Earth wouldn’t be the lively place it is without photosynthesis, which harnesses solar energy to fuel most of the planet’s food webs.
A variety of plants, algae, and cyanobacteria provide this service, but few do it quite like Prochlorococcus, considered to be Earth’s most abundant photosynthetic organism. Tiny even for cyanobacteria, this marine microbe wields outsized influence in its habitats and beyond, contributing nearly a third of the planet’s oxygen production and forming a vital foundation to food webs.
According to a new study, though, Prochlorococcus and its many beneficiaries may be more vulnerable to rising ocean temperatures than previously thought.
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Prochlorococcus are widespread, inhabiting more than 75 percent of sunlit surface waters. They’re most prevalent in and around the tropics, where they’re well-adapted to warm, nutrient-poor conditions.
“Offshore in the tropics, the water is this bright, beautiful blue because there’s very little in it, aside from Prochlorococcus,” says lead author François Ribalet, an oceanographer at the University of Washington.
Given this affinity for heat, some experts think Prochlorococcus may fare well as ocean temperatures continue to rise as a result of burning fossil fuels and loss of carbon sinks.
Yet the new study raises doubts, suggesting hotter isn’t always better for Prochlorococcus.
Their ideal range is 19 to 28 degrees Celsius (66 to 82 Fahrenheit), the authors report, noting many tropical and subtropical waters are forecast to exceed that upper limit within 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,” Ribalet says.
Existing data about these microbes came largely from lab-grown cells, so Ribalet and his colleagues sought new data from wild Prochlorococcus in their element.
“I had really basic questions,” Ribalet says. “Are they happy when it’s warm? Or are they not happy when it’s warm?”
To find answers, the researchers analyzed 800 billion Prochlorococcus-sized cells they encountered during 90 research voyages spanning 13 years.
They did this with a flow cytometer co-developed by Ribalet, designed specifically for detecting tiny phytoplankton like Prochlorococcus.
The researchers measured microbes using a laser in the shipboard device, then applied a statistical model based on established methods for estimating Prochlorococcus growth, all with minimal disturbance of their subjects.
Cell division rates varied by latitude, which the authors linked to variations in water temperature rather than in sunlight or nutrients.
The microbes were at their best in relatively warm water, between 19 and 28 °C, but had surprising difficulty just above that range.
Cell division slowed to a crawl in water warmer than about 30 °C, down to one-third the rate recorded in water at the low end of their tolerance range.
“Their burnout temperature is much lower than we thought it was,” Ribalet says.
Tropical seas are nutrient-poor due to their warmth, which limits the upward cycling of nutrients from deeper waters. Prochlorococcus and other cyanobacteria have adapted in a few ways, including their small size and their bare-bones genome, which is stripped down to essentials.
Despite the benefits of shedding extra baggage, this may have cost the microbes ancient genes related to stress response, potentially now limiting their resilience in the face of rapidly rising temperatures.
That could open a door for Synechococcus, the other cyanobacteria group dominating the tropics and subtropics with Prochlorococcus.
Synechococcus can handle warmer water, but needs more nutrients. If it does capitalize on any future decline in Prochlorococcus, however, we don’t know how that will affect food webs.
“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 says.
The study suggests that by the end of this century, Prochlorococcus productivity could fall by 17 percent in the tropics under a moderate warming scenario, and by 51 percent with more severe warming. Globally, it could drop 10 percent from moderate warming and 37 percent from the more extreme scenario.
“Their geographic range is going to expand toward the poles, to the north and south,” Ribalet says. “They are not going to disappear, but their habitat will shift.”
The study has limitations, the authors note, including a methodology that could mask rare heat-resistant strains. While the data did span various ocean regions, many important tropical areas are still left out.
“This is the simplest explanation for the data that we have now,” Ribalet says. “If new evidence of heat-tolerant strains emerges, we’d welcome that discovery. It would offer hope for these critical organisms.”
The study was published in Nature Microbiology.