Fossil plankton shells have provided new insights into how nitrogen fixation changed in the ocean during a warmer period 3 million years ago. Nitrogen is a vital nutrient for plankton, which form the base of the marine food chain. Two key processes regulate the global nitrogen cycle: nitrogen fixation, where certain bacteria convert atmospheric nitrogen gas (N₂) into a form that marine life can use, and denitrification, where bacteria in oxygen-poor environments convert nitrate back into nitrogen gas. In the Atlantic Ocean, most nitrogen fixation occurs in the tropical North Atlantic, a region that is central to understanding this process. Scientists from the Max Planck Institute for Chemistry (MPIC) previously found that denitrification declined in warmer climates in the past. However, the extent to which nitrogen fixation also changed remained unclear. A new study led by an international team, including researchers from MPIC, has now revealed that nitrogen fixation in the tropical Atlantic dropped significantly during the warm late Pliocene and then increased as the climate cooled into the Pleistocene. This was determined by analyzing nitrogen isotopes in fossilized shells of foraminifera—tiny, single-celled marine organisms whose remains are found in ocean sediments. These fossils are valuable for reconstructing past ocean conditions, as the foraminifera incorporate nitrogen into their shells as they grow. The Pliocene, which lasted from about 5.33 to 2.58 million years ago, was a warm period that preceded the cooler Pleistocene, marked by alternating ice ages and warmer intervals. Researchers compared the nitrogen fixation record in the Atlantic with a denitrification record from the Pacific and found that both processes weakened during the late Pliocene. This is because denitrification removes nitrogen from the ocean and leaves behind phosphorus, which supports nitrogen fixation in nutrient-poor regions. When denitrification slowed in the Pacific, less phosphorus reached the Atlantic, causing nitrogen fixation there to decline as well. This simultaneous weakening of both processes helped maintain a stable overall nitrogen level in the ocean. The study also identified a second pattern: after large ice sheets in the Northern Hemisphere began forming at the end of the Pliocene, about 2.8 million years ago, nitrogen fixation in the Caribbean fluctuated in sync with the 41,000-year Milanković cycle, which describes changes in Earth's axial tilt. These cycles influence the distribution of solar energy and drive climate changes, including ice ages. Researchers linked these fluctuations to sea-level changes: during warm periods, higher sea levels flooded continental shelves, where microbes removed nitrogen and left phosphorus behind, promoting nitrogen fixation. During colder periods, when sea levels dropped, this phosphorus source diminished, causing nitrogen fixation to decline. Recent studies have shown that today, nitrogen fixation in the Atlantic is driven by upwelling in the eastern equatorial region, which brings phosphorus-rich water into the Caribbean. However, during the warmer Pliocene, nitrogen fixation did not respond to changes in this upwelling because the water carried less excess phosphorus. This suggests that the mechanisms controlling nitrogen fixation shift depending on the climate state. The study highlights the sensitivity and complexity of the ocean’s nitrogen cycle, which has important implications for marine ecosystems and is closely tied to global climate patterns.