A Warning Sign of a Mass Extinction Event Is on the Rise, Scientists Say

Toxic microbial blooms thrived during the Great Dying, the most severe extinction in Earth's history, and they are proliferating again due to human activity.
Toxic microbial blooms thrived during the Great Dying, the most severe extinction in Earth's history, and they are proliferating again due to human activity.
Fish die as a result of algal bloom in Florida, 2021. Image: Andrew Lichtenstein / Contributor via Getty Images
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If you live near a freshwater river or lake, odds are good that you have seen warning signs about harmful algal and bacterial blooms posted on its shores. Alarmingly, a new study reports that these blooms may be early indicators of an ongoing ecological disaster, caused by humans, that eerily parallels the worst extinction event in Earth’s history.

Some 251 million years ago, the end-Permian event (EPE), popularly known as the “Great Dying,” wiped out nearly 90 percent of species on Earth, making it the most severe loss of life in our planet’s history.


Ominous parallels of that upheaval are now showing up on Earth, according to a team led by Chris Mays, a postdoctoral researcher and palaeobotanist at the Swedish Museum of Natural History in Stockholm. The researchers found that toxic algal and bacterial blooms during the Great Dying are similar to a recent microbial proliferation in modern lakes and rivers—a trend that has been linked to human activities such as greenhouse gas emissions (especially carbon dioxide), deforestation, and soil loss. 

“We are not there yet,” Mays said in an email, referring to the conditions of the EPE. “There was probably a six-fold increase in carbon dioxide during the EPE, but today carbon dioxide levels haven't yet doubled since pre-industrial times.” 

“But with the present steep increase in carbon dioxide, we're playing catch-up pretty well,” he cautioned. “And the chances of harmful microbial bloom events, along with many other deleterious facets of change (e.g., intense hurricanes, floods, wildfires), also rise...all the way up this steep carbon dioxide slope.” 

The repeated correlation of these blooms with mass extinction events is “a disconcerting signal for future environmental change,” report the researchers in a study published on Friday in the journal Nature Communications. Indeed, there’s a lot of evidence to suggest we are currently in the midst of yet another mass extinction event, caused by humans. 


Not only do microbial blooms transform freshwater habitats into “dead zones” that can choke out other species, thereby increasing the severity of extinction events, they can also delay the recovery of ecosystems by millions of years, the team noted.

Mays and his colleagues reached this troubling conclusion by analyzing fossil records near Sydney, Australia, that were laid down before, during, and after the end-Permian extinction.

Though the exact mechanisms behind the Great Dying are a matter of debate, it was driven in part by an intense bout of volcanic eruptions that sparked a dramatic uptick in global temperatures and greenhouse gases emissions. Wildfires, droughts, and other disruptions swept across the woodlands, causing a collapse of plant life and widespread deforestation. 

The sudden loss of forests, which act as a sink for carbon, created a noticeable “coal gap” during the end-Permian that exposes this long-term interruption in carbon sequestration. Nutrients and soils that had once been metabolized by these botanical ecosystems instead seeped into nearby freshwater habitats, bolstering microbial blooms that were already thriving as a result of higher temperature and atmospheric carbon. 

These microbial communities are an integral part of freshwater ecosystems worldwide, but the effects of human-driven climate change—including wildfires, deforestation, soil loss, and drought—are driving a new bloom boom. 


“The three main ingredients for this kind of toxic soup are accelerated greenhouse gas emissions, high temperatures, and abundant nutrients,” Mays said. “During the EPE and other extreme warming events, volcanic eruptions provided the first two, while sudden deforestation caused the third. Specifically: when the trees were wiped out, the soils bled into the rivers and lakes, providing all the nutrients that the microbes would need.”

“Today, humans are providing all three of the ingredients in abundance,” he noted. “Carbon dioxide and warming are the inevitable byproducts of burning fossil fuels for hundreds of years, and we've provided copious nutrients into our waterways, mostly from agriculture and logging. Together, this mix has led to a sharp increase in freshwater toxic blooms.”

This pattern threatens to spread the reach of toxic sludge and create the kind of dangerous dead zones that contributed to the enormous ecological turmoil, and slow recovery, of the Great Dying. Indeed, Mays’ team drew comparisons between the End-Permian’s blooms and those that are flourishing today, including their texture, filamentous structure, strong fluorescence, and concentrations. 

“The concentrations of algae from the end-Permian event, the worst mass extinction in Earth's history, were as high as some bloom events we see today,” Mays said. “But the EPE blooms occurred without humans helping.”


The team notes that “the optimal growth temperature range” for these freshwater microbes is 20–32°C, which matches the estimated summer air temperatures during the early Triassic, the period that immediately followed the Permian, and is also within the range of projected temperatures at mid-latitudes for the year 2100, according to the study. 

“The beauty of looking at prehistoric extreme warming events, like the end-Permian, is that they provide, arguably, a cleaner signal of the consequences of climate change,” Mays noted. “This is because the fossils and rocks show us the results of warming without additional messy influences from humans” such as “nutrient influx from agriculture, deforestation via logging, extinctions by poaching/overfishing,” and more. 

“As it turns out, you can cause a large number of extinctions simply by releasing a lot of greenhouse gas in a short time frame,” he continued. “It doesn't matter where the gases come from―volcanoes, airplanes, coal-fired power plants―the results may end up the same.”

Clearly, it’s not encouraging to see the same ecological trends of the worst mass extinction event in Earth’s history popping up in freshwater systems all around us. Tracking the continued emergence of these blooms could help scientists predict the environmental costs of the climate crisis in the coming years and decades, which may also include an extremely delayed recovery of the ecosystems lost to the advance of microbial dead zones.

Mays and his colleagues also plan to study the role of wildfires in mass extinction, as well as the burning of crucial carbon sinks such as the wetlands of South America or the peatlands of Siberia.

“As we've seen in the fossil record, without these regions of carbon-dioxide-drawdown, the world can stay intolerably warm for hundreds of millennia,” Mays said. “While wildfires play an important role in some ecosystems, I think most scientists would agree that preventing the burning of carbon sinks should be a global priority if we want to help minimize the long-term impacts of warming.”

“Unlike the species that suffered the mass extinctions of the past,” he concluded, “we have the opportunity to prevent these toxic blooms by keeping our waterways clean and curbing our greenhouse gas emissions.”