Algae seem harmless enough. These precursors to plants thrive throughout the world's waters. But these single-celled plants have global consequences. We can thank them for oxygen in the atmosphere, oil in the lithosphere as well as dead zones in the oceans and now even a dead horse in France.

That's right. The fumes from decomposing algae on a French beach killed a horse and rendered its rider unconscious this past summer. And poisonous tides caused by algal blooms make eating shellfish dicey at times as well as causing mass die-offs of fish, birds and even sea-going mammals. Plus, according to a new theory, that might just be a small taste of the plants' killing ability.

James Castle and John Rodgers of Clemson University think that such algal blooms—triggered by warming water or an increase in nutrients—might be behind the five largest mass extinctions in Earth's history.

Usually asteroid impacts or massive volcanism get the blame, paired with climate change, but it may be the subsequent bloom in toxic algae that really drove death. There's plenty of evidence for such blooms in the geologic record.

Of course, it remains unclear whether algal blooms caused die-offs or merely coincided with them. And it's most likely that some combination of factors, including poisonous algae perhaps, helped wipe out 90 percent of all marine life some 250 million years ago, for example.

The bad news? Such murderous algae is on the march again thanks to a changing climate, creeping northward into newly warmer U.S. waters. Beware the killer algae!

—David Biello