Photo by Brian D. Bill, NOAA Fisheries

Testing the Sea's Mettle
Two UMaine scientists study the nutritional secrets of the world's ocean depths

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In oceans around the world, tiny plants at the bottom of the aquatic food chain are as crucial to the marine ecosystem as grass is to the prairie. And their effects go beyond the sea. They influence atmospheric chemistry, particularly concentrations of carbon dioxide, a greenhouse gas that currently is at the crux of debates over global warming.

In separate projects in the Pacific in 2004, University of Maine oceanographers Fei Chai and Mark Wells boarded ships with colleagues from around the globe to study the physical and chemical factors that control phytoplankton. Their focus: the internal workings of the marine ecosystem. Rather than studying coastal waters where phytoplankton are usually abundant, they go to unusual deep ocean regions where phytoplankton tend to be less productive.

Until recently, some of these areas, comprising about 30 percent of the sea surface, posed a long-standing mystery in marine science. While they appear to have enough of the nutrients that phytoplankton need to grow, the seasonal crop is smaller than expected. Something is holding it back.

That something turns out to involve two critical nutrients — iron and silicate. A major group of phytoplankton, the diatoms, needs both. When one or both of these nutrients are in short supply, diatoms are stuck on idle. They fail to grow and reproduce. Other nutrients such as zinc, cobalt and nickel also play a role in phytoplankton growth, but scientists are just beginning to understand how they all work together.

Research by Wells and Chai stems in part from the so-called iron hypothesis, first published in the journal Nature in 1989 by oceanographer John Martin. For more than a century, scientists puzzled over the phytoplankton deficit in three regions: the equatorial and North Pacific, and the Southern Ocean around Antarctica.

Speculation that iron is key goes back to the 1930s, but it took the development of a new experimental technique to find the answer. Martin created a method to give scientists precise control over iron concentrations in their samples. Using it, he demonstrated that iron was indeed the missing ingredient in those regions.

But Martin didn't stop there. Since growing plants take up carbon dioxide, he also suggested that natural increases in iron inputs to the oceans during the geologic past may have removed enough carbon from the atmosphere to affect global climate, perhaps even contributing to the onset of ice ages.
Martin died in 1993 just as tests of his ideas were getting under way. Since then, scientists have embarked on a dozen experiments in phytoplankton deficit regions to determine how iron and other nutrients promote phytoplankton growth.

"In science, it can take a dozen experiments to understand the fundamental principles. We're just now beginning to understand how iron and other nutrients work in the oceans," says Wells.
 

To the untrained eye, a satellite image of water temperatures in the equatorial Pacific looks like abstract art. Computer enhancement can turn upwelling regions into bright spots where water rises to the surface and brings nutrients and carbon to phytoplankton. Darker areas show the reverse, downwelling regions where water sinks, its nutrient load depleted.

Below the surface, water swirls, and currents shift direction. At about 200 meters (660 feet) down, the prevailing flow has reversed and moves east toward South America. The result is constant turbulence, changing the location of nutrient-rich waters and making it hard to predict just how phytoplankton will respond day to day.

Last December onboard the R/V Revelle out of San Diego, Chai and colleagues from Maine, Hawaii, Oregon and other states studied nutrient levels and phytoplankton growth over a 2,600-mile course across the Pacific. In that vast area, they were looking for the upwelling regions. Their goal was to understand how quickly diatoms and other types of phytoplankton use up the nutrient supply, and how zooplankton graze on the plants, changing the way nutrients are taken up and recycled.

There aren't many direct uptake measurements of how fast diatoms will grow under ambient nutrient limitation conditions. In order to understand these limitations, you have to measure how phyto- plankton, particularly the diatoms, are doing physiologically, says Chai.

In addition to phytoplankton growth, scientists were interested in how iron concentrations change from day to day and from place to place. "This cruise is the first one to measure iron concentrations in the ocean at a large scale. In the past, you would have a few stations. In this one, because of large spatial area coverage, we can get an idea of how iron distribution responds to circulation change and atmospheric deposition," says Chai.

