The ocean’s twilight zone is iron-deficient, limiting bacterial growth and affecting carbon storage. New findings show high siderophore levels in deeper waters, indicating a broader impact on marine biogeochemical cycles and necessitating further study.
Researchers have shed light on the ocean’s twilight zone, revealing that this deep, dark, mysterious region is severely iron deficient. No sunlight reaches this area 200 to 1,000 meters below the sea surface, where levels of iron, a key micronutrient, are so low that the growth of bacteria is restricted. These bacteria compensate by producing molecules called siderophores, which help the bacteria scavenge trace amounts of iron from the surrounding seawater.
The study, recently published in Nature, could change the way scientists view microbial processes in the deep ocean and offer new insight into the ocean’s capacity to absorb carbon.
Iron Deficiency and Its Impacts
“Understanding the organisms that facilitate carbon uptake in the ocean is important for understanding the impacts of climate change,” said Tim Conway, associate professor of chemical oceanography at the USF College of Marine Science, who co-authored the recent study. “When organic matter from the surface ocean descends to the deep ocean, it acts as a biological pump that removes carbon from the atmosphere and stores it in seawater and sediments. Measuring the rates and processes that influence this pump gives us insight into how and where the ocean stores carbon.”
Surprising Discoveries in Deep Waters
To conduct the study, researchers collected water samples from the upper 1,000 meters of the water column during an expedition through the eastern Pacific Ocean from Alaska to Tahiti. What they found in the samples surprised them. Not only were concentrations of siderophores high in surface waters where iron is expected to be deficient but they were also elevated in waters between 200 and 400 meters deep, where nutrient and iron concentrations were thought to have little impact on the growth of bacteria.
“Unlike in surface waters, we did not expect to find siderophores in the ocean’s twilight zone,” said Conway. “Our study shows that iron-deficiency is high for bacteria living in this region throughout much of the east Pacific Ocean, and that the bacteria use siderophores to increase their uptake of iron. This has a knock-on effect on the biological carbon pump, because these bacteria are responsible for the breakdown of organic matter as it sinks through the twilight zone.”
The recent discovery was part of GEOTRACES, an international effort to provide high-quality data for the study of climate-driven changes in ocean biogeochemistry.
Future Research and Implications
The study of siderophores is still in the early stages. Researchers involved in GEOTRACES only recently developed reliable methods to measure these molecules in water samples, and they’re still working to understand where and when microbes use siderophores to acquire iron.
Although the research into siderophores is new, this study demonstrates their clear impact on the movement of nutrients in the ocean’s twilight zone.
“For a full picture of how nutrients shape marine biogeochemical cycles, future studies will need to take these findings into account,” said Daniel Repeta, senior scientist at Woods Hole Oceanographic Institution and co-author of the article. “In other words, experiments near the surface must expand to include the twilight zone.”
Reference: “Microbial iron limitation in the ocean’s twilight zone” by Jingxuan Li, Lydia Babcock-Adams, Rene M. Boiteau, Matthew R. McIlvin, Lauren E. Manck, Matthias Sieber, Nathan T. Lanning, Randelle M. Bundy, Xiaopeng Bian, Iulia-Mădălina Ștreangă, Benjamin N. Granzow, Matthew J. Church, Jessica N. Fitzsimmons, Seth G. John, Tim M. Conway and Daniel J. Repeta, 25 September 2024, Nature.
DOI: 10.1038/s41586-024-07905-z
Funding for this work was provided by the National Science Foundation and the Simons Foundation. The U.S. portion of GEOTRACES is provided by the National Science Foundation.
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