Changing ocean chemistry caused by increasing concentrations of carbon dioxide may reduce the availability of dissolved iron to marine phytoplankton, as the kind shown here.NOAA

Rising acid levels in the world's oceans appear to be robbing the tiny animals that form the bedrock of the marine food web of a vital nutrient. This shift in the ocean's chemistry could reduce populations of phytoplankton, which could touch off a cascade of changes to ocean life.

Roughly one-third of the oceans contain phytoplankton that are limited in their growth by the amount of iron available to them. A study published today in Science, suggested that zone could grow.

"The concept of changes to ocean productivity and ecosystems due to acidification is a very important one to consider," said Ken Buesseler of Woods Hole Oceanographic Institution in Woods Hole, Mass., who was not a part of the study. "If half of the photosynthesis on the planet is in the ocean and if you reduce that because of acidification, that is a big deal."

Ocean acidification is a trickle-down effect of climate change. Higher levels of carbon dioxide in the atmosphere drive more CO2 to dissolve into the ocean, making it more acidic.

In photosynthesis, plants -- including phytoplankton -- convert CO2 from the atmosphere into their tissues, and produce the oxygen that we breathe. Other animals up the food chain feed off of this carbon that was pulled out of the atmosphere by the miniscule plants. A lack of iron appears to slow this process down, which could affect the food supply for other ocean life, and reduce the amount of heat-trapping carbon dioxide the ocean can soak up.

It's still unclear exactly how the new findings will play out in the complex mixture of organisms in the ocean, with many competing changes caused by acidification and warming.

For instance, most of the iron in the ocean comes from dust, and climate change may increase the amount of dust that settles over the ocean, supplying extra iron and counteracting the new findings, Buesseler noted. Or, other organisms that are not limited by iron may fill the voids left by iron-starved phytoplankton.

"We're just at the beginning of research on ocean acidification," Morel said. "This is the first study published of its kind that looks at the uptake of a critical nutrient."

Morel's team, including study lead author Dalin Shi, a graduate student a Princeton, looked at the iron intake of several types of phytoplankton under different levels of acidity, using both lab-made seawater and seawater samples from New Jersey and Bermuda.

Much of the ocean's iron is chemically bound to organic molecules, so phytoplankton can't get at it. Another portion of the iron is available to the organisms. Shi's experiments showed that as conditions become more acidic, the proportion of iron in the available form decreases, slowing the rate at which the phytoplankton can absorb it.

In future work, Morel said, the team plans to look at broader populations of phytoplankton and at water from more locations to understand better how iron starvation will play out ocean-wide.