboratory experiments with exopolymers responsible for 234Th(IV) complexation and extracted from pure cultures of phytoplankton (S. elongatus, E. huxleyi), bacteria (R. gallaeciensis, S. stellata) as well as colloidal organic matter (COM) from the Gulf of
Acknowledgements
We thank Jacques Buffle, Kevin Wilkinson and Alain Reinhardt, CABE, University of Geneva, Switzerland, for stimulating discussions. This work was funded, in part, by the National Science Foundation (BES-0210865, OCE-9906823, 0351559 and 0210865) and the Texas Institute of Oceanography.
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- 1
Current address: National Oceanic and Atmospheric Administration, Office of Ocean Exploration, SSMC3, (R/OE), Rm. 10244, Silver Spring, MD, 20910, USA.
- 2
Current address: Institute of Marine Environmental Chemistry and Ecology, National Taiwan Ocean University, 2 Pei-Ning Road, Keelung, 20224, Taiwan.