Microalgae and Cyanobacteria extracts can be used for the synthesis of spherical silver nanoparticles by the reduction of AgNO under air atmosphere at room temperature. Here, we synthesized AgNPs using extracts of one cyanobacterium (Synechococcus elongatus) and two microalgae (Stigeoclonium sp. and Cosmarium punctulatum). The nature of the AgNPs was characterized by TEM, HR-TEM, EDS, and UV-Vis. Considering the large quantity of functional groups in the ligands of AgNPs, we suppose they could retain ion metals, which would be useful for water decontamination. Thus, their capacity to adsorb iron and manganese at concentrations of 1.0, 5.0, and 10.0 mg L in aqueous solutions was evaluated. All experiments were performed in triplicate of microorganism extract with no addition of AgNO (control) and AgNP colloid (treatment) at room temperature. The ICP analyses showed that the treatments containing nanoparticles were commonly more efficient at removing Fe and Mn ions than the corresponding controls. Interestingly, the smaller nanoparticles (synthesized by Synechococcus elongatus) were the most effective at removing Fe and Mn ions, probably due to their higher surface area:volume ratio. The green synthesized AgNPs proved to be an interesting system for the manufacture of biofilters that could be used to capture contaminant metals in water.