Atomic force microscopy characterization of silver nanoparticles interactions with marine diatom cells and extracellular polymeric substance

This study highlights the capacity of atomic force microscopy (AFM) for investigating nanoparticle (NP) algal cell interaction with a subnanometer resolution. We designed a set of AFM experiments to characterize NP size, shape, and structure to visualize changes in the cell morphology induced by NPs and to characterize NP interaction with the extracellular polymeric substance (EPS). Samples for AFM imaging were prepared using the same protocol-drop deposition on mica and imaged in air. Here we address the interactions of Ag NPs with ubiquitous, lightly silicified marine diatoms Cylindrotheca fusiformis and Cylindrotheca closterium and their EPS. In natural seawater used throughout this study, the single Ag NPs adopted truncated tetrahedron morphology with particle heights of 10, 20, 30, and 40 nm. This size class Ag NPs penetrates the cell wall through the valve region built of silica NPs embedded in organic matrix. The Ag NPs cause a local damage inside the cell without disintegration of the cell wall. The EPS production has been shown to increase as a feedback response to Ag NP exposure and may contribute to detoxification mechanisms. Imaging EPS at high resolution revealed the incorporation of Ag NPs and their aggregates into the EPS-gel matrix, proving their detoxifying capacity.