It Turns Out That Single-Celled Organisms Are Having Sex Too

So much for the stereotype of the chaste, pious diatom. (Wait. Is that not a stereotype?) Scientists examining tiny single-celled organisms say we've been wrong to assume they* lead lives of quiet, sexless desperation. The researchers described their microscopically racy findings in the journal PLOS One.

The tiny specks called diatoms are neither plants nor animals but somewhere in between; they're algae. There are more than 200,000 species living in the water all over the world, each with its own unique and beautiful frustule, or hard silica scaffolding.

To date, scientists have sequenced the genetic code of only two diatoms: the peapod-shaped Phaeodactulum tricornutum and the round, or centric, Thalassiosira pseudonana. The genes of T. pseudonana contained some sex-related code, but researchers believed it was vestigial, like our wisdom teeth or the wings of a kiwi bird.

"Everybody said Thalassiosira pseudonana was asexual, because they'd never seen anything else," corresponding author Kimberly Halsey of Oregon State University said in a statement. "The general thinking was that it just lost the ability or need to go through sex."

Not for lack of trying on our part. Previous experimenters have tried all kinds of things to get the little specks to get busy, from turning off the lights to adding more salt to the water.

"Lab efforts to induce sex in centric diatoms have ranged from sweet talk to torture," Halsey said. These efforts weren't entirely fruitless; once in a while, the diatoms might show some interest. But it wasn't clear why, or if the tiny organisms would do it on their own.

Halsey and her colleagues decided to take another close look at T. pseudonana. They went back over its genome, studying any gene that could be related to sexual activity, then stared long and hard at the diatoms themselves.

What they found surprised them. The diatoms had genes that would allow them to differentiate—that is, to become one sex or another. Lead author Eric Moore says he was startled by this realization.

"In fact, I was convinced my cultures were contaminated before I realized what was actually going on," he said.

Moore, Halsey, and their colleagues also discovered the secret to getting T. pseudonana in the mood: a little aphrodisiac known as ammonia.

While many of us know ammonia as an awful-smelling cleaning agent, it's not hard to find it in the wild, as a component of urine and other waste. Exposing the diatoms to ammonia was all it took to get the cells to start making eggs and sperm.

The authors say the diatoms may be regularly soaking in ammonia and, consequently, getting sexy.

"Our discoveries solve two persistent mysteries that have plagued diatom researchers," Halsey said. "Yes, they have sex, and yes, we can make them do it."

*The single-celled organisms, not the scientists. The sex lives of the latter are none of our business.