The tiny microorganisms Prochlorococcus, which provide about 5% of the world’s photosynthesis, could be severely affected by rising ocean temperatures. New research has shown that this type of cyanobacteria is much more sensitive to warming than previously thought.
Over the course of ten years, an international team of scientists analyzed about 800 billion Prochlorococcus cells during hundreds of ocean voyages. Using the SeaFlow flow cytometer, they were able to establish that these microbes grow optimally at temperatures of 19–29°C. However, at 30°C, the rate of cell division decreases by a third, making them extremely vulnerable to predicted warming. Prochlorococcus plays a key role in tropical marine ecosystems, where there are few nutrients and almost no other microorganisms. But its “genetic minimalism” — the result of millions of years of evolution — makes it incapable of heat tolerance. “Their burnout temperature is much lower than we thought,” says Francois Ribalet from the University of Washington.
If Prochlorococcus numbers decline, they could be replaced by more heat-resistant microbes such as Synechococcus. However, this could dramatically change the marine food webs that have been tied to Prochlorococcus for millions of years.
According to forecasts, with moderate global warming, Prochlorococcus productivity will decrease by 17% in the tropics and by 10% on average globally. In the case of strong warming, losses could reach 51% in tropical regions and 37% globally. The shift in range to the poles does not compensate for the losses in equatorial waters, which threatens the entire ocean ecosystem.
