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June 14th, 2011

Stream paradox solved? Study uncovers why alga feasts where it should starve

In recent decades, the diatom Didymosphenia geminata has emerged as a nuisance species in river systems around the world. Often known as didymo, the single-celled alga is able to colonize and dominate the bottoms of some of the world’s cleanest waterways, forming thick mats.

Didymosphenia geminata in Rio Espolon, Patagonia, Chile. Extensive blooms of didymo were first reported in South America 2010 in Rio Espolon and Rio Futaleufu by river guides. Sarah Spaulding was invited by the Chilean institute Centro de investigacion en ecosistemas de la Patagonia (CIEP) to investigate the blooms. Photo: Sarah Spaulding, July 2010.

The freshwater alga has become notorious. Didymo has taken over low-nutrient rivers in North America and Europe and invaded water bodies in the Southern Hemisphere, including those in New Zealand and Chile. Its blooms alter food webs and have the potential to impact fisheries.

Didymo presented scientists with a puzzle. Algal blooms are usually linked to inputs of nutrients that fuel the growth of the microscopic aquatic organisms. But didymo grows most prolifically in unpolluted streams and rivers with low levels of nutrients like phosphorus, which typically limits growth.

A team of scientists, including Sarah Spaulding (INSTAAR and U.S. Geological Survey) and researchers from South Dakota School of Mines and Technology and South Dakota State University, examined how high algal biomass is formed in low-nutrient conditions. The researchers published their study in the journal Geophysical Research Letters last week.

Sarah Spaulding (INSTAAR and USGS) surveyed the growth of stalked diatoms, including didymo, in the Snake River from Jackson Lake to Moose, Wyoming. The survey, to determine the extent of blooms of stalked diatoms in Grand Teton National Park and appropriate management response, was funded by the National Park Service. Photo: Sarah Rushforth, August 2008.

The scientists conducted their study in Rapid Creek, a mountain stream in western South Dakota where didymo was first observed in 2002. The creek often experiences didymo blooms, with 30 to 100 percent of the streambed covered with didymo over an area up to six miles long.

They discovered that didymo uses a biogeochemical process to scrub phosphorus from flowing waters. It does so with a little help from its friends--in this case, bacteria--which allow it to make use of nutrients like phosphorus.

The stalks of didymo concentrate iron and phosphorus on their surfaces. Bacteria living in the mat then interact with iron to make the phosphorus biologically available to the algae. As didymo mats develop, new stalks grow at the surface and older stalks--with their already-bound phosphorus--are displaced to the mats' inner regions. The process results in abundant phosphorus for cell division, and explains why didymo thrives in low-nutrient rivers and streams.

The results of the study will help scientists and managers identify water bodies susceptible to didymo blooms. “The growth of this diatom in low nutrient rivers has been a paradox,” explains Sarah Spaulding. “By examining the chemistry of the stalk, we think that we are starting to understand how a tiny cell has ecosystem impacts around the globe.”

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