Scientists took water samples at 28 locations on the Equator and along a north-to-south track at 110 degrees west longitude. They put samples into tanks on the Revelle's deck, and monitored phytoplankton growth and nutrient uptake. From some samples, they removed the zooplankton and added iron and silicate to observe the effects on phytoplankton growth.

Chai's primary interest is computer modeling. Over the last decade, he has developed a leading model that simulates cycles of nutrients, including carbon, and phytoplankton dynamics in the equatorial Pacific. Each piece of a model is a mathematical equation. In Chai's case, equations reflect the latest knowledge of how different plankton species take up nutrients as they grow and release them when they die.

Being on the cruise helps scientists like Chai improve their models. "Modelers need to know how data are being collected. We are at a stage where (ocean) modeling can almost do a real-time simulation. Things are getting realistic because you have new data fed into your model with data simulation. Sometimes it's hard to separate (field) data from the model."

The R/V Revelle and Chai's colleagues returned to the equatorial Pacific this past September to repeat their cruise, this time from west to east. Financial support comes from the National Science Foundation and NASA.
 

Wells looks at phytoplankton through the lens of chemistry. When it comes to competition for iron, he sees evidence of a kind of chemical warfare among microorganisms, including phytoplankton, that may be occurring in large areas of the world's oceans. Something odd occurs, says Wells, after iron is added to the ocean. Diatoms and other types of phytoplankton grow but then begin to starve in the midst of plenty, acting as though iron is still in short supply.

Wells' recent focus on iron stems from American participation in a Japanese research program known as SEEDS (Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study), which began in 2001. The goal is to understand changes that occur in phytoplankton communities as a result of adding iron to North Pacific waters.

In July 2004, Wells served as chief scientist on the research ship Kilo Moana out of Honolulu. Joining him were two UMaine graduate students — Eric Roy and Lisa Pickell — and postdoctoral researcher Jennifer Boehme. (UMaine scientist Mary Jane Perry collaborates on the project.)

Also participating were scientists from the University of Western Ontario and San Francisco State University, as well as several members of the Japanese research team. The National Science Foundation and Department of Energy provide financial support.

The Americans' interest stems in part from the first SEEDS experiment in which Japanese scientists recorded the largest phytoplankton bloom of any in the iron fertilization tests. One of the unanswered questions is why diatoms showed signs of nutrient stress before the iron and other nutrients were used up.

Wells and his colleagues think they may know. Soil contains lots of iron, but most of it stays locked up in minerals, as accessible to microorganisms as the gold in Fort Knox. Bacteria and fungi have learned to scavenge what iron is available by building a trap; they create molecules called siderophores that are able to lock up iron. And in some cases, only the organism that built the molecule has the key to unlock it, says Wells.

"It's basically chemical warfare by the bacteria in soils, trying to get the iron. They specifically target iron with these molecules. In some cases, other bacteria have figured out ways to get the iron from molecules that they didn't produce, pirating that iron. It's beginning to look like the same thing may be happening in the ocean," Wells says.

By the time Wells and his colleagues arrived at their appointed location in the North Pacific, the Japanese team had injected iron into the water and was monitoring the growing, roughly 18-square-mile phytoplankton patch.
Operating independently, the two vessels stayed in the patch for 12 days.

The American team analyzed water chemistry, nutrients and microorganism diversity. Assisting their Japanese colleagues on board both vessels, Wells and the other scientists characterized how phytoplankton responded to iron enrichment. They ran experiments to learn how available the iron was in the patch, how diatoms were growing, the rate at which they were coming together in multicellular aggregations and sinking into the deep sea. Through this multistep process, some of the carbon taken up by phytoplankton can be removed from surface waters to be replaced by carbon dioxide from the atmosphere.

Early results suggest that the struggle for iron may indeed follow something like what happens in the soil, although Wells and his colleagues are still evaluating their data. Scientists are planning to return to the Pacific in 2007.

Iron is not a magic bullet for managing ocean ecosystems, Wells and Chai agree. Instead, it's becoming clear that iron works with other nutrients to affect phytoplankton in complex ways that scientists are just starting to unravel.

by Nick Houtman
November-December, 2005

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