Investigating the geochemical impact of burrowing animals: Proton and cadmium adsorption onto the mucus lining of Terebellid polychaete worms

Sediment–water interfaces are the sites of intense chemical activity in natural systems. Burrowing animals extend this interface, and at times coat it with mucous materials they secrete to line their burrows. However, the chemical reactivity of these burrow linings is poorly understood. In this study, we incubated burrowing terebellid worms (Thelepus crispus) for the purposes of harvesting their mucus secretions and investigating its composition and metal adsorptive properties. The mucus was comprised of proteins, carbohydrates, and lipids in an approximate ratio of 200:50:1, and FTIR spectroscopy revealed abundant carboxyl, phosphoryl, hydroxyl, amino, and thiol functional groups typical of mucin-like material. Potentiometric titration data were best fit by a three-site surface complexation model, with a total ligand density of 11.26 ± 1.79 mmol per dry gram, which is relatively high compared to previously-studied humic substances and microbial biomass. Cd adsorption experiments demonstrated that the two most acidic ligands, inferred to represent carboxyl and phosphoryl functional groups, respectively, possessed cadmium stability constants that were comparable to analogous ligands inferred for bacterial surfaces. The third ligand, however, differed from bacterial surfaces in that it likely represents thiol functional groups, and displayed a higher Cd stability constant commensurate with the chalcophile nature of organic sulfur ligands. These results indicate that mucous burrow linings such as those produced by terebellid worms are geochemically important for driving metal adsorption and mineral nucleation reactions at the burrow–water interface.