It had been a long day of PowerPoint presentations and charts and graphs. A ­couple of hours earlier, Nathan Ahlgren ‘99 had been downstairs wrapping up his talk, jazzily titled “Changes in Picocyanobacterial Community Structure across Transitions from ­Oligotrophic to Upwelling Regions off of Costa Rica.” Now, on the fourth floor of the Stata Center, he was letting his hair down. Specifically, he was singing about bacteria.

“You said, ‘Oh no, that’s just noise,’” he wailed, strumming his guitar. “I believe there’s something more … There’s only one way to know … Crank up the gain and let those cells flow …”

At Prochlorococcusfest on May 30 and 31, MIT celebrated an august occasion: the 20th anniversary of the discovery of Prochlorococcus, the organism NPR’s Science Friday dubbed “the most important microbe you’ve never heard of.” The discovery was announced in a 1988 paper published in Nature by MIT professor and microbiologist Penny Chisholm, her former postdoc Rob Olson of the Woods Hole Oceanographic Institution, and several other colleagues. They had found the tiny, ocean-dwelling blue-green bacterium while trolling the sea for another microbe.

“For a while, we were just calling it electronic noise,” recalls Chisholm. “There were these tiny little things that were speaking to us, but we were busy studying something else. But this stuff we thought was noise behaved differently at different depths–which suggested that it was something that was alive.”

Measuring between 0.5 and 0.7 micro­meters, Prochlorococcus is the smallest known ­oxygen-­producing photosynthetic organism. With tens of thousands in every drop of much of the world’s seawater, it might well be the most abundant organism on earth.

The key to its discovery was the flow cytometer, an instrument that counts ­single cells as they flow in liquid past a sensor. Built around a laser focused on a frail glass capillary tube, the finicky, expensive instruments weren’t particularly well suited to the rolling, pitching, vibrating environment of a boat at sea. But having worked with flow cytome­try pioneer Howard Shapiro to set up a cytometer in Chisholm’s lab, Olson–newly arrived at Woods Hole, and still collaborating on research with his former lab–took one of the machines out on the open ocean. Miraculously, he got it to work.

Chisholm and her colleagues thought that perhaps they had found the missing link between bacterial cells and the chloroplasts of higher plants, which are thought to have descended from some ancient bacteria. Genetic analysis proved this hypothesis wrong. But the tiny microbe turned out to be even more important than its discoverers had originally hoped. It accounts for as much as 20 to 30 percent of the photosynthesis in the sea, making it vital to the earth’s atmospheric cycles of carbon and oxygen.

“Every fifth breath you take–thank Prochlorococcus for that oxygen,” Chisholm says.

While Chisholm and Olson’s 1988 paper didn’t get much early notice, it did catch the attention of Washington Post science writer Boyce Rensberger, who wrote an item on it that August. A decade later, Rensberger and Chisholm would become MIT colleagues when he was appointed director of the Knight Science Journalism Fellowships program. As Rensberger packed up his MIT desk in preparation for retirement at the end of June, he chuckled when he recalled the 20-year-old news brief. “The claim was, it was possibly the most abundant organism on earth,” he says. “Reporters–especially science reporters–love superlatives. The most, the biggest, the farthest, the oldest.”

For the dozens of scientists gathered at Prochlorococcusfest, the 20 years since the discovery have been productive. And the researchers proved that they know how to party. After dinner, ­Shapiro led the crowd in the Prochlorococcus Anniversary Song, encoring with the global-warming ditty “Santa Claus Is Going to Drown.” Emcee Luke Thompson, one of Chisholm’s grad students, tossed out stuffed plush microbes, prompting quibbles about whether it was accurate of the toy company to call Biddulphia “scum.” Many Prochlorococktails–which looked suspiciously like margaritas–were downed.

After 20 years studying Prochlorococcus, Chisholm isn’t flagging. “I really am passionate about our field,” she says. “It’s gotten incredibly exciting.”