Model shows long-held constant in ocean nutrient ratio may vary as ecological conditions change

New research shows that what was once considered a universal constant in oceanography could actually vary in the future – depending on the ecological scenarios that affect competition for resources among microscopic marine plants, which play a role in global climate.

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 Alfred Redfield.

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 Princeton.

“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,” Klausmeier added.

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.

Georgia Tech Research News and Research Horizons magazine, along with high-resolution JPEG images, can be found on the Web at http://www.gtresearchnews.gatech.edu.

Media Contacts:
1. Jane M. Sanders, 404-894-2214, or E-mail: jane.sanders@edi.gatech.edu.
2. John Toon, 404-894-6986, or E-mail: john.toon@edi.gatech.edu.

For technical information, contact:
1. Christopher Klausmeier, Georgia Tech, 404-385-4241 or E-mail: Christopher.klausmeier@biology.gatech.edu.
2. Simon A. Levin, Princeton University, 609-258-6880 or E-mail: slevin@eno.princeton.edu.