Chemical composition and relative hydrophobicity of microbial exopolymeric substances (EPS) isolated by anion exchange chromatography and their actinide-binding affinities

Understanding the chemical and physical properties of microbially produced, actinide-carrying biopolymers is essential for the prediction of their relative binding affinities and particle-interaction propensities that are affecting the transport of actinide elements. In addition, they form the basis for their oceanographic applications, such as using POC/Th-234 ratios to determine organic carbon fluxes and for tracing upper ocean particle dynamics. Furthermore, environmental applications of bioflocculation and bioremediation can benefit from such improved understanding. Exopolymeric substances (EPS) in attached and non-attached forms, produced by two bacteria and one alga, were extracted and purified using improved procedures. These biopolymers produced by the two bacteria, which were previously shown to contain strong Th-234 complexing ligands, were further fractionated by semi-preparative HPLC-anion exchange chromatography in order to relate Th(IV)-binding ability to chemical composition. Eluted fractions were further characterized at both semi-molecular and molecular levels. It was found that the EPS consisted of more than one biopolymer, with distinct monosaccharide and amino acid compositions, but very similar molecular weight (-25 kDa). Th-234 binding strength was correlated to their uronic acid to organic carbon ratios, suggesting that uronic acids are the actual binding agents of Th-234, or more likely, proxy compounds for the actual macromolecular Th-234 binding ligands. These compounds were, however, not pure uronic acids, but rather amphiphilic glycoproteins or proteoglycans. Hydrophobic contact areas (HCA) of attached EPS, as measures of their relative hydrophobicity and amphiphilicity, were higher than those of non-attached EPS, and all were highly correlated to the ratios of the FTIR areas of amide-I bands from proteins to those of C-O-C bands from carbohydrates. This suggests that the relative hydrophobicity of the carrier biopolymers, which can be responsible for gel formation, particle aggregation and bioflocculation, was mainly regulated by their relative protein to polysaccharide contents. (C) 2011 Elsevier B.V. All rights reserved.