Every summer, Lake Erie undergoes a striking transformation. The warm, placid waters near its surface create an inviting environment for tiny, plant-like organisms to flourish.

These vibrant blooms of cyanobacteria, often referred to as blue-green algae, may appear innocuous, but they conceal a potent threat beneath their shimmering surface. Certain strains emit dangerous toxins that endanger not only aquatic life but also birds and humans.

In recent years, harmful algal blooms (HABs) have surged in frequency across Lake Erie, leading to dire consequences such as contaminated drinking water and disturbed ecosystems. A notorious incident in 2014 forced Toledo to shut down its water supply due to contamination from the toxin microcystin.

However, another troublesome toxin—saxitoxin—has perplexed scientists since it was first detected in 2007, with researchers struggling to pinpoint its source.

Unveiling the Mystery: The Culprit Behind Saxitoxin!

A breakthrough by researchers at the University of Michigan has finally illuminated this mystery.

Their research zeroed in on a pivotal question: Which cyanobacteria produce saxitoxin in Lake Erie? The answer is Dolichospermum, a resilient genus of cyanobacteria that thrives in nutrient-rich freshwater environments.

Understanding which organisms create specific toxins is crucial, as it empowers scientists to monitor and manage these hazardous blooms effectively.

Saxitoxins, some of the most lethal natural toxins, can inflict severe nerve damage even at minuscule concentrations. Identifying their source is essential for safeguarding public health and wildlife.

Gregory Dick, a professor of earth and environmental sciences, emphasized the significance of this finding. "Knowing which organism produces the toxin enhances our understanding of the environmental conditions that foster toxin production," he explained. This insight could influence future policy and management efforts.

Diving Deeper: Tracking Toxins in Lake Erie

To track down the source of saxitoxin, researchers gathered toxic water samples from Lake Erie during prominent bloom events. These samples contained a complex blend of DNA.

Lead researcher Paul Den Uyl employed a cutting-edge technique known as shotgun sequencing, which captures all DNA from a sample and reconstructs it into complete genomic sequences. This innovative approach allowed Den Uyl to piece together full genomes from bloom samples.

Amidst this genomic treasure, he pinpointed the genetic signature associated with saxitoxin, confirming that specific strains of Dolichospermum harbor the genes required for toxin production.

However, the revelation brings a twist: not all Dolichospermum strains are toxic. Some possess the necessary genes, while others do not. The factors driving this genetic diversity remain a mystery, prompting researchers to examine the lake's environmental conditions for clues.

Climate Change: The Warming Waters of Lake Erie

The study involved samples from various locations across Lake Erie, collected throughout different seasons. Researchers tracked the prevalence of the saxitoxin gene in relation to environmental factors.

A notable trend emerged: warmer waters were often linked to higher levels of the saxitoxin gene. This discovery raises alarms in the era of climate change.

"Our lakes are changing with climate change, and we must ask how these shifts will influence harmful cyanobacterial blooms," Den Uyl cautioned.

Temperature wasn't the only variable; researchers examined nutrient concentrations, particularly ammonium. They uncovered that the saxitoxin gene was less common in areas rich in ammonium.

Dolichospermum's Unique Advantage!

This pattern may be attributed to a distinctive trait of Dolichospermum. Unlike most organisms, Dolichospermum can utilize atmospheric nitrogen, giving it a competitive edge.

The team identified a gene within its genome that allows it to harness nitrogen from dinitrogen gas—a rare ability among aquatic organisms.

"With the complete genome, we gain insights into the organism's capabilities, and Dolichospermum shows potential for acquiring fixed nitrogen from water," remarked Dick.

This extraordinary nitrogen strategy enables Dolichospermum to thrive in low-nitrogen environments, possibly explaining its dominance in areas where other algal species falter.

What Lies Ahead for Lake Erie?

Despite tracking saxitoxin for nearly a decade, predicting long-term shifts in Lake Erie’s ecology remains elusive. Factors like climate, nutrient runoff, and microbial competition play critical roles.

Nevertheless, this discovery lays the groundwork for enhanced monitoring and research.

"Now that we know who generates saxitoxin, we can keep a vigilant eye on these organisms and assess gene abundance over time," Dick concluded.

As lake temperatures continue to climb, the risk of harmful algal blooms increasing in frequency or toxicity looms ever larger. Understanding the organisms involved, their genetic profiles, and the triggers for toxin production is more vital than ever.

This research, published in the journal Environmental Science & Technology, represents a significant advancement in managing one of North America's most cherished freshwater resources.