Low-temperature SEM imaging of polymer structure in engineered and natural sediments and the implications regarding stability

Interpretation of electron micrographs is often used as qualitative support of quantitative data. Conventional and low-temperature scanning electron microscopy (SEM and LTSEM, respectively) have often been used to visualise biofilms and microbial mats and investigate their role in the biogenic stabilisation of sediments. This study investigated the use of LTSEM to image the “binding” of artificial sand and clay systems by extracellular polymeric substances (EPS). Xanthum gum (commercial bacterial EPS) and diatom EPS were mixed with cleaned sand particles and the apparent structure of the polymer in the matrix observed in relation to different stages of sample preparation. In addition, diatoms were added to sand to allow comparison of EPS production “in situ” within the matrix, with that prepared separately and added. The commonly accepted in situ structure of polymer, strands and plates, was found to be a preparatory artefact, as a result of desiccation of the sample prior to freezing, cryofixation and also partial freeze-drying in the LTSEM. The natural structure of the polymer is more likely to be an amorphous gel/sol equilibrium within the inter-particle spaces. In addition, EPS produced by diatoms in situ and examined by LTSEM forms a granular “floc and plate” structure, similar in appearance to fine cohesive particles, rather than the strands and plates previously reported. These results show that caution is needed where a mechanistic interpretation of polymer binding is derived from interpreting LTSEM images.