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 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鈥檛 tolerate it much warmer. Climate models predict that subtropical and tropical ocean temperatures will exceed that threshold in the next 75 years.

鈥淔or a long time, scientists thought Prochlorococcus was going to do great in the future, but in the warmest regions, they aren鈥檛 doing that well, which means that there is going to be less carbon 鈥� less food 鈥� for the rest of the marine food web,鈥� said , a 天美影视传媒 research associate professor of oceanography, who led the study.

in Nature Microbiology on Sept. 8.

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.

鈥淚 had really basic questions,鈥� Ribalet said. 鈥淎re they happy when it’s warm? Or are they not happy when it’s warm?鈥� 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 鈥� called 鈥� 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.

鈥淥ffshore in the tropics, the water is this bright beautiful blue because there鈥檚 very little in it, aside from Prochlorococcus,鈥� 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.

Over millions of years, Prochlorococcus has perfected the ability to do more with less, shedding genes it didn鈥檛 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鈥檛 retrieve stress response genes discarded long ago.

鈥淭heir burnout temperature is much lower than we thought it was,鈥� Ribalet said. Previous models assumed that the cells would divide at a rate that they can鈥檛 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鈥檛 clear how this would impact the food chain.

鈥泪蹿 Synechococcus takes over, it鈥檚 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,鈥� 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%.

鈥淭heir geographic range is going to expand toward the poles, to the north and south,鈥� Ribalet said. 鈥淭hey are not going to disappear, but their habitat will shift.鈥� That shift, he added, could have dramatic implications for subtropical and tropical ecosystems.

Still, the researchers acknowledge the limitations of their study. They couldn鈥檛 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.

鈥淭his is the simplest explanation for the data that we have now,鈥� Ribalet said. 鈥泪蹿 new evidence of heat tolerant strains emerges, we鈥檇 welcome that discovery. It would offer hope for these critical organisms.鈥�

Co-authors include , a UW professor of oceanography; , a senior research scientist in the Center for Sustainability Science and Strategy at MIT; and , 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.

For more information, contact Ribalet at ribalet@uw.edu.

In the frigid waters surrounding Antarctica, an unusual seasonal cycle occurs. During winter, from March to October, the sun barely rises. As seawater freezes it rejects salts, creating pockets of extra-salty brine where microbes live in winter. In summer, the sea ice melts under constant daylight, producing warmer, fresher water at the surface.

This remote ecosystem is home to much of the Southern Ocean鈥檚 photosynthetic life. A new 天美影视传媒 study provides the first measurements of how sea-ice algae and other single-celled life adjust to these seasonal rhythms, offering clues to what might happen as this environment shifts under climate change.

The , published Sept. 15 in the International Society for Microbial Ecology鈥檚 ISME Journal, contains some of the first measurements of how sea-ice microbes respond to changing conditions.

鈥淲e know very little about how sea-ice microbes respond to changes in salinity and temperature,鈥� said lead author , a UW postdoctoral researcher who did the work while pursuing her doctorate in oceanography at the UW. 鈥淎nd until now we knew almost nothing about the molecules they produce and use in chemical reactions to stay alive, which are important for supporting higher organisms in the ecosystem as well as for climate impacts, like carbon storage and cloud formation.鈥�

The polar oceans play an important role in global ocean currents and in supporting marine ecosystems. Microbes form the base of the food web, supporting larger life forms.

鈥淧olar oceans make up a significant portion of the world鈥檚 oceans, and these are very productive waters,鈥� said senior author , a UW assistant professor of oceanography. 鈥淭hese waters support big swarms of krill, the whales that come to feed on those krill, and either polar bears or penguins. And the start of that whole ecosystem are these single-celled microscopic algae. We just know so little about them.鈥�

The tiny organisms are also important for the climate, since they quietly perform photosynthesis and soak up carbon from the atmosphere. Polar algae are especially good at producing sulfur-containing molecules that give beaches their distinctive smell and, when lofted into the air in sea spray, promote formation of clouds that can reduce penetration of solar rays.

Antarctic sea ice, though long stable, is at an this year.

In other oceans, satellite instruments can capture dramatic seasonal phytoplankton blooms from space 鈥� but that isn鈥檛 possible for microbes hidden under sea ice. And Antarctic waters are particularly challenging to visit, leaving researchers with almost no measurements in winter.

In late 2018, Dawson and co-author traveled to , a U.S. research station on the West Antarctic Peninsula. They used a small boat to sample seawater and sea ice at the same nearby sites every three days.

Back on shore, the two graduate students performed 10-day experiments in tanks to see which microbes grew as temperature and salinity were adjusted to mimic sea-ice formation and melt. They also shipped samples back to Seattle for more complex measurements of the samples鈥� genetics and metabolites, the small organic molecules produced by the cell.

Results revealed how single-celled algae deal with their fluctuating environments. As temperatures drop, the cells produce cryoprotectants, similar to antifreeze, to prevent their cellular fluid from crystallizing. Many of the most common cryoprotectant molecules were the same across different microbial lifeforms.

As salinity changes, to avoid either bursting in freshening waters or becoming desiccated like raisins in salty conditions, the cells change the concentration of salt-like organic molecules. Many such molecules serve a dual role as cryoprotectants, to balance conditions inside and outside the cell to maintain water balance.

