SEATTLE (AP) 鈥 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鈥檚 climate, will decline sharply as seas heat up.
in the journal 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 (27.8 Celsius). Many tropical and subtropical and are projected to regularly surpass 86 degrees Fahrenheit (30 Celsius) over that same period.
鈥淭hese are keystone species 鈥 very important ones,鈥 said Fran莽ois Ribalet, a research associate professor at the University of Washington鈥檚 School of Oceanography and the study鈥檚 lead author. 鈥淎nd when a keystone species decreases in abundance, it always has consequences on ecology and biodiversity. The food web is going to change.鈥
These tiny organisms hold a vital role in ocean life
Prochlorococcus inhabit up to 75% of Earth鈥檚 sunlit surface waters and produce about one-fifth of the planet鈥檚 oxygen through photosynthesis. More crucially, Ribalet said, they convert sunlight and carbon dioxide into food at the base of the marine ecosystem.
鈥淚n the tropical ocean, nearly half of the food is produced by Prochlorococcus,鈥 he said. 鈥淗undreds of species rely on these guys.鈥
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. 鈥淓volution has made this very specific interaction,鈥 he said. 鈥淥bviously, this is going to have an impact on this very unique system that has been established.鈥
The findings challenge decades of assumptions that Prochlorococcus would thrive as waters warmed. Those predictions, however, were based on limited data from lab cultures. For this study, Ribalet and his team tested water samples while traversing the Pacific over the course of a decade.
Over 100 research cruises 鈥 the equivalent of six trips around the globe 鈥 they counted some 800 billion individual cells taken from samples at every kilometer. In his lab at the University of Washington, Ribalet demonstrated the SeaFlow, a box filled with tubes, wires and a piercing blue laser. The custom-built device continuously pulls in seawater, which allowed the team to count the microbes in real time. 鈥淲e have counted more Prochlorococcus than there are stars in the Milky Way,鈥 Ribalet said.
Experts warn of 鈥榖ig consequences鈥
Paul Berube, a research scientist at Massachusetts Institute of Technology who studies Prochlorococcus but was not involved in the work, said the breadth of data is 鈥済roundbreaking.鈥 And he said the results fit with what is known about the microbe鈥檚 streamlined genome, which makes it less adaptable to rapid environmental changes.
鈥淭hey鈥檙e at the very base of the food web, and they feed everything else 鈥 the fish eat the things that eat the phytoplankton and we eat the fish,鈥 he said. 鈥淲hen changes are being made to the planet that influence these particular organisms that are essentially feeding us, that鈥檚 going to have big consequences.鈥
To test whether Prochlorococcus might evolve to withstand hotter conditions, Ribalet鈥檚 team modeled a hypothetical heat-tolerant strain but found that even those would 鈥渘ot be enough to fully resist the warmest temperature if greenhouse emissions keep rising,鈥 Ribalet said.
He stressed that the study鈥檚 projections are conservative and don鈥檛 account for the impacts of plastic pollution or other ecological stressors. 鈥淲e actually tried to put forth the best-case scenario,鈥 Ribalet said. 鈥淚n reality, things may be worse.鈥
Steven Biller, an associate professor at Wellesley College, said the projected declines are 鈥渟cary but plausible.鈥 He noted Prochlorococcus form part of the 鈥渋nvisible forests鈥 of the ocean 鈥 tiny organisms most people never think about, but are essential to human survival.
鈥淗alf of all photosynthesis is happening in the oceans and Prochlorococcus is a really important part of that,鈥 Biller said. 鈥淭he magnitude of the potential impact is kind of striking.鈥
Biller, Berube and Ribalet said that while other microbes may compensate somewhat, the broader risks to biodiversity and fisheries are real.
鈥淲e know what drives global warming. There is no debate among the scientific community,鈥 Ribalet said. 鈥淲e need to curb greenhouse gas emissions.鈥
He hopes the findings bring more attention to tropical oceans, which could serve as natural laboratories for warming adaptations and as early warning signals for ecological collapse.
鈥淔or the first time, I want to be wrong. I would love to be wrong,鈥 he said. 鈥淏ut these are data-driven results.鈥
___
Follow Annika Hammerschlag on Instagram
___
The Associated Press receives support from the Walton Family Foundation for coverage of water and environmental policy. The AP is solely responsible for all content. For all of AP鈥檚 environmental coverage, visit
