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What Good is a Diatom

Photo by Howard Lynk.

Exhibition Mount of Arranged Diatoms. Imaged using Darkfield lighting techniques by Howard Lynk, Victorian Microscope Slides. www.victorianmicroscopeslides.com

Breweries, cough syrup, paint, plastic, insecticides, toothpaste, polishes, ... concrete, animal feed, fertilizers, ... dynamite. It is astounding where the remains of diatoms can be found. One might say that many of the wheels making modern life go 'round are studded with the vacant shells of these tiny phytoplankton, which are used as filters, fillers, insulation, and mild abrasives.

Diatom shells (more correctly called frustules) are composed of silica, the essence of glass. Because of this their beauty whipped Victorian era microscopists into giddy covetousness. But most frustules escaped the diatom connoisseurs' slides by sinking to the ocean or lake floor after the diatom died.

Single frustules, typically smaller than the period at the end of this sentence, have mounted up bit by itty bit over the past 200 million years to form large deposits of diatomaceous earth in some places. Diatomaceous earth is mined from areas for industrial purposes and paleoecological studies designed to help scientists construct a picture of what the ecosystems of yesteryear might have looked like. Lake Superior is too youthful to have diatomaceous earth deposits, but it does have sediment. And the diatom frustules in that sediment have a story to tell.

"With the frustules we've sorted and counted, we can show how human decisions influenced water quality in the western arm of Lake Superior," said Amy Kireta, Research Fellow with the University of Minnesota Duluth's Natural Resource Research Institute (NRRI). "Water quality degraded with initial settlement and
watershed development but improved as these stressors lessened. Examining
sediment cores collected from the bottom of Lake Superior, we can look at the past 200 years and say, 'Wow, significant changes on land have, in turn, damaged and improved water quality.'"

The changes Kireta and her colleagues have detected in the diatom community echo the growth of the cities of Duluth and Superior and alterations on the land within the Lake Superior watershed, like those produced through logging and mining. Kireta notes that changes in water quality are now being affected to a greater extent by atmospheric deposition and other factors, such as climate shifts.

One of the unique qualities of Kireta and the research team is their commitment to sharing the research process and the results of the project with others. As the funding for this particular inquiry into 200 years of sediment winds down, she has been working with the Great Lakes Aquarium on a diatom research display and with the University of Minnesota's Visualization Laboratory to create a YouTube video about the project and process.

Graphic by Amy Kireta.

The nature of human activities in the Western Lake Superior watershed and the relative abundance of common historical diatom species have changed since 1850.

"For many visitors, this will be the first time they have heard about or seen diatoms," said Sarah Erickson, Director of Education at the Great Lakes Aquarium. "Peering through a microscope can be a transformative experience that illuminates the majesty and complexity of aquatic ecosystems. In this case, pairing that experience with a research story that speaks to our influence on Lake Superior will surely spark an interesting conversation. We're really looking forward to hearing what the public has to say about Lake Superior diatoms and this research once our new display is in place this winter."

Diatoms supply the oxygen in every fourth breath you take. Diatoms are critical in the ecological food chain of streams, lakes and oceans. It's thought diatoms could help slow Earth's current warming trend; scientists have suggested that seeding the ocean with iron to stimulate diatom reproduction could help remove carbon dioxide from circulation. They also inspire artists, designers, and Euan Reavie, Senior Research Associate with NRRI. Reavie and Kireta are co-principal investigators on Sea Grant's investigation into the extent to which Lake Superior's sediments can show how humans altered Lake Superior over time. Reavie said, "Diatoms are the most powerful tool in our paleolimnological arsenal to tell us about the past."

What good is a diatom in Lake Superior? See for yourself. Visit the Great Lakes Aquarium in Duluth, Minnesota; let Dr. Euan Reavie show you how he works with diatoms on YouTube at: www.youtube.com/mnseagrant; read "What have diatoms revealed about the ecological history of Lake Superior?," an article in Aquatic Ecosystem Health and Management 14 (4), 2011 by Reavie and Allinger, available at www.ingentaconnect.com/content/tandf/uaeh/2011/00000014/00000004/art00008.

Reproductive head-scratcher: How does a thing that lives in a veritable glass box reproduce?

The main way diatoms beget more diatoms is through a "shrinking division" mode of asexual reproduction. Diatom frustules are constructed like a box; one half is lodged within the other. After the cell divides, the frustule bursts open and a new half shell is formed to fit WITHIN the original shell. This means that one of the two new diatoms is the same size as the parent while the other one is slightly smaller. It could also mean that diatoms are doomed to become tinier and tinier until they vanish EXCEPT for the fact the small diatoms escape oblivion through sex. Sexual reproduction allows for the growth of a large zygote (in the diatom world this is called an auxospore) that splits the parent diatom in half as it continues to grow. Eventually the new diatom secretes its own frustule. The details of how sexual reproduction occurs depends on whether the frustule is round or if it is elongated. Round diatoms become male or female; long ones produce gametes that look similar.

Graphic by Russell Habermann.

By Sharon Moen
January 2013

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