In the small, freshwater streams of South Dakota, a horrible menace is creeping along the rockbeds. Known as rock snot, it's a form of algae that clings to rocks and spreads rapidly until it takes over the entire ecosystem of the stream, often killing off local plants and microbes. The stuff has spread from North America all the way to New Zealand. But what's really weird is that rock snot can grow even in rivers that appear to have almost no nutrients in them at all. What is helping this algae grow so fast? Is it feeding on our fear?

A group of scientists wanted to find out. So they set about studying Didymosphenia geminata (its scientific name), also called Didymo, in one South Dakota waterway called Rapid Creek. What they discovered was that the algae could actually suck extra nutrients from water that appeared to be nutrient-free. And they do it by creating vast bacterial farms beneath the mats of rock snot. Essentially, the algae uses other life forms to create food for it.


In a release about the study from Geophysical Letters, we find out more:

Blooms of Didymo, also known as "rock snot," says scientist P.V. Sundareshwar of the South Dakota School of Mines and Technology, are made up of stalks that form thick mats on the beds of oligotrophic, or low-nutrient, streams and rivers. Sundareshwar is the paper's lead author.

"In recent decades, human activities have led to many uncommon environmental phenomena," he says. "Now we have Didymo."

The freshwater diatom has become notorious. Didymo has taken over low-nutrient rivers in North America and Europe. It has also invaded water bodies in the Southern Hemisphere, including those in New Zealand and Chile.

Didymo thrives in Rapid Creek through biogeochemical processes in biofilms in the mats. As Didymo mats form, new stalks develop at the surface and older stalks—which have already bound phosphorus—are displaced to the mats' inner regions.

Phosphorus is available to Didymo thanks to the activity of the bacteria that live inside these mats.
"This study solves the puzzle of how Didymo can produce such large blooms in low-nutrient rivers and streams," says Tim Kratz, program director in NSF's Division of Environmental Biology.

"It has uncovered the fascinating mechanism by which Didymo 'scrubs' phosphorus from a stream or river," says Kratz, "then creates a microenvironment that allows microbes to make this nutrient available for Didymo's growth."


In a world where plants create farms made of bacteria, anything can happen. No wonder these diatoms are so notorious!

Read the full scientific article in Geophysical Letters.