The marine nitrogen cycle

Abstract

• Early during the 20th century, Alfred Redfield determined that marine phytoplankton help to set the ratio of nitrogen and phosphorus in the deep sea; he also framed a still-unsettled debate over which one of them is the primary limiting nutrient in the sea. • Many marine diazatrophs, including long-recognized Trichodesmium and also more recently identified diazotrophs that are smaller than 10 μm, are globally important contributors to nitrogen input. • Limiting nutrients for nitrogen fixers appear to differ in different ocean regions and depend on the relative supply of iron versus phosphorus. • Some experts believe that there are feedback processes that maintain overall fixed global nitrogen within relatively strict limits.