Assistant Professor of Biochemistry
Nils Kroger | Georgia Institute of Technology
Masters of Sunlight and Minerals
In a deep blue sea of tiny floating factories, diatoms are the stars. They spend their days manufacturing a fifth of the world's oxygen. In their spare time, they bioengineer their famous rock-hard shells (as in diatomaceous earth) out of sturdy little silica nanospheres.
Leading the applause for them is Nils Kröger of Georgia Tech. He's out to learn how this brownish-green, single-celled microscopic master of photosynthesis fabricates its sophisticated cell walls so quickly, using nothing but sunlight and a few minerals.
Applications? Well, diatom shells make excellent filters for beer, which Kröger, as a native of Marburg, Germany, could appreciate, and for swimming pools (diatom granules can filter at 3 to 4 microns, while a sand filter traps at 20 microns). More important may be making things like better sensors, flow-through reactors and devices that need an enzyme built into diatom walls as a catalyst.
From a decade of work, Kröger, who holds positions in the School of Materials Science and Engineering and the School of Chemistry and Biochemistry, knows about putting enzymes to work in diatoms, what he calls enzyme mobilization. "Enzymes are proteins, encoded by genes," he says, "so if you have the gene to encode for an enzyme, you can incorporate it into an organism." Another word for that work is biomineralization.
You could, for example, test blood for a toxic substance by running it through diatom granules where an enzyme has been put to work. "We need to find the limits of this approach," he says. "How can we improve the techniques?"
But enough about applications; Kröger says his work is mainly about basic biology.
For decades, the diatom's creativity has baffled the biochemists who study them as incredible factories creating what industry could match only with high-temperature and high-pressure devices, and far higher costs per unit.
Diatoms, the most prominent algae in oceans and freshwater with a biomass equal to all the world's rain forests, have fascinated biochemists for centuries, ever since the first microscopes were invented.
As a graduate student in Germany, Kröger came upon the diatom challenge. It was an unexplored research path -- no data to build on -- but he plunged in. To get started, he examined the nanoscale biology of how the diatom produces cell walls, in just an hour or so, one microstructure at a time.
"If humans did the same, with same precision, it would take them enormous time and money and equipment," he said. "Here, a single cell makes inorganic structures, relying on the information in protein, in their genes."
A crucial ingredient in their walls is silicic acid - for the element silicon. To make their silica nanosheres, they use what Kröger calls biosilicification. He was among the first to figure it out.
It was tricky at the start. He had to grind up their shells and inventing ways to clean the shells with strong acids while leaving the organic molecules behind. "I thought if I was lucky, some proteins involved in silica formation might be stuck to the silica," he said. As luck had it, he identified the coordinating role of protein molecules called silaffins, or polycationic peptides, that organize the diatoms' shell building. Silaffin was his word for the molecules' "silica affinity."
If you think that's enough big words to learn, imagine the diatom devising all this when the Earth was new, and when it needed walls fast, for protection from predators. Kröger has watched them in his lab begin to cook up nanospheres within seconds. Now that's nanotechnology.
Education Component
Nils Kröger plans to recruit students to science by smoothing the critical points where they get lost: coming out of high school and moving from college to Ph.D. work. He plans work with high teachers to create summer workshops on diatom biology and mineralization by proteins, hoping "to get kids fired up for research in general."
"Diatoms are so beautiful," he says. "I am amazed by looking at them. The students can get a handle on micro techniques for genetic engineering."
He will with community college students on a course on biologically inspired material synthesis, to attract them to chemistry and materials science. "Novel things are attractive for students," he says. "With the beautiful diatoms, we want all educational levels to be asking: How do they do that?"
