UM Bio Station Scientists Make Deep Sea Discovery in Ocean’s Twilight Zone

The ocean’s twilight zone is deep, dark, and—according to recent research—iron deficient. No sunlight reaches this region 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. To compensate, these bacteria produce molecules called siderophores, which help the bacteria scavenge trace amounts of iron from the surrounding seawater.

Recently published in Nature, a new study, “Microbial iron limitation in the ocean’s twilight zone” co-authored by University of Montana Flathead Lake Biological Station (FLBS) Postdoctoral Research Associate Lauren Manck and FLBS Aquatic Microbial Ecology Professor Matt Church, could change the way scientists view microbial processes in the deep ocean.

The study’s findings also offer insight into the ocean’s capacity to absorb carbon, which plays an important role in naturally managing carbon dioxide levels in the earth’s atmosphere.

Near the surface of the ocean, communities of planktonic algae use carbon dioxide, dissolved in ocean water, as they photosynthesize and grow. Therefore, oceans act as significant sinks for atmospheric carbon dioxide, because when these algae die they descend into the deeper waters of the ocean where the carbon in their bodies is either consumed and recycled by bacteria or stored in deepest regions of the oceans for long periods of time.

“Understanding the amount of carbon that the ocean holds is crucial for understanding Earth’s climate,” said Manck. “The growth of bacteria in the dark ocean determines the efficiency of the ocean’s carbon storage mechanism, so it is important to understand the factors, such as nutrient availability [e.g., iron], that control the growth of bacteria.”

To conduct the study, a team of 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.

The samples revealed the presence of siderophores, along with some surprising results. Siderophores are produced by bacteria when iron availability in the environment is very low—so low that it may be the limiting factor to their growth.

Not only did the samples reveal that concentrations of siderophores were high in surface waters—where iron is expected to be deficient—but siderophores were also elevated in waters between 200 and 400 meters deep, where iron concentrations were previously thought to have little impact on the growth of bacteria.

 “The discovery of siderophores at a large scale in the dark ocean significantly expands the region of the ocean where we think iron availability is impacting the uptake and storage of carbon,” said Manck. “Understanding these processes better will also help us better predict how the ocean will respond to future change.   

A conductivity, temperature, and depth (CTD) rosette, specially designed to collect water with trace amounts of iron and other metals without contamination, is deployed during a SCOPE expedition in the North Pacific Ocean.

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. Additional experiments in the North Pacific, as a part of the Simons Collaboration of Ocean Processes and Ecology (SCOPE), showed that not only was the concentration of siderophores high between 200 and 400 meters, but bacteria also took up siderophores from the environment at faster rates at depth than at the surface. This confirms the complete cycle of siderophore production and utilization by bacteria in this region of the ocean.

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 and carbon 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.”

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.

This release was adapted from an original release from the University of South Florida College of Marine Science: https://www.usf.edu/marine-science/news/2024/deep-sea-discovery-shines-light-on-life-in-the-twilight-zone.aspx