Carbon nanostructures are known to serve as a scaffold for growth of efficient water-splitting nanocatalysts. Here we have used Tetraselmis algal cells as both a renewable and sustainable carbon source and a biotemplate to synthesize low-cost oxygen evolution reaction (OER) NiFe oxide nanocatalysts. The nanocatalysts were grown on Tetraselmis-derived carbonized cells (cCells), a three-dimensional (3D) carbon scaffold, by two approaches. In the first approach, the catalyst components were loaded on cells prior to carbonization (pre-loading method). The nanocatalysts were then grown by using silica precursor tetramethoxysilane to generate NiFe(OH)(x) @Cell-SiO2 composites. Further pyrolysis produced NiFe oxides on electrically conductive cCells (NiFeOx @cCell). The cCells were reduced graphene oxide with 3D morphology. In the second approach, the cCells were synthesized first, followed by a hydrothermal reaction with the catalyst precursors (post-loading method). In comparison with the pre-loading method, the post-loading method enabled to load more nanocatalysts on individual cCells, which were highly efficient, with OER performance superior to that of the benchmark OER catalyst Ir/C. Our current methods may offer a new opportunity for using microalgae-derived, highly efficient water-splitting nanocatalysts for hydrogen production. (C) The Author(s) 2018. Published by ECS.