By Carl Zimmer
Over the weekend, an entire city was brought to its knees by pond scum.
Toledo, Ohio, gets its drinking water from the western end of Lake Erie. A bloom of bacteria formed there last week, producing a dangerous toxin called microcystin. City officials warned half a million residents against drinking municipal water. At high doses, the toxin can cause liver failure.
The microbes that terrorized Toledo, known as cyanobacteria, are actually a worldwide menace.
Fertilizers and other pollutants, the consequences of modern agriculture and fossil fuel production, are flowing into rivers and lakes, promoting the growth of these waterborne bacteria. The result is a worldwide rise in cyanobacterial blooms.
Cyanobacteria are prehistoric organisms, and they were making these toxins billions of years before humans were around. The question that puzzles scientists is: Why?
“We only call them toxins because they’re toxic to us,” said Hans W. Paerl, an environmental scientist at the University of North Carolina at Chapel Hill. By understanding the ancient history of cyanobacteria, Dr. Paerl and other scientists hope they can help fight the new threat posed by these microbes.
The spherical fossils of cyanobacteria date back two billion years, and scientists suspect that the microbes first evolved a billion years before that. They use sunlight to grow, absorbing carbon dioxide while releasing oxygen. It’s likely that the earliest cyanobacteria transformed the planet’s atmosphere three billion years ago, flooding it with oxygen for the first time.
When scientists first realized that many species of cyanobacteria made toxins, they were puzzled. The molecules have to be important, because they require a lot of effort to build. To create microcystin, for example, cyanobacteria first need to produce ten different proteins, which then assemble the toxin from smaller parts.
Initially, many scientists suspected that cyanobacteria used toxins to defend themselves against predators. Tiny aquatic animals that feed on bacteria avoid toxin-producing cyanobacteria if they can.
But there was a big problem with the defense hypothesis: Animal predators didn’t evolve until about 600 million years ago. Genetic studies have shown that cyanobacteria were probably making toxins from their earliest days.
Recent research has revealed that microcystin has other uses. “It’s like a Swiss Army knife,” said Elke Dittmann, a microbiologist at the University of Potsdam.
Although cyanobacteria need sunlight to grow, for example, they risk being overexposed as they bob on the surface of the water — the extra radiation can damage their molecules. Inside the cells of cyanobacteria, microcystin grabs onto several different proteins and supports them under the stress so they can function normally.
It’s also possible that cyanobacteria use toxins as a kind of molecular pantry. When there’s an abundance of nitrogen and carbon around them, the microbes can use them to build microcystin. In lean times, they can break down the molecules of microcystin and use the atoms to build new compounds.
Yet another possibility is that the microbes use toxins as signals. When cyanobacteria die, they release microcystin, and scientists have found that remaining cyanobacteria respond to the molecule by making more of their own. It’s even possible that microcystin may attract other species that the cyanobacteria depend on for survival.
For all that microcystin may do for cyanobacteria, it does us no good at all. The molecule is absorbed by cells in the liver, where it latches onto certain proteins and brings our biochemistry to a halt. In high doses, the toxins can be fatal, but even at low doses, they may cause long-term damage.
“That’s just an unfortunate byproduct,” said Tim G. Otten, a microbiologist at Oregon State University.
Dr. Otten and other researchers have learned a great deal about microcystin in recent years, but they still know very little about the hundreds of other toxins that cyanobacteria can make — from the functions they perform inside the bacteria to the risk they pose to humans. “We’ve barely scratched the surface,” Dr. Otten said.
We may even be able to harness the toxins to help us, rather than hurt us. Some studies suggest that the molecules can latch on to proteins in cancer cells and kill them.
Unfortunately, we may also be exposed more often to the harm that toxins can cause us. In their three-billion-year history, cyanobacteria have never quite experienced the conditions they have today, in which huge amounts of nitrogen and phosphorus pumped into rivers and lakes allow the microbes to flourish in huge numbers.
As the climate warms, cyanobacteria may become even more prevalent. So may their ancient poisons.
“They’re always going to be around,” Dr. Otten said. “We’re just making it easier and easier for them to proliferate.”
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