Questions rise about seeding for ocean C02 sequestration

DOE Argonne National Laboratory: A new study on the feeding habits of ocean microbes calls into question the potential use of algal blooms to trap carbon dioxide and offset rising global levels. These blooms contain iron-eating microscopic phytoplankton that absorb C02 from the air through the process of photosynthesis and provide nutrients for marine life. But one type of phytoplankton, a diatom, is using more iron that it needs for photosynthesis and storing the extra in its silica skeletons and shells, according to an X-ray analysis of phytoplankton conducted at the U.S. Department of Energy’s Argonne National Laboratory. This reduces the amount of iron 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,” said Ellery Ingall, a professor at the Georgia Institute of Technology and co-lead author on this result.  “Everyone else in line gets nothing; the person’s decision affects these other people.”

Because of this iron-hogging behavior, the process of adding iron to surface water – called iron fertilization or iron seeding – may have only a short-lived environmental benefit. And, the process may actually reduce over the long-term how much C02 the ocean can trap.

Rather than feed the growth of extra plankton, triggering algal blooms, the iron fertilization may instead stimulate the gluttonous diatoms to take up even more iron to build larger shells. When the shells get large enough, they sink to the ocean floor, sequestering the iron and starving off the diatom’s plankton peers.

Over time, this reduction in the amount of iron in surface waters could trigger the growth of microbial populations that require less iron for nutrients, reducing the amount of phytoplankton blooms available to take in C02 and to feed marine life....

A phytoplankton bloom in Shark Bay off the West coast of Australia, NASA image