Warming seas could halve populations of crucial phytoplankton, study warns
Decade-long sampling shows Prochlorococcus — a foundational marine microbe that produces oxygen and base food — may decline sharply in tropical oceans as surface waters warm
A decade-long global survey of ocean microbes found that Prochlorococcus, the smallest and most abundant phytoplankton on Earth, could decline by as much as half in tropical oceans over the next 75 years if surface waters routinely exceed about 82 degrees Fahrenheit (27.8 Celsius), according to a study published Monday in Nature Microbiology.
Researchers cautioned that many tropical and subtropical sea surface temperatures are already trending above historical averages and are projected to regularly surpass 86 degrees Fahrenheit (30 Celsius) over that period, amplifying the risk to a microbe that helps sustain marine food webs and contributes substantially to global oxygen production.
Lead author François Ribalet, a research associate professor at the University of Washington’s School of Oceanography, described Prochlorococcus as a keystone organism. The microbe inhabits as much as 75 percent of the ocean’s sunlit surface waters, produces about one-fifth of the planet’s oxygen through photosynthesis, and is responsible for nearly half of primary production in some tropical regions, the study said. “These are keystone species — very important ones,” Ribalet said. “And when a keystone species decreases in abundance, it always has consequences on ecology and biodiversity. The food web is going to change.”
The findings overturn a long-standing expectation that Prochlorococcus would generally benefit from warmer seas. Earlier predictions were largely based on limited laboratory cultures. To better assess how natural populations respond, Ribalet and colleagues deployed a custom instrument they call SeaFlow as part of more than 100 research cruises across the Pacific over roughly a decade. The continuous-sampling device allowed the team to count roughly 800 billion individual Prochlorococcus cells from samples taken at every kilometer, providing an unprecedented field data set on abundance and temperature sensitivity.
Paul Berube, a research scientist at the Massachusetts Institute of Technology who studies Prochlorococcus and was not involved in the study, called the breadth of the data “groundbreaking.” He said the results are consistent with known features of the microbe’s streamlined genome, which may limit its ability to adapt to rapid environmental change. “They’re at the very base of the food web, and they feed everything else — the fish eat the things that eat the phytoplankton and we eat the fish,” Berube said. “When changes are being made to the planet that influence these particular organisms that are essentially feeding us, that’s going to have big consequences.”
To test potential evolutionary responses, the researchers modeled a hypothetical heat-tolerant strain of Prochlorococcus. Even under that scenario, the modeling indicated the strain would not be sufficient to offset population declines if greenhouse gas emissions continue to drive ocean warming. The team said its projections are conservative and do not incorporate additional stressors such as plastic pollution, acidification, nutrient shifts or other ecological pressures that could exacerbate declines.
Steven Biller, an associate professor at Wellesley College, described the projected losses as “scary but plausible,” noting that Prochlorococcus makes up part of the ocean’s “invisible forests” that perform much of the planet’s photosynthesis. He and other experts emphasized that while other phytoplankton groups might partially compensate for reduced Prochlorococcus abundance, they are not perfect functional substitutes for the microbe’s role in tropical food webs.
The potential decline of Prochlorococcus raises concerns for biodiversity and fisheries that depend on productivity at the base of the marine food chain. Changes in the abundance and distribution of foundational microbes can cascade through ecosystems, altering the availability of food for a wide range of organisms and affecting the regionally important services those ecosystems provide.
Ribalet said the study’s results underscore the need to reduce greenhouse gas emissions and pay closer attention to tropical oceans as natural laboratories for warming-related change. “We know what drives global warming. There is no debate among the scientific community,” he said. “We need to curb greenhouse gas emissions.” He added that tropical regions could act as early warning systems for broader ecological shifts as the climate warms.
The study’s authors said further research will be needed to monitor how ongoing warming and other human-caused stressors interact to shape the future of ocean productivity. For now, the data-driven projection that a foundational marine microbe may sharply diminish in some of the warmest parts of the ocean presents a new and tangible concern for scientists and resource managers tracking the ecological impacts of a changing climate.