Inga Lokey holds diatom mats in South Boulder Creek.
Inga Lokey holds diatom mats in South Boulder Creek. ( Sarah A. Spaulding US Geological )

Scientists have long been puzzled by the ability of a slimy algae known as "rock snot" to bloom (and bloom and bloom) in clear, low-nutrient streams like Boulder Creek, leaving thick, slick mats on the stream beds.

Now, a team of scientists, including a Boulder researcher, has discovered a mechanism that allows rock snot -- formally known as Didymosphenia geminata -- to thrive in areas where similar kinds of algae only grow modestly. Didymosphenia geminata, often shortened to Didymo, is part of a family of single-celled algae called diatoms.

"Diatoms pretty much grow everywhere, whether it's really clean water or waters that are relatively impacted," said Sarah Spaulding, a researcher at the University of Colorado's Institute of Arctic and Alpine Research. "Generally, the more nutrients you get in the water, the more biomass you get.

"What's different about this diatom is that we found it would make a large amount of biomass where there weren't many nutrients in the water."

This has allowed rock snot, an invasive species in North America, to explode in streams that have not historically supported large algae blooms. In a study published in the journal Geophysical Research Letters, the scientists explain how Didymo does it.

The key is Didymo's stalks, which keep the algae attached to the rocks. The stalks are able to collect the limited phosphorous nutrients available in the stream on their surfaces along with iron. Then, bacteria that live in the rock snot mats interact with the accumulated iron to make the phosphorous easier for the algae to use.


"Even though the phosphorous is low in concentration, the stalk strips it right out of the water and concentrates it right on the stalk," said Spaulding, who also works as an ecologist for the U.S. Geological Survey. "We found that the stalk also strips iron out of the water, which is crucial to part of the process."

The study may help land managers identify which streams may be susceptible to rock snot invasions.

"It also has the potential to lead to discoveries that may stem this organism's prolific growth in rivers around in the world," said P.V. Sundareshwar, lead author of the study and a researcher at the South Dakota School of Mines and Technology.
