The PETRA IV project at DESY (Deutsches Elektronen Synchrotron) aims at upgrading the PETRA III synchrotron radiation facility to a unique synchrotron light source with an ultralow-emittance and diffraction limited up to X-rays of 10 keV. An optimization of the girder structure mainly supporting the magnets is essential to ensure a stiff machine and thus a stable particle beam. Therefore, an innovative, biologically inspired girder structure has been developed. Several structural elements were inspired by marine plankton organisms showing light, stiff, and vibration optimized structures [1,2]. The girder development process is based on a topology optimization to disclose an optimum material distribution. The subsequently built beam-shell model combines biologically-inspired structural elements such has irregular honeycombs, smooth connections, branching structures, and torsional-rigid elements. A following cross section optimization revealed optimum cross section diameters and thicknesses for all beam and shell elements. The resulting optimized girder structure has been successfully fabricated using the casting technology.