Elsevier

Marine Chemistry

Volume 100, Issues 3–4, 1 August 2006, Pages 337-353
Marine Chemistry

Binding of thorium(IV) to carboxylate, phosphate and sulfate functional groups from marine exopolymeric substances (EPS)

https://doi.org/10.1016/j.marchem.2005.10.023Get rights and content

Abstract

Th(IV) isotopes are important proxies in oceanographic investigations, and are used as tracers of particle dynamics and particulate organic matter (POC) fluxes out of the euphotic zone through the use of 234Th/POC ratios. These approaches rely on empirically determined and variable POC to 234Th ratios, which might be controlled, in parts, by the abundance of exopolymeric substances (EPS). EPS contain acidic polysaccharides (APS) and are excreted by both phytoplankton and bacteria. To this end, radiotracer experiments with EPS from microbial cultures were conducted to determine the binding environment of 234Th(IV)-binding ligands in colloids and suspended particles in marine systems. In these experiments, the 234Th distribution during isoelectric focusing (IEF) and polyacrylamide gel electrophoresis (PAGE) was related to the functional group composition of EPS and of colloidal organic matter (COM) isolated from the Gulf of Mexico (GOM) using cross-flow ultrafiltration. EPS was extracted from phytoplankton (Emiliania huxleyi and Synechococcus elongatus) and bacteria (Sagittula stellata and Roseobacter gallaeciensis) cultures by repeated alcohol precipitation. Phosphate and sulfate concentrations were determined using ion chromatography (IC). IEF profiles indicated that 49% to 65% of the 234Th-labeled EPS from plankton and bacteria as well as COM samples from the GOM was found concentrated below pH of 4, near an isoelectric point, pHIEP, of about 2. The carboxylic acid maxima for extracted EPS and COM samples appeared close to the pHIEF of 234Th(IV). The phosphate maximum appeared at the same pHIEF as 234Th(IV) for EPS from R. gallaeciensis and S. elongatus. The sulfate maximum was found at the same pHIEF as 234Th(IV) for EPS from S. elongatus and COM. The molecular weight (MW) of the strongly Th(IV)-binding ligand varied from 1 to 14 kDa, depending on the species, but was about 10 kDa in COM. Thus, depending on the species of plankton or bacteria, the MW and specific functional group composition of the strongly 234Th(IV)-binding amphiphilic biomolecule can vary. Therefore, different acidic functional groups can, at times, contribute to the binding of Th(IV) to the EPS chelating ligand, which can also have different MWs. This implies that the binding environment for Th(IV), which is present at total concentrations at least a million times lower than the acid functional groups, consists of strong polydentate chelate complexes in clustered structures of carboxylate, sulfates and/or phosphates. The combination of strongly chelating groups and amphiphilicity gives this biomolecule the unique properties of a “sticky” ligand.

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.

References (65)

  • T. Doco et al.

    Determination of neutral and acidic glycosyl-residue compositions of plant polysaccharides by GC–EI–MS analysis of the trimethylsilyl methyl glycoside derivatives

    Carbohydr. Polym.

    (2001)
  • G.M.S. El Shafei

    The polarizing power of metal cations in (hydr)oxides

    J. Colloid Interface Sci.

    (1996)
  • T.M. Filisetti-Cozzi et al.

    Measurement of uronic acids without interference from neutral sugars

    Anal. Biochem.

    (1991)
  • H.E. Grotjan et al.

    Ion chromatographic method for the analysis of sulfate in complex carbohydrates

    J. Chromatogr.

    (1986)
  • L.-D. Guo et al.

    The distribution of colloidal and dissolved organic carbon in the Gulf of México

    Mar. Chem.

    (1994)
  • L.D. Guo et al.

    Trace metal composition of colloidal material in estuarine and marine environments

    Mar. Chem.

    (2000)
  • L. Guo et al.

    Re-examination of cross-flow ultrafiltration for sampling aquatic colloids: evidence from molecular probes

    Mar. Chem.

    (2000)
  • L.-D. Guo et al.

    234Th scavenging and its relationship to acid polysaccharide abundance in the Gulf of México

    Mar. Chem.

    (2002)
  • K. Hirose et al.

