It Produces 50% of Food in the Ocean. And It's in Trouble

For decades, scientists believed Prochlorococcus, the smallest and most abundant phytoplankton on Earth, would thrive in a warmer world. But new research suggests the microscopic bacterium, which forms the foundation of the marine food web and helps regulate the planet's climate, will decline sharply as seas heat up. A study published Monday in Nature Microbiology found Prochlorococcus populations could shrink by as much as half in tropical oceans over the next 75 years if surface waters exceed about 82 degrees Fahrenheit, per the AP.

Many tropical and subtropical sea surface temperatures are already trending above average and are projected to regularly surpass 86 degrees Fahrenheit over that same period. That means "the food web is going to change," said François Ribalet, a research associate professor at the University of Washington's School of Oceanography and the study's lead author. Prochlorococcus inhabit up to 75% of Earth's sunlit surface waters and produce about one-fifth of the planet's oxygen through photosynthesis. More crucially, Ribalet said, they convert sunlight and carbon dioxide into food at the base of the marine ecosystem.

"In the tropical ocean, nearly half of the food is produced by Prochlorococcus," he said. "Hundreds of species rely on these guys." Though other forms of phytoplankton may move in and help compensate for the loss of oxygen and food, Ribalet cautioned they are not perfect substitutes. "Evolution has made this very specific interaction," he said. "Obviously, this is going to have an impact on this very unique system that has been established." He stressed that the study's projections are conservative and don't account for the impacts of plastic pollution or other ecological stressors. "We actually tried to put forth the best-case scenario," Ribalet said. "In reality, things may be worse."