Acidifying ocean may stifle phytoplankton

Chemical changes in seawater make a key nutrient less available to these organisms

Depending on nature — or at least phytoplankton in the ocean — to absorb humans’ ever-increasing emissions of planet-warming carbon dioxide probably won’t solve the problem, new research suggests.

A few scientists have long argued that increasing CO2 levels will stimulate long-term carbon-sopping plant growth, but the idea hasn’t proven true for land plants (SN: 12/16/00, p. 396). Now, new findings suggest that the notion won’t hold for tiny ocean plants either, thanks to one of the nagging side effects of carbon dioxide emissions — the gradual acidification of the ocean’s surface waters. Research by oceanographer Dalin Shi and his colleagues at Princeton University hints that rising CO2, instead of providing extra nutrients for phytoplankton, may actually curb the growth of these organisms, which form the base of the ocean’s food chain. The team reports these findings online January 14 and in an upcoming Science.

In their tests, the researchers studied how acidification, a decline in ocean pH, affects the ability of phytoplankton to take up dissolved iron, another nutrient required for growth. The scientists measured growth rates of four species of the marine microorganisms — including two that Shi described as “the lab rats of phytoplankton” — in ocean water with pH values that ranged from 8.8 to 7.7. On average, the pH of ocean surface waters today is about 8.08, says Shi.

Across large swaths of the ocean, phytoplankton are already starved for iron, Shi says. And the team’s research suggests that acidification will make things worse: If ocean pH drops by about 0.3 units over the next century — the acidification expected if CO2 emission trends continue — iron uptake by phytoplankton could drop by between 10 and 20 percent, the data suggest. Ironically, even though more-acidic waters are able to hold increased amounts of dissolved iron, a larger percentage of that nutrient would be chemically bound to organic matter dissolved in the water and therefore unavailable to nourish phytoplankton, Shi says.

Shi and his colleagues “did a really good job of looking at how organic material will bind up the iron,” says Ken Bruland, a chemical oceanographer at the University of California, Santa Cruz. In certain parts of the ocean, especially the biologically productive yet iron-limited seas surrounding Antarctica, the phytoplankton-stifling effect could be substantial, he notes. Some studies have shown a decline in CO2 uptake by those seas in recent decades (SN: 5/26/07, p. 333).

The new research does a good job of considering the effects of ocean acidification on today’s phytoplankton but doesn’t necessarily reflect how all phytoplankton will respond in the long term, says Paul J. Harrison, a biological oceanographer at the Hong Kong University of Science & Technology. As the oceans gradually acidify, some species might be able to adapt to absorb iron even more efficiently. If not, he notes, those species may be replaced by more-efficient ones that are present only in small numbers today but could proliferate under more-acidic conditions.

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