Marine diatoms are a class of microalgae that possess cell walls composed of silica nanoparticles. These organisms actively assimilate silicic acid Si(OH)4 from seawater, polymerize silicic acid to silica nanoparticles by a protein-mediated precipitation process, then assemble the silica nanoparticles into intricate patterns that constitute the cell wall microarchitecture of the diatom frustule. The biomineralization capacity of marine diatoms, Nitzschia frustulum, was harnessed to biologically manufacture silicon oxide / germanium oxide nanocomposite materials. Germanium was incorporated into living diatom cell mass by a two-stage cultivation process. The micro- and nanostructures of biogenic oxide nanocomposite were characterized by TEM with EDS, and XRD. Furthermore, photoluminescence (PL) measurements were performed on these biogenic nanocomposites with different post processing to study their optoelectronic properties. Strong blue photoluminescence was observed from samples treated with H2O2 and oxygen plasma. A clear blueshift was observed from the biogenic oxides with the addition of germanium. It is believed that self-trapped exciton affected by quantum confinement effect is responsible for the PL from these biogenic oxide nanocomposites.