The photosynthetic activity inhibition ratio (RIA^ph) and the growth inhibition ratio (RI^g) of the <i>A</i>. <i>lobatus</i> cells treated with AgNP concentrations at every hour/day of the experiment and the concurrent chlorophyll fluorescence intensity (^chlFI) and biomass (B) of the control cells; the statistical significance at the 0.05 level.

<p>The photosynthetic activity inhibition ratio (RIA^ph) and the growth inhibition ratio (RI^g) of the <i>A</i>. <i>lobatus</i> cells treated with AgNP concentrations at every hour/day of the experiment and the concurrent chlorophyll fluorescence intensity (^chlFI) and biomass (B) of the control cells; the statistical significance at the 0.05 level.</p

The biomass of <i>A</i>. <i>lobatus</i> cells under AgNP exposure; the biomass was expressed using chl <i>a</i> concentration.

<p>The biomass of <i>A</i>. <i>lobatus</i> cells under AgNP exposure; the biomass was expressed using chl <i>a</i> concentration.</p

Silver nanoparticles as a control agent against facades coated by aerial algae—A model study of <i>Apatococcus lobatus</i> (green algae)

<div><p>Aerial algae are an important biological factor causing the biodegradation of building materials and facades. Conservation procedures aimed at the protection of historic and utility materials must be properly designed to avoid an increase of the degradation rate. The aim of the present study was to investigate the effect of silver nanoparticles (AgNP) synthetized with features contributing to the accessibility and toxicity (spherical shape, small size) on the most frequently occurring species of green algae in aerial biofilms and thus, the most common biodegradation factor–<i>Apatococcus lobatus</i>. Changes in the chloroplasts structure and the photosynthetic activity of the cells under AgNP exposure were made using confocal laser microscopy and digital image analysis and the estimation of growth inhibition rate was made using a biomass assay. In the majority of cases, treatment with AgNP caused a time and dose dependant degradation of chloroplasts and decrease in the photosynthetic activity of cells leading to the inhibition of aerial algae growth. However, some cases revealed an adaptive response of the cells. The response was induced by either a too low, or—after a short time—too high concentration of AgNP. Taken together, the data suggest that AgNP may be used as a biocide against aerial algal coatings; however, with a proper caution related to the concentration of the nanoparticles.</p></div

Extinction of chlorophyll fluorescence emission in the <i>A</i>. <i>lobatus</i> cells treated with AgNP concentrations.

<p>The digital images were captured every hour/day of the experiment using the Leica TCS SP8 Confocal Laser Microscope.</p

Digital imaging of the changes in <i>A</i>. <i>lobatus</i> chloroplasts under AgNP exposure (20 ppm); cross-sections in 3-D were performed using the Leica TCS SP8 Confocal Laser Microscope and the LAS-AF 3.3.0.10134 software.

<p>a) control samples; b) chloroplasts after 1 hour of AgNP exposure; c) chloroplasts after 24 hours of AgNP exposure; d) chloroplasts after 1 week of AgNP exposure.</p

The range of chlorophyll fluorescence intensity of <i>A</i>. <i>lobatus</i> cells under AgNP exposure; the fluorescence was measured using the Leica TCS SP8 Confocal Laser Microscope equipped with WLL of 488 nm.

<p>The range of chlorophyll fluorescence intensity of <i>A</i>. <i>lobatus</i> cells under AgNP exposure; the fluorescence was measured using the Leica TCS SP8 Confocal Laser Microscope equipped with WLL of 488 nm.</p

Percentage of the growing inhibition rate and the biocidal effect of AgNP against aerophytic algal cells after 14 days of AgNP exposure.

<p>Percentage of the growing inhibition rate and the biocidal effect of AgNP against aerophytic algal cells after 14 days of AgNP exposure.</p

Electrodynamic and hydrodynamic characteristics of AgNP.

<p>a) Zeta potential of AgNP at different pH; b) DLS of AgNP.</p