Nitrogen triggers extra-toxic algal blooms in Lake Erie

[USA] Over the last 20 years, Lake Erie has been heaving up ever-larger blooms of cyanobacteria that are so toxic they shut down Toledo, Ohio’s drinking water system in 2014. Scientists say that a warming climate and farming practices that deliver more soluble phosphorus are driving the blooms, but they can’t explain precisely why certain types of cyanobacteria dominate, and why the blooms sometimes produce toxins and other times don’t. A new study reveals that Microcystis, the most toxic type, uses nutrient-scavenging genes to outcompete other algae when phosphorus is low and boosts toxin output when nitrogen levels are high, suggesting that lake managers may want to add nitrogen limits to existing phosphorus controls (Environ. Sci. Technol. 2015, DOI:10.1021/acs.est.5b03931).

Cyanobacteria are photosynthetic bacteria that thrive in lakes polluted by excess phosphorus. Some types, such asAnabaena, can capture atmospheric nitrogen and convert it to a form available for growth, whereas Microcystis must use dissolved nitrogen species in the water to meet its needs. Anabaena and Microcystis produce variable levels of microcystin, a potentially deadly toxin that attacks the liver. Since Microcystis is the main culprit behind noxious blooms on Lake Erie, scientists would like to know how to defeat it.

In Western Lake Erie, where the Maumee River carries high loads of farm runoff, blooms are dominated at times by Anabaena and other times by Microcystis. Previous Lake Erie studies have observed that Anabaena dominates in high phosphorus environments near the highly polluted mouth of the river, while Microcystis is most abundant in regions away from the river, which have lower phosphorus and high nitrogen concentrations. To get detailed information on how cyanobacteria respond to nutrients at the cellular level, Christopher J. Gobler, an aquatic ecologist at Stony Brook University, and his team decided to use genetic analysis to see if nutrients turned on genes that could affect the distribution of cyanobacteria and their ability to form toxic blooms.

The scientists cruised through a bloom from the mouth of the Maumee River to the Lake Erie Islands, sampling lake water. Back in the lab, the researchers analyzed the samples for concentrations of nitrogen and phosphorus, the composition of cyanobacteria, and levels of gene expression.

Under high phosphorus concentrations at the mouth of the river, Anabaena showed enhanced expression of nitrogen fixation genes and dominated the blooms, Gobler says. But offshore, where phosphorus is lower, Microcystis was the most abundant cyanobacteria, gaining a competitive advantage by ramping up expression of genes for phosphorus uptake. Under the relatively high nitrogen levels found throughout Lake Erie, Microcystis also upregulated genes for compounds that discourage grazing by zooplankton, its main predator. When scientists added nitrogen to Microcystis populations in an ecosystem-based incubation experiment, the nutrient triggered expression of microcystin-synthesizing genes and increased microcystin production.

“This study shows that Microcystis can thrive even when phosphorus is low, and that when it is saturated with nitrogen it makes more toxins,” Gobler says. The results are important for lake managers because they suggest that “management plans limited to phosphorus-load reduction not only will not reduce the risk of harmful blooms, but may promote a shift to a community dominated by toxic strains of Microcystis,” says David A. Culver, an aquatic ecologist at Ohio State University not involved with the work.

Gobler’s study is valuable because it shows how the environment influences gene expression, toxin production, and bloom composition, says Stephen R. Carpenter, an aquatic ecologist at the University of Wisconsin. “But their results don’t support the call for limiting nitrogen,” he says. Nitrogen is difficult to manage in a lake because if levels are low, species like Anabaena will fix atmospheric nitrogen and then release it to feed blooms of Microcystis, he says. The bottom line, however, is that scientists are sure that harmful blooms in lakes can be controlled by reducing phosphorus inputs. The U.S. and Canada are currently mulling policies aimed at a 40% cut in phosphorus flowing into Lake Erie.

 

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