Kristen Buck (left) directs the collection of water samples for iron analysis on a GEOTRACES cruise in the Gulf of Mexico.
CEOAS scientists join GEOTRACES to unlock the secrets of iron in the ocean
By Nancy Steinberg
Which chemical elements are absolutely necessary for life?
The ones that might leap to mind are the celebrities of the periodic table like hydrogen, carbon, nitrogen and oxygen. This is a pretty good list, but there are other essential elements that just don’t get the attention of these main characters. Let’s give a little love to critical trace elements, those lesser-known building blocks of life found in very low concentrations, including iron, zinc and manganese.
An international scientific program called GEOTRACES has set out to increase our knowledge of these rare but important elements in the marine environment. By collecting data on multiple long research cruises throughout the world’s oceans, GEOTRACES scientists aim to determine the concentrations of trace elements over space and time, and learn how they got there, how they move around and how all of those parameters are changing as the climate changes.
Two CEOAS faculty members, Rene Boiteau and Kristen Buck, are participating in the GEOTRACES program, both with a particular interest in iron and how it is used by organisms at the base of the food web. Iron is critical for life in the ocean — and nearly all life on Earth, actually — but it is also in short supply, at least in forms that organisms can use. Nearly one-third of the world’s surface oceans are iron limited: They have plenty of the other elements necessary for phytoplankton to thrive, but the lack of available iron keeps phytoplankton in check. “The goal of GEOTRACES was to answer these very basic, zero-order questions about where metals are coming from in the ocean, how much is there, and what processes help move it around,” Boiteau explains.
Both Boiteau and Buck are using GEOTRACES data collected in the Southern Ocean, one of the largest iron-limited environments on Earth. Using complementary scientific approaches, they hope to answer many questions about how iron is used and cycles around the global ocean.
Boiteau is interested in defining the sources of iron to the ocean, which include sediments on the seafloor, hydrothermal vents and deposition from dust that is wafted by windstorms. A particularly intriguing aspect of constraining these iron sources for Boiteau is understanding the distribution of microbial iron-binding molecules, called siderophores. Iron minerals that enter the ocean are nearly insoluble in seawater, but they must be dissolved for cells to take up and use — this is one reason that iron is so scarce and limits algal growth in so much of the ocean. Microbes that live in iron-limited environments have evolved a solution to this problem: They produce siderophores on their outer cell walls, unique to each species, that grab and dissolve iron in seawater and bring it into the cell. With the iron in a usable form, it can move into the rest of the food web, from plankton to fish to whales.
Boiteau has developed sophisticated methods for characterizing individual siderophores — and there are hundreds of them — and examining how they affect microorganisms’ competition for iron. Using GEOTRACES water samples, Boiteau and his lab will look at where siderophores in the surface of the Southern Ocean come from and how they enable various organisms to dissolve and use scarce iron resources.
“We’re really excited to understand how the microbial competition for iron and other metals in the Southern Ocean plays out,” Boiteau says.
Kristen Buck’s GEOTRACES work focuses on other aspects of iron in the Southern Ocean. For example, she is interested in how iron moves around the global ocean on what is referred to as the ocean conveyor belt. Ocean water forms distinct masses that circulate around the planet in a predictable way based on temperature and salinity. Cold, dense water at the poles sinks to the ocean bottom, warming and rising as it heads towards the equator. Nutrients and trace elements come along for the ride, being transformed by biological processes as they go. Buck is intrigued by the fact that there are strong iron-binding ligands (a broader term for metal-binding molecules that includes siderophores) throughout the water column in the North Atlantic, but in the Pacific, the stronger ligands are found only at the surface of the ocean and not at depth. Why is the North Atlantic so rich in ligands, including siderophores? Buck thinks the difference is due to changes that occur as water masses move on the conveyor belt.
“Randie Bundy (University of Washington) and I propose to examine the hypothesis that water masses formed in the Southern Ocean provide an important source of ligands to the global ocean,” she says. So, they’ll start by examining ligand concentrations at their hypothesized source.
“The Southern Ocean is the hub of the ocean conveyor belt and global ocean circulation,” Buck says. “Water masses formed here extend into the other ocean basins and are known to be an important source of nutrients to far-away surface waters.”
The unprecedented sampling resolution in the GEOTRACES program will help both researchers in their quest to learn about the supply and chemical forms of iron in the Southern Ocean. The resulting iron availability impacts the ocean’s ability to take up carbon, especially critical in this time of climate change. Buck notes, “I usually get close to a thousand samples from a GEOTRACES cruise. Considering that my entire Ph.D. dissertation probably comprised less than 100 samples, it’s a huge increase in resolution. It’s like we’re seeing the ocean in color for the first time, and now we have to figure out what the colors mean.”
Sampling for metals from a metal ship
Salinity? We can measure that. Nutrients? We can take samples all day for nitrogen. Carbon is a piece of cake. But sampling for metals in the ocean presents some tough challenges.
One challenge is sample contamination. Because concentrations of metals in the ocean are so low to begin with, the sample collection and analytical process is very sensitive to contamination. Culprit number one is the massive metal ship, with a hull coated in metal-based anti-fouling paint, used to collect water samples. And then there’s the metal cable used to drop the sampling equipment into the ocean, and the metal fixtures on analytical equipment.
“There was a period where there was quite a bit of debate about how much iron was in a bottle of seawater,” Rene Boiteau says. “Measurements spanned many orders of magnitude,” thanks to this contamination problem.
Now oceanographers know how to address the issue. First, they replace metal fixtures and lines with Teflon and Kevlar. They tow the water sampling equipment a distance from the ship, avoiding the metal hull, the paint and the engine emissions. And they create a makeshift clean lab on board swathed in plastic sheets and containing no metal.
These iron-clad solutions do wonders, making the scientists confident that, as Kristen Buck says, “our samples represent the chemistry of the ocean and not the fact that the ship showed up there that day.”
Watch more in-depth videos about GEOTRACES on the STRATA YouTube playlist.