Climate Change Threatens The Ocean's Smallest But Most Important Microbe

In the vast oceans, even the smallest living things play a big role in keeping marine life alive. These microscopic organisms, known as Prochlorococcus, are a type of blue-green algae, or cyanobacteria. Even though they are very tiny, they are the most photosynthesizing organisms on Earth, providing nutrients for many sea creatures, from the smallest plankton to large whales. These organisms can be found in over 75% of the ocean's surface waters.

Scientists once believed Prochlorococcus would be able to handle the warming of the planet because they are most active in warm tropical waters. However, a recent study shows this may not be true. The research warns that increasing ocean temperatures could push Prochlorococcus close to its limit, which could have serious effects on ocean ecosystems. The study was published in Nature Microbiology and funded by organisations including the Simons Foundation and the Massachusetts Institute of Technology (MIT).

Why Prochlorococcus Matters

Prochlorococcus is responsible for about 5% of the world's photosynthesis. This means it plays an important role in producing oxygen and absorbing carbon dioxide. It also supports the food chain by providing food for small sea creatures, which are then eaten by larger animals.

But new research shows that Prochlorococcus actually functions best in water temperatures between 18°C and 30°C (66°F to 86°F). When the water is warmer than 30°C, the microbes find it difficult to survive and reproduce.

The study's lead researcher, Professor François Ribalet from the University of Washington, explained that in the warmest parts of the ocean, Prochlorococcus populations are not doing well. This could mean less food and less carbon cycling for the whole marine food web, from tiny creatures to large fish and mammals.

The findings show that when the temperature rises above 30°C, their reproduction rate drops significantly, only about a third of the rate they have at 18°C. This means that when ocean temperatures become too warm, Prochlorococcus populations shrink.

Why Warm Water is Tough for Prochlorococcus

Ocean mixing is important because it brings nutrients from deep water to the surface. However, in warmer waters, ocean mixing slows down and the microbes struggle to grow. Tropical waters are often clear and blue because they have fewer nutrients, but this organism has adapted to survive in these conditions.

Prochlorococcus adapted by simplifying its genetic code. It kept only the essential genes needed to survive in warm, low-nutrient waters. However, this adaptation has a downside to it. The microbes have lost many genes that could help them tolerate heat stress. Now, with ocean temperatures rising rapidly due to climate change, Prochlorococcus finds it harder to cope than scientists had hoped.

Could Another Microbe Take Their Place?

Another type of cyanobacteria, called Synechococcus, is larger and has a more complex genome, which helps it tolerate warmer water. However, it needs more nutrients to survive. If Prochlorococcus populations decline, Synechococcus might take over some of their role. But it is unclear if this change would affect the marine food chain.

Why This Matters 

Scientists found that under extreme warming, the decline in Prochlorococcus could range between 10% and 37%, depending on how much the oceans warm. Interestingly, as temperatures rise, Prochlorococcus may expand their range towards the poles, moving north and south to cooler waters. They probably won't disappear, but their habitat will shift.

The researchers admit there are limits to their study. They couldn't sample every ocean area or every strain of Prochlorococcus. There may be heat-tolerant strains they haven't found yet and they may be able to survive warming better than expected.

Understanding how climate change affects them is crucial for predicting the future of marine ecosystems and the health of our oceans.

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