Nanoparticles have various physicochemical properties attributed by their small dimensions. The global production of nanoparticles lead to a raised concern for their environmental impacts. Algae are of highly ecological importance functioning as primary producers for almost all aquatic life. In the present thesis, interactions of nanoparticles with different fresh water algal strains were examined. First, the toxicity and uptake of citrate-coated silver nanoparticles (AgNP) and AgNO3 were examined in the alga Euglena gracilis, which has no cell wall but a pellicle. Secondly, to examine the role of algal cell wall in determining the particles interactions with algae, four strains were selected, including Euglena gracilis, Haematococcus pluvialis, and Chlamydomonas reinhardtii wild type and a cell wall free mutant. Their interactions with polystyrene nanoparticles (PSNP) of two sizes, 50 nm (PSNP50) and 500 nm (PSNP500), were investigated. Third, interactions of three differently coated AgNP with alkaline phosphatase (AP), an extracellular enzyme used for phosphorus acquisition, were assessed. The selected coatings were citrate (CIT), polyvinylpyrrolidone (PVP) and gelatin (GEL). Exposure to AgNP and AgNO3 for 1-2 hours led to decrease in photosynthetic yield, in a concentration-dependent manner, and changes in cell morphology in E. gracilis. Based on total silver added, AgNP were less toxic than AgNO3. Concentrations causing a 50% reduction in photosynthetic yield were 1.9 µM and 85 for AgNP and AgNO3, respectively. Damaging effects of AgNP were completely prevented by cysteine, suggesting that the toxicity of AgNP was mediated by Ag+ ions. Uptake studies showed that the maximal cell-associated silver was higher in AgNP compared to AgNO3, and the higher silver level was shown to correspond to particles adsorbed to the pellicle. By examining four algal stains, it was found that no strain internalized PSNP, emphasizing the role of algal cell walls as barrier for nanoparticle uptake. Interactions of PSNP with algae were found to be unique for each strain, and depend on particle size. PSNP50 were associated with E. gracilis cells displaying a non-homogenous distribution on pellicle. In H. pluvialis, PSNP50 distributed homogenously around the cells. The wild type and cell wall free mutant of C. reinhardtii cells exposed to PSNP50 were found to clump together packed in the extracellular polymeric substances (EPS). The particles were associated with the EPS. The larger PSNP500 were observed to interact only with the two C. reinhardtii strains. Taken together, these results indicate that the algal cell walls hinder the crossing of nanoparticles. Assessing the sorption of AP to AgNPCIT, AgNPPVP, and AgNPGEL showed that the physiochemical properties of both the particle coatings and the enzyme were determinant for the binding. The enzyme adsorbed to AgNPCIT and AgNPPVP, leading to a 10% and 70% coverage of the particle surface area, respectively. No adsorption was found in the case of AgNPGEL. The three types of AgNP decreased AP activity, however, the inhibitory effects only occurred when the AgNP were added after addition of the substrate to the enzyme, not vice versa. AgNO3 did not affect the AP activity. Thus, the results of this study indicate particle-specific effects due to interactions with the AP-substrate intermediate.