The biochemical composition of benthic mucilaginous aggregates collected from the south-western coastal waters of Sardinia in the Mediterranean Sea was analysed. Acidic glycoproteins surrounding the included microorganisms, mainly diatoms, were found to be the predominant fraction. The aggregates, diluted and finely dispersed by ultrasonic treatment, bound with ammoniated ruthenium oxychloride (ruthenium red), a specific ligand for the acidic glycoproteins. When concentrated crude aggregates were used ruthenium red was bound both chemically and by physical entrapment. The data were consistent with the Langmuir ligand binning isotherm. The extent of physical adsorption displayed a clear pH dependence. Physical adsorption was predominant at acidic pH, suggesting that the stabilization of the supramolecular structure of the aggregates also depends on the degree of protonation of the acidic glycoresidues. A structural model of the aggregates is proposed, where the mucus matrix containing acidic binding sites acts as an intercellular glue, providing a molecular sieve-like filtering capacity to the aggregates dispersed in seawater.
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