Will ocean acidification erode the base of the food web?

Acidification of the world’s oceans is already damaging coral reefs and could produce other unexpected chemical and biological consequences. Princeton University researchers now report that at low pH, phytoplankton take up less iron, a key nutrient needed for photo-synthesis and growth. The results, reported in the 5 February 2010 issue of Science, suggest ocean acidification could have a profound impact on these tiny one-celled plants, which reside at the bottom of the food web and support commercially important fisheries.

Seawater becomes more acidic when atmospheric carbon dioxide (CO2) absorbed by the water is converted into carbonic acid. The acidity of oceans is changing very rapidly. The hydrogen ion concentration of surface ocean water (a reflection of pH) is now about 30% higher than it was 200 years ago, according to William Sunda, a research chemist at the National Oceanic and Atmospheric Association in Beaufort, North Carolina, while atmospheric concentrations of CO2 have risen by about 38%. Most of the research focus has been on how ocean acidification negatively impacts marine creatures, such as mollusks and corals, that form shells or exoskeletons from calcium carbonate [EHP 116:A292–A299 (2008)]. Little attention has been paid to how increasing acidity changes the chemistry and biological availability of essential nutrients such as iron.

In the current study, Dalin Shi, Francois M. M. Morel, and colleagues at Princeton University measured the uptake of iron in Thalassiosira weissflogii, Thalassiosira oceanica, Phaeodactylum tricornutum, and Emiliana huxleyi. As the researchers lowered the pH of model laboratory culture media from 8.6 to 7.7, they observed a significant decrease in the rate of iron uptake by all species. A similar trend occurred when laboratory phytoplankton were placed in natural seawater collected off the New Jersey coast and the open ocean near Bermuda. The average iron uptake rate decreased by 10–20% between the highest- and lowest-pH conditions in natural seawater. “The average pH of ocean water today is 8.08,” says Shi, a graduate student in oceanography.

Potera, C., 2010. Will ocean acidification erode the base of the food web? Environmental Health Perspectives 118(4):157-157. Article.

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