For Immediate Release
The future of these plants, called phytoplankton,
is important because they exist at the base of the marine food web and
represent a large source of food for fish. Also, they affect global climate
by using atmospheric carbon dioxide, a greenhouse gas.
Phytoplankton depend upon nitrogen and phosphorus to grow and, ultimately,
replenish the supply of these nutrients in the ocean. Since the 1930s,
scientists have known that the average nitrogen-to-phosphorus (N:P) nutrient
ratio of phytoplankton closely mirrors the N:P ratio in the ocean - 15:1
for the plants and 16:1 for the water. Scientists accepted this as a constant
called the Redfield ratio, named after the late Harvard University scientist
But researchers at the Georgia Institute of Technology and Princeton
University designed a mathematical model based on phytoplankton physiology.
It shows a broad range of N:P ratios are possible depending on the conditions
under which species grow and compete. This research - part of a larger
biocomplexity research project led by Professor
Simon A. Levin at Princeton -- is published in the May 13 edition
of the journal Nature.
"The take-home message is that this finding reinforces what some
researchers have been saying lately - that N:P is not so fixed,"
said lead author Christopher
Klausmeier, a Georgia Tech assistant professor of biology and former
postdoctoral fellow at Princeton. Other authors are Elena
Litchman, also of Georgia Tech, and Tanguy Daufresne and Levin of
"This shows the range of ratios within which we could expect the
ocean to change in the future," Klausmeier said. "Right now
we have 16:1, but 500 years from now, if we have a different mix of growth
conditions, then it might change the overall N:P needs of the phytoplankton
community and the ocean."
Under two extreme conditions - one with few resources because of increased
competition and the other with abundant nutrients - researchers determined
the optimal strategies that phytoplankton use to allocate the cellular
machinery - namely ribosomes and chloroplasts -- for nutrient uptake.
Ribosomes assemble two proteins that take up nitrogen and phosphorus.
Chloroplasts gather energy from the sun.
"When competing to the very end, then the optimal strategy has a
lot of resource acquisition machinery, but not much assembly machinery,"
Klausmeier explained. "In that case, there aren't many ribosomes,
and therefore not much phosphorus. So if you have a small amount of phosphorus,
you have a high N:P ratio. This strategy is best for competition to equilibrium.
"In the other scenario, where nutrients are very available, you
have a lot of ribosomes. Then you have a lot of phosphorus and therefore,
a low N:P ratio. This is optimal under exponential growth conditions,"
Given these optimal strategies, researchers were able to determine the
N:P needs of species competing at the extremes. "These two scenarios
set the endpoints of what happens in reality," Klausmeier explained.
"In the real world, it's a mix of conditions."
From a literature review earlier in the study, they found that N:P ratios
among different species vary from 7:1 to 43:1 - with one oddity requiring
a 133:1 ratio. Results from modeling the optimal strategies mirror this
range of ratios, Klausmeier said, in contrast with the long-accepted constant
ratio of N:P in the ocean.
"The 16:1 Redfield ratio has been used too dogmatically by some
scientists," Klausmeier said. "It has been treated as an optimum
ratio, but that's not what Redfield intended. He has been misunderstood
and oversimplified. This ratio is an average that is subject to change."
As is the case in many other ecological studies, researchers in this
study had to confront the natural variability found in nature.
"This is a very ecological story," Klausmeier noted. "One
thing that frustrates ecology and makes it tough is that there's a lot
of natural variability. We want to explain the variability, not just the
average number. So this problem turned out to be more complicated because
of the variability."
Klausmeier's findings have broader implications, as well, because of
the roles phytoplankton play in the ocean ecosystem and across the globe.
"Phytoplankton do half of the planet's primary production,"
Klausmeier explained. "They capture energy from the sun and have
a big role in biogeochemical cycles -- how elements cycle through the
biosphere. Phytoplankton have a main role in the carbon cycle. They need
carbon dioxide to grow, so they suck it out of the atmosphere, controlling
its presence there. And that ties into global climate."
Klausmeier believes his study contributes to a better understanding of
global biogeochemical cycles. "It's important for us to understand
global climate and how it might change in the future," he added.
"And ocean life, such as phytoplankton, is a big player in climate."
This study was funded by grants from the National Science Foundation and the Andrew Mellon Foundation for Levin's biocomplexity project. Biocomplexity refers to studies of ecological and evolutionary systems as a whole.
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