Expeditions Uncover the Major Role of a Micronutrient

Expeditions Uncover the Major Role of a Micronutrient

As with humans, iron is an essential micronutrient for life in the ocean, and the tiny plant-like phytoplankton that form the base of the marine food web rely on it to grow. Though it's abundant on land (the fourth most common element in the Earth's crust in fact), iron tends to exist in far lower concentrations in the ocean - hence why scientists refer to it as a "trace" metal.

Levels are low enough that iron is the limiting, or controlling, nutrient that ultimately determines biological productivity and phytoplankton growth across roughly a third of the ocean surface. That means iron plays an outsized role in the ocean's ability to absorb carbon from the atmosphere and support rich, thriving ecosystems.

Researchers from Bigelow Laboratory for Ocean Sciences are part of an international team combining field expeditions and lab experiments to understand how different kinds of iron cycle through the marine environment - across time and space - with the ultimate goal of creating models that better reflect iron's critical role in ocean biology.

"One of the central questions in our field is how the productivity of the ocean is going to change," said Senior Research Scientist Ben Twining. "If you want to be able to predict what's going to happen in the future, you need to understand at this granular level of detail, how iron is moving around, what happens when it gets into the ocean, and how long it stays there."

Challenging an Iron Paradigm

Twining was previously involved in a project, led by a researcher at the University of Liverpool, to understand how the iron cycle fluctuates seasonally. Though their fieldwork was stymied by Covid, that work led to a significant discovery, published in Nature in 2023, suggesting that there is a pool of iron in the ocean that models don't account for.

For phytoplankton to be able to benefit from iron in the water column, the mineral needs to be dissolved or in a "bioavailable" particulate form that can redissolve or break down easily so the phytoplankton can use it. Otherwise, that iron will eventually sink away from the sunlit surface.

The long-held view was that organic molecules, called ligands, capture and bind to iron atoms so they can keep iron in that dissolved form that's more readily absorbed by phytoplankton. Yet, Twining and his colleagues showed that more iron was sinking than expected. They proposed that some iron, rather than being bound by ligands at the surface, is clustered into these tiny iron oxide particles called colloids. That iron precipitates out of seawater into solid particles that sink beyond the reach of phytoplankton to the deep sea in a process termed the "colloidal shunt."

"We care about the particulate form of iron because that's when the iron sinks and is lost from the surface where the sun and plants are - and where carbon uptake happens," Twining said. "This colloidal shunt appears to control how much time iron and other nutrients spend in the surface before they sink, which affects how much carbon and how much new growth that iron can support."

Multiple Sites for Multiple Forms of Iron

That discovery inspired the current project, which is jointly funded by the National Science Foundation and the UK Natural Environment Research Council.

Building on the previous work, the team is trying to understand how these different kinds of iron are formed, what role they each play in the iron cycle, and how those processes vary depending on season and ocean conditions. Since the chemical reactions involved in creating these tiny colloids depend on oxygen and dissolved organic carbon, they're also interested in how those other molecules influence the transformations of iron.

Answering those big questions requires a mix of methods and a lot of data. So far, they've completed three expeditions and have two more planned before the end of the year. Postdoctoral Scientist Susanna Michael is, in fact, in the midst of preparing for the fourth cruise next month.

On each expedition, the research team collects thousands of liters of water - running much of it through complex filtering processes designed to avoid metal contamination - to measure dissolved organic carbon, dissolved metals, and ligands. For each bottle, they also preserve a filter that catches iron particulates, which Michael describes like grounds trapped on a coffee filter. They're also trying to develop similar methods for identifying the tiny colloid particles that are so small they flow through most filters and can get lumped with the dissolved measurements.

Meanwhile, the team is tracking different physical parameters, like dissolved oxygen, and preparing phytoplankton samples for later analysis at the synchrotron, a specialized particle accelerator that can measure the number of iron atoms in an individual cell. They've also set up several incubation experiments as well, adding different amounts of iron to samples of water to track how the iron transforms chemically in the absence of phytoplankton.

Most of the expeditions focus on a handful of sites in the Sargasso Sea around Bermuda where large eddies, big seasonal swings, and occasional dust plumes from the Sahara lead to rapid blooms in phytoplankton interspersed with long "droughts" in nutrients. The region also has the advantage of providing almost 30 years of additional reference data collected through the Bermuda Atlantic Time Series.

At each site, the team samples from the surface down to 3,000 meters, drawing water from several spots along the way to capture how productivity and dissolved oxygen fluctuate through the water column. Each depth adds another layer of detail as to how iron cycles in different conditions.

The most recent cruise in May was unique in that they also sailed across the Atlantic to the rich waters of Cape Verde, where an upwelling of cold, nutrient-rich water and a steadier stream of iron-laden Saharan dust have created a global hotspot for marine biodiversity

"Going from Bermuda to Cape Verde, you get a shift in the amount of dust, the temperature, the productivity, oxygen levels, and iron," Twining said. "There's a lot of different gradients and patterns we're trying to capture to tease apart everything happening."

Looking Ahead

Eventually, the goal is to build models of global ocean productivity, and how it's changing, that reflect this more complete understanding of this mighty micronutrient. For now, they're beginning to analyze the samples from the last several cruises, which will be a significant undertaking given the number of samples they have to work with - and a significant accomplishment given how hard many of those samples were to come by, Twining and Michael say. Field-based observations, though, have been promising.

"We've been around Bermuda across several seasons, and have this data from one month across the entire ocean basin, and have seen some of the shifts in water conditions and phytoplankton growth that we were hoping for, which is affirming," Michael said. "Collecting these samples isn't trivial, so the fact that we appear to be capturing the variety over space and time we were looking for is great. It makes one very excited to start digging into the analysis."

Photo Captions

Photo 1: Project collaborators deploy a tow fish to collect trace metal clean water (Credit: Ben Twining).

Photo 2: Ben Freiberger, senior faculty research assistant at Oregon State University (left), and Bigelow Laboratory Senior Research Scientist Ben Twining examine an incubation experiment (Credit: Susanna Michael).

Photo 3: Filter towers in the clean lab aboard the research vessel (Credit: Ben Twining).

Photo 4: Dust blowing in from the Sahara Desert, which is a major source of iron to the open ocean (Credit: Susanna Michael).

Photo 5: The group off Cape Verde in May 2026.