The limited availability of biocompatible metal oxide photocatalysts that are responsive to visible light remains a major challenge in the development of environmentally friendly biomedical materials, as most conventional photocatalysts are active only under ultraviolet (UV) irradiation, which exhibits low tissue penetration. The functional performance of metal oxide nanohybrids can be tailored through heterojunction formation via green synthesis strategies to control morphology, crystal structure, optical properties, and biological activity under visible-light irradiation. In this study, ZnO-MgO-CuO oxide nanohybrids were synthesized using a green synthesis approach employing amino cellulose extracted from the red macroalga Gelidium pusillum as a natural reducing and stabilizing agent. Two molar ratio compositions were prepared (Zn:Mg:Cu = 1:1:1) which consist of (0.1 M: 0.1 M: 0.1 M) and (0.2 M: 0.2 M: 0.2 M) to evaluate the effect of precursor concentration on structural evolution, band gap modulation, and biomedical performance, with single oxides (ZnO, MgO, and CuO) used as reference materials. XRD, FTIR, TGA-DTA, SEM-EDX, and UV-Vis-DRS analyses confirmed the formation of a mixed-phase crystal structure (hexagonal-cubic-monoclinic) after calcination at 850 °C, with rod-like, cubic, and spherical morphologies (∼70 nm) and visible light responsive band gap energies in the range of 2.62-2.82 eV. Biological evaluations revealed that ZMC-2 exhibited the highest antioxidant activity (58.2%; IC50 = 46.3 ± 0.01 μg/mL. Antibacterial activity under visible-light irradiation against Staphylococcus epidermidis and Salmonella typhosa demonstrated inhibition zones up to 35 ± 0.04 mm. These findings highlight the potential of green synthesized ZnO-MgO-CuO nanohybrids as multifunctional materials for biomedical-oriented applications. © 2026 Elsevier Ltd and Techna Group S.r.l. All rights are reserved, including those for text and data mining, AI training, and similar technologies.