Magnetite iron oxide (Fe3O4) nanoparticles (NPs) are key materials applied in many different fields of modern technology. The potential environmental impact of these NPs is of great concern. In this study, initially the effect of Fe3O4 NPs size (20 and 40 nm) as well as bulk (>100 nm) at 200 mg L(-1) on Picochlorum sp. (Trebouxiophyceae, Chlorophyta) is investigated during the different growth phases. The most inhibitory NPs were then chosen to assess their effects at different concentrations. The 20 nm NPs at 200 mg L(-1) were found to significantly reduce the viable cell concentration and chlorophyll a content during the exponential growth phase compared to the other particle sizes. However, the 20 nm NPs at different concentrations were found to promote algal growth during the late growth stages (stationary and decline phases) compared to the control. Additionally, algae were found to accelerate the aggregation and sedimentation of nanoparticles into the medium and therefore can be considered as potential organisms for bioremediation of nano-pollution.