    Thorium-particulate matter interaction. Thorium complexing capacity of oceanic particulate matter: theory

    Geochim. Cosmochim. Acta

    (1994)
  • K. Hirose et al.

    The vertical distribution of the strong ligand in particulate organic matter in the North Pacific

    Mar. Chem.

    (1998)
  • K. Hirose et al.

    Strong ligands for thorium complexation in marine bacteria

    Mar. Environ. Res.

    (2001)
  • C.C. Hung et al.

    Spectrophotometric determination of total uronic acids in seawater using cation-exchange separation and pre-concentration by lyophilization

    Anal. Chim. Acta

    (2001)
  • C.-C. Hung et al.

    Distribution of carbohydrate species in the Gulf of México

    Mar. Chem.

    (2003)
  • T.L. Ku et al.

    Uranium in the open ocean: concentration and isotopic composition

    Deep-Sea Res.

    (1977)
  • G.G. Leppard

    The characterization of algal and microbial mucilage and their aggregates in aquatic ecosystems

    Sci. Total Environ.

    (1995)
  • G.G. Leppard

    Colloidal organic fibrils of acid polysaccharides in surface waters: electron–optical characteristics, activities and chemical estimates of abundance

    Colloids Surf., A Physicochem. Eng. Asp.

    (1997)
  • M. Mecozzi et al.

    The humin structure of mucilage aggregates in the Adriatic and Tyrrhenian seas: hypothesis about the reasonable causes of mucilage formation

    Mar. Chem.

    (2005)
  • S.B. Moran et al.

    Distribution of Th-230 in the Labrador Sea and its relation to ventilation

    Earth Planet. Sci. Lett.

    (1997)
  • J.W. Murray et al.

    Export flux of particulate organic carbon from the central equatorial Pacific determined using a combined drifting trap–234Th approach

    Deep-Sea Res. II

    (1996)
  • S.M. Myklestad et al.

    A sensitive and rapid method for analysis of dissolved mono- and polysaccharides in seawater

    Mar. Chem.

    (1997)
  • K.L. Nash et al.

    Interaction of humic and fulvic acids with Th(IV)

    J. Inorg. Nucl. Chem.

    (1980)
  • M.S. Quigley et al.

    Sorption irreversibility and coagulation behavior of 234Th with marine NOM

    Mar. Chem.

    (2001)
  • Cited by (61)

    • Stickiness of extracellular polymeric substances on different surfaces via magnetic tweezers

      2021, Science of the Total Environment
      Citation Excerpt :

      Under low salt conditions, the negative charges on the EPS are exposed to the surrounding sea water and can form stereo-electric interactions, which drive charged molecules to repel each other (Li et al., 2013; Meng and Liu, 2016; Shiu et al., 2014). Since more negative charges are exposed on the surface of EPS polymers (Alvarado Quiroz et al., 2006; Xu et al., 2011), the alteration of ionic strength by cations, such as those of calcium and magnesium, influence the intermolecular interactions within the polymeric matrix, which can lead to a change in stickiness. However, more detailed experiments would be required to further investigate the influence of other characteristics, such as environmental temperature and shear force, on the relative stickiness of EPS polymers.

    • The Carbon:<sup>234</sup>Thorium ratios of sinking particles in the California current ecosystem 1: relationships with plankton ecosystem dynamics

      2019, Marine Chemistry
      Citation Excerpt :

      Respiration of particle-attached bacteria is a substantial source of flux attenuation in the twilight zone and likely leads to preferential degradation of carbon (relative to thorium) and hence a decrease in C:234Th ratios with depth (Buesseler et al., 2006; Simon et al., 2002). This simple conceptual view may be complicated, however, by microbial interactions including quorum sensing (Hmelo, 2017; Mislan et al., 2014) and production or breakdown of thorium binding ligands by bacteria (Hirose and Tanoue, 2001; Quiroz et al., 2006; Santschi et al., 2003). Furthermore, the presence of flux feeders, including Rhizaria that are abundant in the CCE (Biard et al., 2018; Stukel et al., 2018), may lead to flux attenuation without a substantial change in C:234Th ratios.

    View all citing articles on Scopus
    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.

    View full text