Iron Oxides Easily Reduce To Bioavailable Form When Trapped In Ice

Inside clouds or on ice sheets in Antarctica, iron oxide dust particles embedded in ice crystals may reduce quickly into a form needed by phytoplankton, a new study reports. This bioavailable iron fuels algal photosynthesis, which pulls carbon dioxide from the atmosphere into the oceans, an important process in regulating climate.

Phytoplankton use iron in the active sites of their photosynthetic and nitrogen-fixing proteins. Phytoplankton can't access iron oxides in the insoluble Fe(III) oxidation state, but they can grab the more soluble Fe(II) oxidation state.

Most of the oceans' iron starts as mineral dust blown from deserts. This dust mostly contains Fe(III), which can transform to Fe(II) when a photon-excited electron reduces it. The electron comes either from an iron oxide particle's conduction band or from an organic acid ligand.

After working at the Korean Polar Research Institute's Dasan station in Ny-Alesund, Norway, environmental photochemist Wonyong Choi, of Pohang University of Science and Technology, in South Korea, hypothesized that ice might influence this process.

"In polar regions, the sun constantly irradiates the surface for six months and most of the ground is covered in ice and snow," he says. "So I wondered what kind of unique chemistry might happen in that environment."

In his laboratory in South Korea, Choi and his co-workers compared the photoreduction of iron oxides trapped in ice to that of particles suspended in liquid water. They irradiated both samples with ultraviolet light for 48 hours in the presence of formic acid, an organic acid common in cloud water droplets. The researchers then observed greater than 10 times more Fe(II) ions in the ice than in liquid water. In a similar comparison using natural sunlight outside the Ny-Alesund station, they found that ice enhanced photoreduction about five-fold over liquid water (Environ. Sci. Technol. DOI: 10.1021/es9037808).

Photoreduction may occur more quickly in ice, the scientists propose, because of a concentration effect.  As water freezes, it pushes iron oxide particles and organic acids out of the ordered ice lattice and concentrates them into narrow channels between ice crystals called grain boundary regions. Even at freezing temperatures, these regions remain liquid-like. By bringing iron oxide particles closer to electron donors, such as organic acids or other iron oxide particles, Choi says, ice makes charge transfer more efficient.

Ice's role in iron photoreduction "certainly isn't anything anyone else talked about before," says William G. Sunda, a  marine biogeochemist at the National Oceanic and Atmospheric Administration's Beaufort Laboratories, in North Carolina. Because phytoplankton depend on bioavailable iron to absorb carbon dioxide from the atmosphere, understanding how ice affects photoreduction could help scientists model the influence of algal photosynthesis on climate change, especially in the Southern Ocean where iron is limiting.

Also, Sunda says, ice loss in polar regions caused by warmer global temperatures could lead to less iron present in its bioavailable form. That shrinkage, in turn, could slow phytoplankton growth and disrupt climate regulation. But Choi warns that he and his colleagues have yet to confirm that this laboratory mechanism has importance in the environment. Until then, he hesitates to speculate about any implications for climate change.