Greedy Diatoms Refuse to Share, Put Planet at Risk

Algal blooms may not be as effective as once thought in trapping carbon dioxide, according to a new study on the feeding habits of ocean microbes.

Published in the journal Nature Communications, the report reveals that while these blooms contain iron-eating microscopic phytoplankton that absorb the known greenhouse gas through photosynthesis, one type, called a diatom, is far greedier than the rest. Using more iron than it needs for the process, it stores the extra in its silica skeletons and shells, which then reduces the amount left over to support the carbon-eating plankton.

“Just like someone walking through a buffet line who takes the last two pieces of cake, even though they know they’ll only eat one, they’re hogging the food,” Ellery Ingall, a professor at Georgia’s Institute of Technology and co-lead author, said in a press release. “Everyone else in line gets nothing; the person’s decision affects these other people.”

Such iron-hogging behavior, the researchers hypothesize, may mean that the process of adding iron to surface water, called iron fertilization or iron seeding, may only have a short-lived environmental benefit by reducing the amount of carbon dioxide the ocean can trap.

Thus, rather than feed the growth of extra plankton that then trigger algal blooms, iron fertilization may instead stimulate the gluttonous diatoms to absorb even more iron to build ever-larger shells that, once they reach a certain size, simply drop to the ocean floor while its peers are left to starve.

Over time, this reduction in the amount of iron in surface waters could result in the growth of microbial populations requiring less iron for nutrients, reducing the amount of phytoplankton blooms available to take in carbon dioxide and serve in its other crucial role as food for marine life.

In order to discover just how much iron was getting rapped in those sinking skeletons, the scientists relied on X-ray studies at the Advanced Photon Source at the U.S. Department of Energy’s Argonne National Laboratory.

“Being able to use X-rays and see the element content of individual microscopic phytoplankton has completely altered our perspective on how these organisms use iron and how that could affect CO2 levels,” Ingall said.

In all, Ingall and his team estimate that 2.5 milligrams of iron are removed from every square meter of surface water in the Ross Sea annually and sequestered in silica skeletons on the ocean floor, which roughly equals the amount of iron deposited every year into the body of water through snow melt, dust and upwelling of seawater.

This same process may be occurring in the Southern Ocean where it very well may be having a greater impact as the region dictates the nutrient mix for the rest of the world’s oceans through migratory current patterns, the researchers warn.

“This gap in our knowledge, combined with renewed interest in iron fertilization as an approach to the current climate crisis, makes it crucial that we have an improved understanding of iron cycling in marine life,” Ingall said.

The study was supported by the National Science Foundation and the Swedish Antarctic Research Program.