Effects of aluminum ion on particle sizes and surface charges of exopolysaccharides from Rhizobium tropici and pH effects

Exopolysaccharides (EPS) are ubiquitous and important extracellular macromolecular biopolymers produced by soil microorganisms. Aluminum (Al) is a major element in earth crust and Al toxicity is one of the major limitations inhibiting plant growth in acidic soils. EPS strongly affects soil microaggr...

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Bibliographic Details
Published in:Rhizosphere Vol. 26; p. 100713
Main Authors: Larson, Steve L., Ballard, John H., Runge, Kauri A., Zhang, Huimin, Breland, Benjamin R., Nick, Zachary H., Vroman, Ethan Tyler, Weiss, Charles A., Han, Fengxiang X.
Format: Journal Article
Language:English
Published: Elsevier B.V 01-06-2023
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Summary:Exopolysaccharides (EPS) are ubiquitous and important extracellular macromolecular biopolymers produced by soil microorganisms. Aluminum (Al) is a major element in earth crust and Al toxicity is one of the major limitations inhibiting plant growth in acidic soils. EPS strongly affects soil microaggregate structure, soil moisture and bioavailability of nutrients and heavy metals. The objectives of this study are to investigate the effect of Al ion on particle sizes and zeta potential of EPS and EPS-Al complexes. Results show that Al ions strongly interact with EPS and the interaction was affected by pH. Al ion binding in EPS was involved in multilayer binding sites, i.e., inside/within the macromolecular and intermolecular bridging. This was confirmed by Al binding isotherm, the change of the particle size and zeta potentials of EPS-Al complexes. Al ions significantly increased the particle sizes and zeta potentials of EPS-Al became less negative. At the lower molar ratio of Al/EPS, EPS-Al may form mono-complexation within molecule while at higher molar ratios EPS-Al may form dimers/multimerization/lateral association of dimers of EPS, significantly increasing the particle sizes. When the molar ratio of binding Al/EPS was 72, the zeta potential of EPS-Al complexes was almost zero. With continuing increase in Al ion/EPS molar ratios to 336, EPS-Al complexes reached the positive charge at +2.0 mV. The particle sizes of EPS-Al complexes correlated well with the zeta potentials. pH significantly affected the particle sizes and zeta potential of EPS alone and EPS-Al complexes. The size of the EPS particles decreased with pH while the particle size of the EPS-Al complexes increased with pH. pH not only controlled protonation and deprotonation of carboxylic and phenol functional groups of EPS, but also shifted the speciation of Al ions, thus significantly controlling particle sizes of EPS and EPS-Al. This study provides an insight understanding of the interaction of Al3+ with EPS for controlling toxicity of Al ion in water and soils affected by microbial-excreted exopolysaccharides.
ISSN:2452-2198
2452-2198
DOI:10.1016/j.rhisph.2023.100713