Greedy Algae May Thwart Ocean Fertilization Efforts

Another study shows iron fertilization may not be a great solution to climate change.
This photo shows the ship on which researchers traveled while collecting plankton samples in Antarctica. Layers of brown ice, like those in this photo, often contain diatoms. Image courtesy Georgia Institute of Technology

One idea for removing excess carbon dioxide from the Earth’s atmosphere may not work as well as advertised, according to a new study. The problem arises from extra-greedy diatoms, a type of algae with structured silica bodies.

In the past few years, some researchers and enthusiasts have proposed dumping iron into the ocean as a strategy for mitigating climate change. Last fall, a California businessman even did a little of his own ocean fertilization, drawing condemnation from legal experts. The idea is that the iron acts as a fertilizer, encouraging the growth of photosynthetic plankton that, like land plants, absorb carbon dioxide. When the plankton die, they sink to the seafloor. In that way, they’re supposed to sequester away excess carbon forever.

What would actually happen if you fertilized the ocean isn’t well studied, but one new piece of research suggests that the iron’s effects would be short-lived. In the long run, iron fertilization may even decrease the amount of carbon dioxide-absorbing algae that live in the ocean. Recently, another research team also found that fertilizing the ocean may not work, as the plankton could run out of nitrogen.

For the newer study, oceanographers from several U.S. institutions studied phytoplankton off the coast of West Antarctica. There, they found, diatoms take iron from the ocean and put it in their shells at a high rate. They even seem to take up more than they need. “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, an earth scientist at the Georgia Institute of Technology who went to collect the phytoplankton, said in a statement.

Iron that enters the Antarctic Ocean via snowmelt and dust can barely keep up with the diatoms’ appetite, Ingall and his colleagues wrote in a study they published on Monday in the journal Nature Communications.

This could mean that if someone were to dump iron into the ocean, much of it would be taken up by diatoms. That might be fine at first. Diatoms are photosynthetic, so they absorb carbon dioxide. When they die and fall to the bottom of the ocean, however, they take the iron they ate with them, trapped in their silica shells.

After an initial bloom from iron fertilization, diatoms may leave other plankton types with less iron, reducing the size of carbon dioxide-absorbing plankton blooms, according to Argonne National Laboratory. Argonne scientists worked on this study by analyzing diatom silica structures for their iron content.

Studies like this help scientists understand the consequences of ocean fertilization without having to actually dump anything into the water, which is controversial among conservationists and may violate international law.

[Argonne National Laboratories, Georgia Tech]