ERC CoG DeepTrace will advance a ground-breaking mechanistic, analytical, and predictive understanding of how the life-supporting metals cycle across Earth's deep-sea ecosystems such as the Black Sea and hydrothermal vents, developing the application of metal nanoparticles as tracers of planetary redox habitability and bridging the study of Earth's Oceans with the Ocean Worlds of the Solar system. DeepTrace will tap the potential of tracing redox metals as one of the best opportunities in the next decade for detecting life in Ocean Worlds, and accelerate improved parametrizations of metal cycles for better prediction of Earth’s marine ecosystems under multi-stressors such as deoxygenation, warming, and biodiversity loss.