OBJECT OF THE MONTH - February, 2009

"Rock Snot" (Didymosphenia sp.)

Three large Didymosphenia take up the majority of this picture, but you can also see several smaller diatom species clinging to the Didymo stalks. Didymosphenia is commonly called "rock snot."

Diatoms are an extremely abundant type of unicellular photosynthetic algae that inhabit all aquatic, and even some terrestrial, habitats. Fossil records show that diatoms have been very common and widespread since the Jurassic, about 150 million years ago.
Like plants, diatoms have chloroplasts and chlorophyll for harvesting energy from the sun. Unlike plants, or any other organism, diatoms have a cell wall made of silica. Under a microscope, this special cell wall looks much like an ornate glass case.
Within an aquatic habitat such as a lake, diatoms fill virtually every ecological niche and have a huge impact on the ecosystem itself. Many diatoms live attached to rocks or to other algae, float in the open water column, or move within sediments. No matter their physical location within an ecosystem, diatoms form the vast majority of the foundation of the aquatic food web, affecting many other aquatic and even some terrestrial organisms.
Three different images of Didymosphenia. Single cell at the end of a stalk. Notice the small epiphytes (diatoms which grow on plants) near the base of the cell (left). Two cells with detail of stalk and especially attachment to the base of the cells (center). Low magnification view of the algal community associated with Didymo (right).
Despite the vast diversity within diatoms (estimates of 60,000 to 100,000 extant species), very few people outside of the study of them could name any.
However, there is one diatom in particular that has garnered local, national, and international attention: Didymosphenia! Unfortunately, much of its press focuses on Didymosphenia's ability to "over-colonize" streams and become a nuisance.
A single Didymosphenia.
Despite a less than pristine reputation, there is another side to Didymosphenia's story. "Didymo" grows on mucilaginous stalks in order to climb above the other algae on the river bottom to compete for light and other resources. These long, slimy, branched stalks have earned Didymo the nickname "Rock Snot". However, these stalks provide habitat for epiphytic diatoms (diatoms that grow on plants) as well as habitat for other aquatic life such as small invertebrates.
Despite its large overall size, Didymosphenia has a very delicate arrangement of puncta (openings in the cell wall) that are less than 0.5 microns, as shown in this scanning electron micrograph.
Didymo cells can approach 200 micrometers in length, or about twice the width of a human hair! This makes it one of the larger diatoms in the University of Colorado Museum of Natural History's collections.
Please enjoy these photographs of Didymosphenia, a beautiful diatom, despite its tarnished image! To learn more about diatoms and other algae, visit the new BioLounge exhibit on the lower level of the Museum.