Is algae secret ingredient in mass extinctions?

By Rachael Rettner

updated 6:47 p.m. ET Oct. 29, 2009

Mass extinctions that wiped entire species off the face of the Earth in a relative blink of the eye are often blamed on catastrophic occurrences, such as an asteroid crash or large volcanic eruption. But a new hypothesis points to a different culprit: lowly algae.

In the past 540 million years, five massive extinctions are thought to have killed off, in each case, some 50 percent to 90 percent of animal species. A new study suggests that toxins from algae played a major role in all five extinctions, including the most recent and most well-known — the death of the dinosaurs 65 million years ago. The idea was presented at the annual Geological Society of America meeting Oct. 19.

But could primitive algae really have taken down the dinosaurs? Well, the jury is still out, though some scientists are skeptical of the algae-killing idea, saying evidence is at best lacking and to point the finger at just one culprit for one mass extinction, let alone five, makes little sense.

And while no one can ever know for certain what caused these extinctions, some theories are more accepted than others. For instance, most scientists think an asteroid impact was the major factor in the dinosaur extinction, while a volcanic eruption was a big contributor to the Permian extinction, the largest of the five. As of now, the killer-algae hypothesis doesn't fit into the "accepted" category.

Toxic algae
Algae are simple organisms that get their energy from the sun and lack many features found in plants, such as roots and leaves. Some algae species produce toxins that are harmful to other aquatic organisms and even us. For instance, one group of algae called dinoflagellates can release neurotoxins that act on nerve cells.

When nutrients abound, the algae and other primitive microbes can grow rapidly and can aggregate to form dense populations, known as algae blooms. Such outbreaks of toxic algae can have devastating effects on ecosystems, killing fish, birds, marine mammals and even people.

The most problematic group of toxin-producers are cyanobacteria, commonly known as blue-green algae. While cyanobacteria are not technically algae — they were reclassified from algae to bacteria — they can produce their own energy from the sun, and some researchers still place them in the algae group. Also, their "blooms," which cover the water with a blue-green film, are referred to as algal blooms.

Clemson University researchers James Castle, a geologist, and John Rodgers, an environmental toxicologists, wanted to find out if such algal blooms that are harmful today could have posed a threat millions of years ago and possibly contributed to extinctions.

First, they did a literature search, turning up reports of an increase in fossilized stromatolites, or dome-shaped rocks with layers of cyanobacteria known as "microbial mats," during four out of five of the mass extinctions.

Then, the team compared the structure of modern-day cyanobacteria with ancient cyanobacteria, finding the species had not changed much over millions of years.

"Since they've changed very little in their structure, and they make toxins today, we propose that they did so in the past," said Castle.

While they didn't find an increase in fossilized algae at the time of the dinosaur extinction, the authors suggest that another form of algae, one that doesn't leave fossils behind, could have contributed to that extinction. They suggest so-called planktonic algae, which produces toxins in the soil, could have found its way into the animals' diets. The toxins can also become airborne, providing another way to poison species.

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