Testing the Sea's Mettle
Two UMaine scientists study the nutritional secrets of the world's
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
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
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
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
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
for more stories from this issue of UMaine Today Magazine.