Algal blooms — the thing that many geoengineering advocates could take up vast amounts of carbon dioxide and save us from climate change — could instead be the culprit behind some of the world’s worse mass extinctions.

Toxic algae can exist just about anywhere there’s water, usually in small concentrations. When water warms suddenly, though, or a rapid injection of dust or sediment provides a large source of “food,” toxic algae can bloom in such profusion that it kills thousands of fish and poisons shellfish or even humans.

In fact, just such toxic blooms could have occurred during the five largest mass extinctions in Earth’s history, according to James Castle and John Rodgers, two researchers at Clemson University. Each time a large die-off occurred in the past, Castle and Rodgers found a spike in the number of fossil algae mats called stromatolites strewn around the planet.

Castle was expect to present the research today at the annual meeting of the Geological Society of America in Portland, Oregon.

“If you go through theories of mass extinctions, there are always some unanswered questions,” Castle said. “For example, an impact — how does that cause species to go extinct? Is it climate change, dust in the atmosphere? It’s probably not going to kill off all these species on its own.”

But as the nutrient-rich fallout from, say, a massive meteorite impact lands in the water, it becomes food for algae. They explode in population, releasing chemicals that can act as anything from skin irritants to potent neurotoxins. Plants on land can pick up the compounds in their roots, and pass them on to herbivorous animals.

If the theory is right, it answers a lot of questions about how species died off in the ancient world. It also raises concerns for how today’s algae may damage the ecosystem in a warmer world.

“Algae growth is favored by warmer temperatures,” Castle said. “You get accelerated metabolism and reproduction of these organisms, and the effect appears to be enhanced for species of toxin-producing cyanobacteria.”

He added that toxic algae in the United States appear to be migrating slowly northward through the country’s ponds and lakes, and along the coast as temperatures creep upward. Their expanding range portends a host of problems for fish and wildlife, but also for humans, as algae increasingly invade reservoirs and other sources of drinking water.

“(T)his hypothesis gives us cause for concern and underscores the importance of careful and strategic monitoring as we move into an era of global climate change,” the researchers write in their study.

“Scientists from around the world have been sending us data that support our hypothesis and our concern about the future,” Rodgers said. “I look forward to the debate this work will generate. I hope it helps focus attention on climate change and the consequences we may face.”

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