May 13, 2009...6:55 am
Science of the Seasons: Tiny particles have big role in stream ecology.
By Dr. David Wartinbee, for the Redoubt Reporter
Have you ever stepped into a stream and had your foot slip on a flat rock? Maybe you really slipped, lost your balance and took a bath you hadn’t planned? If you spend enough time around streams, you probably will have this experience, or perhaps your own special version of embarrassment. The cause of these slippery surfaces is a bunch of microscopic, photosynthetic organisms called diatoms.
Diatoms are made of two paired frustules — their external cell wall — or “tests” of silicon dioxide (also known as glass) that fit together much like the petri dishes we use in microbiology. These miniscule frustules may only be 20 to 200 microns in length (one micron equals one millionth of a meter), and often have intricate, consistent openings or striations on the surface. When they reproduce, one new cell gets the outer frustule and the other cell gets the inner frustule. Each new cell then replaces the specific-size frustule they are missing by extracting dissolved silicon from the water. This way, the diatom cell size stays the same.
These are minute photo-synthetic organisms. They capture energy from light and combine it with carbon dioxide and hydrogen from water to form energy-rich carbohydrates or sugars. As part of this photosynthetic chemical reaction, diatoms split water to get the needed hydrogen, and they then release oxygen into the water. The carbohydrates produced by diatoms make them an important food source for many aquatic invertebrates.
A large number of diatoms are found floating near the surface of ponds, lakes and oceans so they can absorb the light they need for photosynthesis. Other members of this large, diverse group of algae will attach themselves to substrate surfaces, like rocks or stream and lakeside vegetation. Sometimes you can see a fuzzylike layer on the surface of aquatic vegetation, which is most likely a mass of diatoms. Those attaching to various substrates use gelatinous layers and stalks to hold themselves in place. It is these gelatinous coverings that make rocks so slippery in the stream.
Diatoms are an integral part of any stream or lake food web. They are a favored food item for a great many aquatic insects and other aquatic invertebrates. Once the cell components are digested, the silicon frustules are eliminated. The diatoms continue to reproduce on stream rocks as insects continually graze on them. One can think of it kind of like a miniature pasture with insects taking the place of grazing horses and cows.
In various experiments, stream ecologists have found that if the insects are prevented from feeding on the diatoms, the community becomes thick and overgrown, kind of what the pasture looks like when we fence out cattle.
In northern latitude stream systems, like those streams above the Brooks Range, diatoms can be the major source of energy for the entire stream. This occurs because there is very little riparian vegetation and only limited amounts of in-stream vegetation available as possible invertebrate food.
The frustules that pass through the insect gut will mostly remain intact and may build up in the bottom sediments of streams, lakes or ponds. In graduate courses in stream ecology that I used to teach, we would extract the guts of a variety of insects and examine the contents with a scanning electron microscope. We would photograph the diatom contents and could then compare the preferred diets of different aquatic insects.
By looking at the bottom sediments of a pond or lake, ecologists can determine which diatoms were living there in the past.
By taking layered core samples of lake bottoms, scientists can determine changes in the populations of diatoms over a time period. Those changes in diatom communities can then be used to infer climatic and community conditions in the past.
In some areas of the oceans, or areas of ancient oceans, there are so many frustules in the sediments that they are mined for industrial uses. Because of their very small size and the porosity of the material, the silicon frustules can be used as micro-filters. Many swimming pool filters or home fish-tank filters use “diatomaceous earth” as the primary material the water is passed through.
Diatomaceous earth has also been used as an insecticide because the tiny fragments of frustules (remember, they are made of glass) stick to the insect and abrade the exoskeleton as the insect moves. Eventually, the exoskeleton springs a leak, the insect dehydrates and dies.
While diminutive in size, diatoms are key players in virtually every aquatic environment. They can be a major producer of oxygen, a big-time consumer of carbon dioxide, a critical food source for many invertebrates and the reason many of us have lost our footing while wading in a stream.
David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River watershed.