The efficiency of our oceans to take up excess CO2 depends on nutrient availability, among which iron (Fe) plays a critical role. A shallow (~500 m) hydrothermal Fe source has been discovered during the OUTPACE cruise in the Western Tropical South Pacific, and resulted in high concentrations (>4 nM) of dissolved Fe up to the photic (~0-150 m) layer. However to date, the potential impact of such input on regional Fe budgets and biogeochemical cycles of C, N, P has never been studied. TONGA is a multidisciplinary project dedicated to the study of the control of ocean productivity and carbon sequestration by such micronutrients of hydrothermal origin. This project gathers the geoscience, oceanographic and atmospheric communities (90 scientists from 20 international institutions) and is based both on a 37-days oceanographic cruise in the WTSP and modelling work. TONGA has been endorsed as a GEOTRACES process study and received a letter of support from the IMBER international programme.