Study may explain extreme algae growth in streams where they should starve

Joint Release

WASHINGTON—A species of freshwater algae that lives in streams and rivers is able to colonize and dominate the bottoms of some of the world’s cleanest waterways-precisely because they are so clear, according to a new study.

The alga, called Didymo for Didymosphenia geminata, does so with a little help from its friends—in this case, bacteria—which allow it to make use of nutrients like phosphorus.

Blooms of Didymo, also known as “rock snot,” are made up of stalks that form thick mats on the beds of oligotrophic (low-nutrient) streams and rivers, says scientist P.V. Sundareshwar of the South Dakota School of Mines and Technology in Rapid City.  Sundareshwar is the lead author of the paper, published on 28 May inGeophysical Research Letters, a journal of the American Geophysical Union.

“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 oligotrophic rivers in North America and Europe.  It has also invaded water bodies in the Southern Hemisphere, including those in New Zealand and Chile.

Because its blooms alter food webs and have the potential to impact fisheries, “Didymo presents a threat to the ecosystem and economic health of these watercourses,” says Sundareshwar.

Algae blooms are usually linked with the input of nutrients that fuel the growth of microscopic aquatic plants.  Didymo’s ability to grow prolifically in waters where nutrients such as phosphorus are in short supply had puzzled scientists.

Environmental managers had tried to mitigate Didymo blooms and predict their spread.  But how the diatoms sustained such high growth in oligotrophic systems was unknown.

In the study, Sundareshwar and colleagues revealed that Didymo is able to concentrate phosphorus from the water.

The scientists conducted their research in Rapid Creek, an unpolluted mountain stream in western South Dakota where Didymo was first observed in 2002.  The creek regularly has Didymo blooms, with 30 to 100 percent of the streambed covered with Didymo over an area up to ten kilometers (6.2 miles) long.

Didymo thrives in Rapid Creek through biogeochemical processes in biofilms in the mats.  As Didymo mats develop, 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 the National Science Foundation’s (NSF) 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.”

The concentration of phosphorus on Didymo mats far exceeds the level that was expected based on the nutrient content of surface waters, says Sundareshwar. “The ability of the mats to store phosphorus is in turn tied to the availability of iron in the water.”

Didymo cells adsorb (collect on their surfaces) both iron and phosphorus.  Then bacterial processes in the mat interact with iron to increase the biological availability of phosphorus.

The process results in abundant phosphorus for cell division, “and hence,” says Sundareshwar, “a resolution to the paradox of Didymo blooms in oliogotrophic streams and rivers.”

The result may help managers identify water bodies susceptible to Didymo blooms, and develop management strategies.

“It also has the potential to lead to discoveries that may stem this organism’s prolific growth in rivers around in the world,” says Sundareshwar.

This study was funded by NSF and the State of South Dakota Carbon Scientist fund.