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You Are Not the Only One Who Needs Iron

Photonics Spectra
May 2007
Hank Hogan

It is easy to look out over the ocean and think that nothing could be less like a desert, yet the two share some things in common. For one, life may struggle in either region whenever a vital ingredient becomes scarce: In the desert, it is water; in the ocean, especially near Antarctica, it is iron.

However, there is another link, noted Constant M.G. van den Berg, a professor in the earth and ocean sciences department at the University of Liverpool in the UK. That connection lies in the way in which iron ends up in the water. Some of it reaches the ocean after starting out in the desert. “The source of iron is mostly atmospheric dust,” he said.

No matter where the iron comes from, though, van den Berg and fellow researcher Luis M. Laglera would like to know what happens to it. Photonics plays a key role in uncovering that information. For example, the scientists used chemiluminescence, the emission of light from a chemical reaction, to measure the effect of sunlight on iron concentrations in seawater.

Previous work had indicated that ferrous iron [Fe(II)] should be stable for hours when produced by the exposure of ferric iron [Fe(III)] to sunlight. That is important because Fe(II) is more soluble in water and presumably more available to microorganisms than Fe(III). However, this team’s research showed that photochemically produced Fe(II) was reoxidized to Fe(III) with a half-life of less than one minute. “That was a big surprise,” van den Berg said.

In making their measurements, the researchers traveled to the Sverdrup Research Station at Ny-Alesund, located on the Arctic island of Svalbard, where they made use of the water and natural sunlight, eliminating the need for simulations of either.

First they collected seawater and stored it overnight in the dark to deplete any Fe(II) present. They then pumped it past an Fe (II) concentration chemiluminescence-based detector from Waterville Analytical LLC of Maine, carefully controlling the speed of the flowing water.

Others at the polar station measured the total solar irradiance using a radiometer from the Eppley Laboratory Inc. of Newport, R.I. The Liverpool researchers used a spectrophotometer from Ocean Optics Inc. of Dunedin, Fla., to measure the spectra on land and 5 m below the ocean surface. In evaluating the effect of sunlight, they either allowed it all through down to the 300-nm wavelength, or they used filters to block light below 315, 354 and 362 nm. They measured the resulting Fe(II) concentration using chemiluminescence.

They found that 35 percent of the photochemically produced Fe (II) resulted from light at wavelengths below 315 nm, 30 percent from light below 360 nm and 35 percent from the rest. The study indicated that the photochemical reaction produces two transient species: an initial Fe(II) that was quickly reoxidized into an inorganic Fe(III). Van den Berg noted that both forms might be available to marine microorganisms.

He said that more seawater testing is planned, including looks at the reduction/oxidation cycle of iron in other regions. “The ultimate goal is to better understand the biogeochemistry of iron to marine and other microorganisms. This is another cog in the wheel of reactions, so to speak.”

Environmental Science and Technology, April 1, 2007, pp. 2296-2302.


GLOSSARY
chemiluminescence
A chemical reaction involving the production of light. The reaction of ethylene with ozone is chemiluminescent.
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