The biogeochemical cycles proceed through biological, geological and chemical interactions along hydrological, gaseous, and mineral "trade routes." Among the most ecologically important and well known are the element cycles of carbon (C), nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S), as well as the water (H2O) cycle. One biogeochemical cycle that is often overlooked, however, is Earth's iron (Fe) cycle (Figure 1).