Multicomponent heteronanostructures offering catalytic and optical properties have applications across various fields. Photonic crystals (diatom frustules) coated with Au and copper chalcogenide domains represent a unique nanosystem that integrates multiple plasmon resonances from guided-mode resonance, conduction electrons, and valence holes in a single nanoscale system. In this work, we fabricate an enzyme-assisted photonic crystal-enhanced plasmonic nanosystem using a live diatom (Thalassiosira pseudonana) for surface-enhanced Raman scattering (SERS) quantification of sarcosine, an early stage prostate cancer (PCa) biomarker. The biosynthesized heteronanostructure was constructed by coating bimetallic nanoparticles (Au/Cu X ) on the diatom frustule via a two-stage cultivation process. A silaffin peptide-tagged sarcosine oxidase (SoX) was designed for specific substrate recognition and oriented conjunction. The components were coupled into a single entity (SoX-immobilized Au/CuX-coated frustule, BioNPS) to overcome the interenzyme distance and analyte trade-offs between mass transport. The sarcosine detection by BioNPS outperforms suspended bimetallic nanoparticles and single NP-coated diatom frustules. The improvement is attributed to the coupling of photonic frustule guided-mode resonance to the localized surface plasmonic resonance of bimetallic NPs via both electromagnetic and CT mechanisms. The cascade reaction in close proximity greatly enhances the catalytic efficiency by 5.47-fold compared to the solution-phase assay. The biochem nanosystem precisely detects tiny sarcosine concentration changes in the urine samples of PCa patients and healthy individuals. As a proof of concept, the in vivo fabrication of photonic/plasmonic heterostructures with tunable properties holds great promise for noninvasive biomarker screening.