The results show that under short-term temperature and salinity changes, community structure in each sample remained stable while adjusting the production of protective molecules. Different microbe species showed consistent responses to changing conditions. This should simplify modeling future responses to climate change, Young said.

Results also hint that the production of omega-3 fatty acids may decline in lower-salinity environments. This would be bad news for consumers of krill oil supplements, and for the marine ecosystem that relies on those algae-derived nutrients. Future research now underway by the UW group aims to confirm that result 鈥� especially with the prospect of increasing freshwater input from melting sea ice and glaciers.

鈥淲e鈥檙e interested in how these sea-ice algae contend with changes in temperature, salinity and light under normal conditions,鈥� Dawson said. 鈥淏ut then we also have climate change, which is completely remodeling the landscape in terms of when sea ice is forming, how much sea ice forms, how long it stays before it melts, as well as the quantity of freshwater input from glaciers. So we’re both trying to capture what’s happening now, and also asking how that can inform what might happen in the future.鈥�

The study was funded by the National Science Foundation, the Simons Foundation, and the Alfred P. Sloan Foundation. Other co-authors are Anitra Ingalls, Jody Deming, Joshua Sacks and Laura Carlson at the UW; Natalia Erazo, Elizabeth Connors and Jeff Bowman at Scripps Institution of Oceanography; and Veronica Mierzejewski at Arizona State University.

For more information, contact Dawson at hmdawson@uw.edu or Young at youngjn@uw.edu.

Like many of life鈥檚 challenges, it turns out that dental plaque is all about how you respond.听听

A team of microbiologists, immunologists and periodontists in the 天美影视传媒鈥檚 School of Dentistry are expanding upon their recent discovery that people鈥檚 gums respond to plaque with three distinct types of inflammation. The team has received a from the National Institutes of Health to better understand each of听those three responses. 听

The team discovered that people fall into three main types of responses to bacteria in plaque, including a new type of what UW microbiologist Jeffrey McLean called 鈥渟low responders.鈥� That discovery added new depth to the field鈥檚 understanding of gingivitis 鈥� the swelling, redness and bleeding in the gums that occurs when plaque builds up on and below the gumline. 听

Left untreated, gingivitis can lead to periodontitis, an irreversible condition that eats away at gum tissue and the bone that supports teeth. Periodontitis has been linked to an increased risk of heart and lung disease and other systemic diseases in humans.听

Gingivitis research could also deepen our understanding of inflammation in the rest of the body, McLean said, which can be difficult to study in real time.听听

鈥淲e think eventually, knowing someone鈥檚 responder type could also relate to their response to other things. Even, potentially, the virus that causes COVID,鈥� said McLean, an associate professor in the Department of Periodontics. 鈥泪蹿 you鈥檙e a certain type of responder, you might have that response to other viral infections, too.鈥� 听

The team will use the new grant to explore the specific mechanisms that control gingival inflammation. Researchers will identify the specific bacteria, fungi, viruses and metabolites associated with different responder types. Then they will attempt to understand what causes such vastly different inflammation responses.听听

鈥淲e don鈥檛 know if it鈥檚 your prior history, or if that鈥檚 your response type. Those are the questions we try to answer eventually,鈥� McLean said. 鈥淏y knowing there鈥檚 three major response types, we can now dig in and find out what makes them different and what鈥檚 the basis of why they鈥檙e responding differently.鈥�听

That research will rely upon the time-tested model of experimental gingivitis 鈥� the only model that allows researchers to create, and immediately reverse, inflammation in healthy human subjects. Participants will undergo a full dental cleaning, and then stop brushing several of their teeth for 21 days. As plaque builds up and inflammation sets in, researchers will take samples from both sides of participants鈥� mouths. After three weeks, participants will receive another cleaning, and the inflammation will recede.听听

Previously, scientists believed there were two types of responses to plaque below the gumline: Some people鈥檚 gums responded to plaque with strong, swift inflammation and redness, while other people鈥檚 gums had a more muted response.听听

In 2021, the researchers . They showed that some people accrue dental plaque much more slowly than others, meaning it takes longer for their gums to become inflamed. Once that inflammation kicks in, however, slow responders鈥� gums become just as inflamed as the strong responders鈥� gums. They also found unique molecular signatures in the other responder types.听

These discoveries open a path to develop treatments and products specifically designed for different response types 鈥� for example, a toothpaste that replicates the bacterial conditions found in slow responders鈥� mouths could help strong responders stave off gingivitis.听

Knowing your specific responder type might also change how you maintain good oral hygiene. Slow responders, for example, may not need to visit a dentist as often as those with stronger, quicker inflammation responses.听听

Those discoveries won the 2022 , given to the year鈥檚 most outstanding research in the field.听听听

This trial will be led by principal investigators McLean and Rich Darveau, with co-investigators Diane Daubert and Yung-Ting Hsu, all of the UW School of Dentistry. The trial will be conducted in the UW Regional Clinical Dental Research Center in the Health Sciences Building with clinical site investigators Marilynn Rothen and Mary K Hagstrom. The award, from the National Institute of Dental and Craniofacial Research, also includes collaboration with the University of Texas Health Science Center at San Antonio.听

For more information, contact McLean at jsmclean@uw.edu.听