The oceanic iron cycle has evolved significantly over the past three decades, with new insights into iron sources, recycling, and its role in marine productivity. Iron is a critical trace element that influences ocean productivity, carbon sequestration, and atmospheric CO₂ levels. The iron hypothesis, proposed by John Martin, highlighted the importance of iron in controlling phytoplankton growth, particularly in high-nitrate, low-chlorophyll regions, which cover 25% of the world's ocean. Iron-enrichment experiments have shown that adding iron to these regions stimulates phytoplankton growth, enhancing carbon drawdown and dimethyl sulphide production. Iron also plays a role in nitrogen fixation by diazotrophs in nutrient-poor waters. Iron sources include dust, coastal sediments, sea ice, hydrothermal fluids, and atmospheric deposition. Iron is rapidly recycled in the upper ocean by bacteria, which release iron-binding ligands to keep iron in solution. Sinking particles scavenge iron from solution, creating a balance that determines dissolved iron concentrations. Iron's biogeochemical cycle is influenced by factors such as ocean circulation, iron-binding ligands, and the fate of particulate iron. Colloidal and dissolved iron differ in reactivity, with colloidal iron being more readily scavenged. Iron supply mechanisms vary regionally, with the Southern Ocean having multiple sources such as dust, glacial melt, and hydrothermal activity. Iron recycling is mediated by biological processes, including bacterial and grazers' roles in regenerating iron. The 'ferrous wheel' describes the rapid cycling of iron between dissolved and particulate forms, driven by microbial activity. Iron's fate in the ocean is influenced by remineralization, scavenging, and the interplay between dissolved and particulate iron. Iron biogeochemical models have advanced, incorporating complex interactions between iron, nutrients, and climate. These models help predict how changes in iron supply and ocean circulation will affect marine productivity and carbon cycling. Understanding the iron cycle is crucial for assessing the impacts of climate change on oceanic biogeochemical processes. The field of iron biogeochemistry continues to evolve, with ongoing research into iron sources, cycling, and its role in global climate systems.The oceanic iron cycle has evolved significantly over the past three decades, with new insights into iron sources, recycling, and its role in marine productivity. Iron is a critical trace element that influences ocean productivity, carbon sequestration, and atmospheric CO₂ levels. The iron hypothesis, proposed by John Martin, highlighted the importance of iron in controlling phytoplankton growth, particularly in high-nitrate, low-chlorophyll regions, which cover 25% of the world's ocean. Iron-enrichment experiments have shown that adding iron to these regions stimulates phytoplankton growth, enhancing carbon drawdown and dimethyl sulphide production. Iron also plays a role in nitrogen fixation by diazotrophs in nutrient-poor waters. Iron sources include dust, coastal sediments, sea ice, hydrothermal fluids, and atmospheric deposition. Iron is rapidly recycled in the upper ocean by bacteria, which release iron-binding ligands to keep iron in solution. Sinking particles scavenge iron from solution, creating a balance that determines dissolved iron concentrations. Iron's biogeochemical cycle is influenced by factors such as ocean circulation, iron-binding ligands, and the fate of particulate iron. Colloidal and dissolved iron differ in reactivity, with colloidal iron being more readily scavenged. Iron supply mechanisms vary regionally, with the Southern Ocean having multiple sources such as dust, glacial melt, and hydrothermal activity. Iron recycling is mediated by biological processes, including bacterial and grazers' roles in regenerating iron. The 'ferrous wheel' describes the rapid cycling of iron between dissolved and particulate forms, driven by microbial activity. Iron's fate in the ocean is influenced by remineralization, scavenging, and the interplay between dissolved and particulate iron. Iron biogeochemical models have advanced, incorporating complex interactions between iron, nutrients, and climate. These models help predict how changes in iron supply and ocean circulation will affect marine productivity and carbon cycling. Understanding the iron cycle is crucial for assessing the impacts of climate change on oceanic biogeochemical processes. The field of iron biogeochemistry continues to evolve, with ongoing research into iron sources, cycling, and its role in global climate systems.