The Carlisle Mosquito Online

Friday, November 30, 2007


Biodiversity Corner: Stauroneis

On November 4, I was fortunate enough to spend the day in a workshop at the Harvard

Stauroneis are seen in this photo taken by a scanning electronic microscope. Four Stauroneis are among the other diatom fragments. (Photo by Karen Ponader)

Herbarium learning about diatoms. Workshop participants were invited to bring in their own material to examine. I collected samples of water at three locations in Great Brook Farm State Park near the North Road canoe launch and one sample from the Conant Land.

In the workshop, we spent most of the time looking at prepared slides of named species of diatoms from the herbarium collection. Diatoms are microscopic single-celled algae. I became rather excited when I recognized some of these same forms in the pond water I had brought from Carlisle, and I felt quite proud of myself for having collected them. I did not know how common they are. Robert Edgar, our workshop leader and professional diatomist, put things in perspective for me with, "It's actually quite difficult to not collect diatoms."

Name. One of the diatoms from Carlisle that took my fancy was from the genus Stauroneis. The name comes from the Greek word "stauros" meaning stake or pole and has come to be a term for cross. Diatoms in this genus have a very distinctive cross-shaped mark on their cell walls. The word diatom comes from the Greek "diatomus" meaning cut in half. This is a good descriptive name since each diatom has two symmetrical parts (called valves) which make up the cell wall. Think of a Petri dish where the dish itself is one valve, and the cover is the other.

What is a diatom? Diatoms are a very successful life form living in both fresh and marine
This Stauroneis was photographed through a microscope lens. It is about 200 microns long. (Photo by Kay Fairweather)
water where they may float as plankton, form a layer on top of the sediment, or cling to other surfaces like water plants, shellfish, and rocks. They can also be found in damp habitats on soil and on moisture-loving plants like mosses — basically anywhere there is water and sunlight. They are so abundant that it is estimated they are responsible for 25% to 35% of the planet's carbon fixing. The two-part cell wall is made of silica — you could say that diatoms live in glass houses. The silica is laid down in the cell wall in intricate patterns which can be used to identify the different species of which there are over 50,000. Silica does not break down easily and so diatoms in core samples from lake and ocean beds can be used to construct part of the ecological history of the habitat.

Distinguishing characteristics. Before you can identify a diatom you need to know if your view is one of the valve surface or if it's a side view of the girdle bands connecting the two valves. The cross in Stauroneis shows up in the valve view. The valves of this species are more or less leaf-shaped. Despite being photosynthetic, diatoms do not have the characteristic green color of chlorophyll. Their chloroplasts are golden brown and live diatoms take on this color.

Close encounters. Most of us have encountered diatoms in the form of diatomaceous earth, also called diatomite, which is the chalky white or gray fossilized remains of diatoms. It is very finely porous and is used as a filter in beer- and wine-making and in swimming pool filters. Its absorbent properties make it useful as a pesticide, as an agent for toxic waste cleanup and as an additive in cat litter; it is abrasive and is used in toothpaste and facial scrubs; and perhaps its most famous use is that of Alfred Nobel, who used it to stabilize nitroglycerin and create dynamite.

Sources. Robert Edgar, professional diatomist at the Farlow Herbarium, Common Freshwater Diatoms of Britain and Ireland, Environment Agency, Bristol, England at

Submissions for the Biodiversity Corner are encouraged from everyone. What are you finding? Send a message to Kay Fairweather at

2007 The Carlisle Mosquito