Unexpected Deep-Sea Discovery Sheds Light on Life in the Twilight Zone

The ocean’s twilight zone is deep, dark, and, according to new research, iron-poor. No sunlight reaches this region, 200 to 1,000 meters below the sea surface, where levels of iron, an essential micronutrient, are so low that bacteria growth is limited. To compensate, these bacteria produce molecules called siderophores, which help them scavenge trace amounts of iron from the surrounding seawater.

A Nature A paper detailing these unexpected findings in the Pacific Ocean could change the way scientists think about microbial processes in the deep ocean and offer new insights into the ocean’s ability to absorb carbon.

“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 in the USF College of Marine Science, a co-author of the recent study.

“As organic matter from the ocean surface sinks to the depths, 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.”

To conduct the study, the researchers collected water samples from the top 1,000 meters of the water column during an expedition across the eastern Pacific Ocean, from Alaska to Tahiti. What they found in the samples surprised them.

Not only were siderophore concentrations high in surface waters where iron would be expected to be deficient, they were also high in waters between 200 and 400 meters deep, where nutrient and iron concentrations were thought to have little impact on bacterial growth.

“Unlike surface waters, we did not expect to find siderophores in the ocean’s twilight zone,” Conway said.

“Our study shows that iron deficiency is high in bacteria living in this region, across much of the eastern Pacific Ocean, and that the bacteria use siderophores to increase their iron uptake. This has a domino effect on the biological carbon pump, because these bacteria are responsible for breaking down organic matter as it sinks into the twilight zone.”

This recent discovery is part of GEOTRACES, an international effort to provide high-quality data for the study of climate-induced changes in ocean biogeochemistry.

The study of siderophores is still in its early stages. GEOTRACES researchers have only recently developed reliable methods to measure these molecules in water samples, and they are still working to understand where and when microbes use siderophores to acquire iron.

Although research on siderophores is new, this study clearly demonstrates their impact on nutrient movement in the ocean’s twilight zone.

“To get a complete 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 paper.

“In other words, experiments conducted near the surface must extend to include the twilight zone.”