Seaweed capped zinc oxide nanoparticles (ZnO NPs) are synthesized from four zinc source precursors with brown seaweed of Padina tetrastromatica by precipitation method. The prepared ZnO NPs were characterized by means of FT-IR, XRD, HR-SEM, UV-vis, FLS, TGA and BET. The XRD pattern indicated a hexagonal wurtzite structure of ZnO with high crystallinity and nanocrystalline size. The SEM images show a variety of the as-synthesized hexagonal zinc oxides including crystals, rods, plates and star of ZnO NPs. BET analysis confirmed the presence of a mesoporous network in ZnO NPs samples, with high surface area (21.9184-27.369 m(2)/g) and pore size (16.6856-30.808 nm). Here, seaweed plays a superior role in the development of porous ZnO nanostructures. The photocatalytic activities of the as prepared ZnO NPs have been evaluated by photodegradation of anionic and cationic dyes Drimarene Turquoise Blue S-G (DTB) and Methylene Blue (MB) investigated under the direct sunlight irradiation. Finally, the potential toxicity of synthesised material was also investigated against Staphylococcus aureus and Escherichia coli. All of the seaweed capped ZnO NPs showed good photocatalytic and antibacterial performance. In particular, the sample prepared from zinc nitrate exhibited superior performance compared to other zinc source based ZnO NPs and pure ZnO. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.