# Effects of Nanoparticles on Algae: Adsorption, Distribution, Ecotoxicity and Fate
## 2 Adsorption of NPs by Algae  
As a model organism for studying the biological toxic effects of NPs, it is necessary to investigate the adsorption capacity of algae to NPs. The adsorption capacity of algae to NPs has been tested in some research. The results are summarized in Table 1. For Ag NPs, the accumulated Ag concentrations in the algae _Chorella vulgaris_ reached 1200-3300 \(\upmu\)g/g dry weight after 4 h of exposure to 2 mg/L [30], and the accumulated Ag concentrations in the algae _Raphidocelis subcapitata_ reached 45.0 \(\upmu\)g/g dry weight after an exposure of 24 h to 15 \(\upmu\)g/L and 93.7 \(\upmu\)g/g dry weight after exposure of 24 h to 30 \(\upmu\)g/L [31]. These results indicated that algae can absorb a large amount of NPs. However, the absorption capacities of different species of algae to different NPs varied in a wide range. Meanwhile, the absorption capacity was also related to other reaction parameters, such as the concentration of NP suspension and the time of algal exposure to NPs. In the study of algal adsorption dynamics, two kinetic models, the pseudo-first order model and the pseudo-second order model, are often used to analyze the kinetics of algal adsorption to NPs. Two isotherm models, the Langmuir and Freundlich models, are widely used to fit the adsorption isotherm experimental data [35; 36]. In general, the studies of adsorption kinetic and isotherm models show that the adsorption of algae to NPs is best fitted to a pseudo-second order kinetic model and Langmuir isotherm model [35; 37; 38]. The initial concentration of NPs in water influenced the adsorption of algae to NPs. With an increase in initial concentration, the adsorption capacity of algae to NPs increased [37]. The adsorption process was very fast at the initial stage, but afterwards the rate of adsorption decreased slowly until it reached saturation [37]. The amount of NPs adsorbed by algae from the water varied significantly with the change of solution pH. The relatively strong acidic pH ranges can thus enhance the adsorption of algae to NPs [36]. At the same time, the absorption of algae to NPs was affected by the shape, biomass dose and physicochemical properties of algae [35]. For the uptake of NPs into algae, NPs need to move across two barriers (i.e., cell walls and plasma membranes) [25]. For algae, the relatively thick and tough cell walls are generally considered as the first barrier to prevent internalization of NPs. The algae cell walls are semipermeable, and usually porous in their structure [26]. The diameter of these pores is in the range of 5-20 nm [39; 40]. Generally, only the NPs in the size smaller than the maximal pore size can easily pass through the cell wall, while limiting the passage of larger molecules [30]. The main component of the cell wall in algal cells is cellulose, which also usually contains glycoproteins and polysaccharides [26]. These components can act as binding sites to promote the adsorption of NPs by algae [41; 42]. Once the NPs pass through the cell wall, NPs will encounter the second barrier--the plasma membrane. Endocytosis and passive diffusion are considered to be the main pathway for NPs to cross the bilayer lipid membrane [43; 44]. In addition, the permeability of the cell wall may change during cell cycling, with the newly synthesized cell wall being more permeable to NPs, thereby increasing the uptake of NPs by algal cells [30; 45].