Sunset aboard the Thomas G. Thompson, a University of Washington-operated research vessel equipped for ocean voyages. The instrument visible on the left is a water sampler that can collect from different depths, the SeaFlow flow cytometer was also aboard, but not pictured here.Kathy Newer/University of Washington
Among the tiniest living things in the ocean are a group of single celled microbes called Prochlorococcus. They are cyanobacteria, also known as blue-green algae, and they supply nutrients for animals all the way up the food chain. Over 75% of surface waters teem with Prochlorococcus, but as ocean temperatures rise, researchers fear that the water might be getting too warm to support the population.
Prochlorococcus is the most abundant photosynthesizing organism in the ocean, accounting for 5% of global photosynthesis. Because Prochlorococcus thrive in the tropics, researchers predicted that they would adapt well to global warming. Instead, a new study finds that Prochlorococcus prefers water between 66 and 86 degrees and doesn't tolerate it much warmer. Climate models predict that subtropical and tropical ocean temperatures will exceed that threshold in the next 75 years.
"For a long time, scientists thought Prochlorococcus was going to do great in the future, but in the warmest regions, they aren't doing that well, which means that there is going to be less carbon - less food - for the rest of the marine food web," said François Ribalet, a University of Washington research associate professor of oceanography, who led the study.
Their results were published in Nature Microbiology on Sept. 8.
Researchers cataloged Prochlorococcus abundance using SeaFlow continuous flow cytometry along the path of the lines shown. The water in yellow areas hovers around 86 degrees while the temperature at the poles is closer to 32.
In the past 10 years, Ribalet and colleagues have embarked on close to 100 research cruises to study Prochlorococcus. His team has analyzed approximately 800 billion Prochlorococcus-sized cells across 150,000 miles to figure out how they are doing and whether they can adapt.
"I had really basic questions," Ribalet said. "Are they happy when it's warm? Or are they not happy when it's warm?" Most of the data comes from cells grown in culture, in a lab setting, but Ribalet wanted to observe them in their natural habitat. Using a continuous flow cytometer - called SeaFlow - they fired a laser through the water to measure cell type and size. They then built a statistical model to monitor cell growth in real time, without disturbing the microbes.
Results showed that the rate of cell division varies with latitude, possibly due to the amount of nutrients available, sunlight or temperature. The researchers ruled out nutrient levels and sunlight before zeroing in on temperature. Prochlorococcus multiply most efficiently in water that is between 66 and 84 degrees, but above 86, rates of cell division plummeted, falling to just one-third of the rate observed at 66 degrees. Cell abundance followed the same trend.
In the ocean, mixing transports nutrients to the surface from the deep. This occurs more slowly in warm water, and surface waters in the warmest regions of the ocean are nutrient-scarce. Cyanobacteria are one of the few microbes that have adapted to live in these conditions.
"Offshore in the tropics, the water is this bright beautiful blue because there's very little in it, aside from Prochlorococcus," Ribalet said. The microbes can survive in these areas because they require very little food, being so small. Their activity supports most of the marine food chain, from small aquatic herbivores to whales.
This image, captured by an electron microscope, displays individual Prochlorococcus cells. Each blob is a microbe, measuring just 500 nanometers in diameter. For reference, the width of a single human hair is around 100,000 nanometers.Natalie Kellogg/University of Washington
Over millions of years, Prochlorococcus has perfected the ability to do more with less, shedding genes it didn't need and keeping only what was essential for life in nutrient-poor tropical waters. This strategy paid off spectacularly, but now, with oceans warming faster than ever before, Prochlorococcus is constrained by its genome. It can't retrieve stress response genes discarded long ago.
"Their burnout temperature is much lower than we thought it was," Ribalet said. Previous models assumed that the cells would divide at a rate that they can't sustain because they now lack the cellular machinery to cope with heat stress.
Prochlorococcus is one of two cyanobacteria that dominate tropical and subtropical waters. The other, Synechococcus, is larger, with a less streamlined genome. The researchers found that although Synechococcus can tolerate warmer water, it needs more nutrients to survive. Should Prochlorococcus numbers dwindle, Synechococcus could help fill the gap, but it isn't clear how this would impact the food chain.
"If Synechococcus takes over, it's not a given that other organisms will be able to interact with it the same way they have interacted with Prochlorococcus for millions of years," Ribalet said.
Climate projections estimate ocean temperatures based on greenhouse gas emission trends. In this study, the researchers tested how Prochlorococcus might fare in moderate- and high-warming scenarios. In the tropics, modest warming could reduce Prochlorococcus productivity by 17%, but more advanced warming would decimate it by 51%. Globally, the moderate scenario produced a 10% decline while warmer forecasts reduced Prochlorococcus by 37%.
"Their geographic range is going to expand toward the poles, to the north and south," Ribalet said. "They are not going to disappear, but their habitat will shift." That shift, he added, could have dramatic implications for subtropical and tropical ecosystems.
Still, the researchers acknowledge the limitations of their study. They couldn't examine every cell or sample all bodies of water. Their measurements are based on pooled samples, which could mask the presence of a heat-tolerant strain.
"This is the simplest explanation for the data that we have now," Ribalet said. "If new evidence of heat tolerant strains emerges, we'd welcome that discovery. It would offer hope for these critical organisms."
Co-authors include E. Virginia Armbrust, a UW professor of oceanography; Stephanie Dutkiewicz, a senior research scientist in the Center for Sustainability Science and Strategy at MIT; and Erwan Monier, co-director of the Climate Adaptation Research Center and an associate professor in the Department of Land, Air and Water Resources at UC Davis.
This research was funded by the Simons Foundation and other government, foundation and industry funders of the MIT Center for Sustainability Science and Strategy.
