Marine microbial nitrogen supply reduced under warmer climate

Fossil plankton shells show that three million years ago, under warmer climate, nitrogen fixation was reduced

• About three million years ago: During the warm late Pliocene, nitrogen fixation in the tropical Atlantic declined significantly caused by slowed denitrification in the Pacific Ocean.
• Comparison to today: The warm climate in the late Pliocene, with temperatures higher than today’s, is geologically comparable to warming in the 21st century.
• In a warmer future: the new study suggest that the total nitrogen balance would remain stable.
• Climate archive foraminifera: Evidence from nitrogen isotopes in foraminifera skeletons in drill cores from the tropical Atlantic.

Nitrogen is a key nutrient as it fuels the growth of plankton, which forms the basis of the marine food web. Two opposing processes determine the global nitrogen cycle: Nitrogen fixation and denitrification. 

Nitrogen-fixing cyanobacteria convert nitrogen gas (N2) dissolved in water into a bioavailable form. This occurs primarily in warm, nutrient-poor surface waters. In the Atlantic, nearly 90 percent of nitrogen fixation occurs in its tropical North, making it a key region for understanding this process. 

In other ocean regions nitrogen is removed from the ocean back into the atmosphere through denitrification, where nitrate is reduced by denitrifying bacteria back to N2 gas. This process occurs particularly in oxygen-depleted zones, which today are most extensive in the Pacific Ocean. 

In previous studies  researchers from the Max Planck Institute for Chemistry (MPIC) showed that denitrification in the water column, the body of seawater between the surface and the sea floor, decreased globally under past warmer climate conditions (https://www.mpic.de/5260848/ocean-oxygenation). However, it was previously unclear whether and how nitrogen fixation also changed during these warmer periods, which is important because the balance between these two processes maintain how much nitrogen is available for the global ocean’s biological processes.

Reconstructed nitrogen fixation rate

An international research group led by the MPIC has now been able to demonstrate, using drill cores from the ocean sediment, that natural nitrogen fixation in the tropical Atlantic was significantly reduced during the warm late Pliocene, and then increased into the cooler Pleistocene. The scientists analysed nitrogen isotopes in foraminifera fossil shells from ocean sediment cores in the tropical North Atlantic that lived during the Pliocene and early Pleistocene. Foraminifera are microscopic fossils that are ideally suited for reconstructing past climate conditions and ocean currents. The fossils of these organisms, which incorporate small amounts of nitrogen into their calcareous skeletons as they grow, can still be found today in marine sediments.

The Pliocene is a geological epoch that began about 5.33 million years ago and ended 2.58 million years ago and was globally warmer than today. This warm climatic period was followed by the cooler Pleistocene, which was characterized by alternating glacial and interglacial periods, due to the cyclic formation and disappearance of Northern Hemispheric ice sheets.

Phosphorus-rich seawater boosts nitrogen fixation

When comparing nitrogen fixation record in the Atlantic with recently published denitrification record in the Pacific, the researchers found that both processes weakened during the late Pliocene. They attribute this to the tight link between the two processes: When nitrogen is removed through denitrification, the residual ocean water is enriched with the nutrient phosphorus. This excess phosphorus is then distributed through ocean currents and promotes nitrogen fixation in in the nutrient poor parts of the ocean, where nitrogen fixing organisms are able to thrive. 

When denitrification in the Pacific Ocean slowed during the Pliocene, less phosphorus reached the Atlantic Ocean as a result, causing nitrogen fixation to decline there as well. The simultaneous decrease in both processes meant that the ocean's main nitrogen source and main nitrogen sink shrank together, keeping the total amount of nitrogen in the ocean relatively stable.

"Whether the ocean's nitrogen cycle remains balanced under climate perturbations has been debated for decades. In a warmer future, we suspect both processes will slow down, so the ocean would fix less nitrogen but would also lose less. Because these changes largely cancel each other out, the ocean’s overall nitrogen balance could stay relatively stable, though the amount of nutrients supplied to a given region may differ from today", says Maayan Yehudai, postdoctoral researcher at the Max Planck Institute for Chemistry and first author of the study now published in Nature Communications. 

Nitrogen fixation paced by the ice ages

The records of the Max Planck researchers also revealed a second pattern. After large Northern Hemisphere ice sheets began to grow at the end of the Pliocene, about 2.8 million years ago, nitrogen fixation in the Caribbean rose and fell synchronized with the approximately 41,000-year Milanković cycle in the tilt of Earth's axis. The Milanković cycles are periodic variations in the Earth's orbit around the Sun. They alter the amount and distribution of solar energy on Earth and play a key role in determining cold and warm periods, as well as the rhythm of the ice ages.

The team links this natural rhythm to sea level changes: during warm interglacials, higher sea level flooded the continental shelves, where microbes in the sediment removed nitrogen from the water and left phosphorus behind, which is another way to locally fuel more nitrogen fixation; when sea level fell during glacials and exposed the shelves, this phosphorus source shrank and nitrogen fixation declined.

A different Atlantic today

“Our findings help us to put the mechanisms that drive Atlantic N2 fixation today in a longer-term context”, says Alfredo Martínez-García, group leader at Max Planck Institute for Chemistry and senior author of the study. Recent research (https://www.mpg.de/25659684/the-driver-of-sargassum-blooms-in-the-atlantic-ocean) led by his group showed that during the past decades, nitrogen fixation in the Atlantic Ocean has been driven by upwelling in the eastern equatorial Atlantic. This delivers phosphorus-rich water that currents carry into the Caribbean. 

The new study shows that in the warmer Pliocene, nitrogen fixation did not seem to respond to equatorial upwelling changes, because the upwelled water then carried far less excess phosphorus. This means that the driving force controlling Atlantic nitrogen fixation shifts between warmer and cooler climates. In other words, the mechanism depends on the background climate state.

The ocean's nitrogen cycle is a highly sensitive and dynamic system, according to the study. It has a significant impact on marine ecosystems and is closely linked to the climate.