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Effect of silver nanoparticles on marine diatom – an AFM study

Silver nanoparticles (Ag NP) were found to reduce cell-specific growth rate, photosystem II quantum yield, and chlorophyll-a content of marine and freshwater algae1. These toxic effects were accounted for by the release of Ag+ ion. The toxicity of NP themselves remains an open question, as the NP-cell wall interactions were not precisely analyzed. The exopolymeric substances (EPS) production has been shown to increase as a feedback response to Ag NP exposure and may thus contribute to detoxification mechanisms. Algae typically have glycoproteins and polysaccharides in their cell walls. However, diatoms represent a special case, with cell walls composed of hydrated silicon dioxide. Principal question is: can nanoparticles enter the diatom cell or does the diatom cell wall acts as a barrier for entering nanoparticles? We have chosen to study the effect of Ag NP on ubiquitous, weakly silicified marine diatom Cylindrotheca fusiformis. In our previous AFM studies we performed a detailed morphological and nanomechanical characterization of Cylindrotheca spp. cell wall and its EPS release at a single cell level2, 3. Thus, we have tools to monitor the NP-cell wall interaction and to verify if the released EPS has a detoxifying capacity. AFM imaging was performed after 24 h-exposure at 103 Ag NP/cell in filtered seawater. The primary NP diameter was 60 nm. After the exposure the cells were placed on mica using drop deposition method modified for phytoplankton samples2 for imaging in air. The panel illustrates the effect of Ag NP to the C. fusiformis cell and Ag NP accumulation within EPS. The Ag NP can enter the diatom cell through the valve region causing local damage inside the cell without the cell wall disintegration. This can be understood in the light of the organosilica structures of the Cylindrotheca spp. cell wall3. More experiments are needed to strengthen the results.

marine diatom; nanoparticles; AFM; silver; EPS

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