By James Ashworth
First published 8 September 2025
One of the world’s most common microbes isn’t immune to the effect of climate change.
Prochlorococcus produces vast amounts of oxygen, but rising ocean temperatures are predicted to cause sharp declines in its productivity.
Warming waters could impact the fundamentals of life in the ocean.
New research has looked at the impact of higher ocean temperatures of Prochlorococcus, a bacteria which carries out as much as 5% of all the world’s photosynthesis. As a result, it’s responsible for as much as 20% of oxygen in the oceans, and plays key roles in the cycle of other nutrients and minerals.
It was expected that this microbe would benefit from a hotter world, but the new study shows that its temperature limit might be much closer than thought. At temperatures above 28°C, levels predicted to be exceeded regularly by the end of the century, Prochlorococcus growth declines sharply.
The research anticipates that if global temperatures rise by around 2°C, the productivity of this crucial microbe could fall by around 17%. If temperatures rise by 4°C, it could face declines as steep as 51%.
While it’s possible that Prochlorococcus could adapt, lead author Dr François Ribalet says that these declines would cause untold changes in marine ecosystems.
“A 17-51% decline in Prochlorococcus production would translate to approximately 3-10% reduction in total oceanic oxygen,” François explains. “However, this impact is likely overestimated since other photosynthetic organisms show compensatory increases in our models.”
“What’s more concerning than the oxygen reduction are the cascading effects through marine food webs. Prochlorococcus has co-evolved with numerous other microorganisms over millions of years, and its declines could create a domino effect through the microbial communities that control fundamental biogeochemical cycles.”
The findings of the study were published in the journal Nature Microbiology.
How is Prochlorococcus affected by global warming?
The ocean is full of many enormous animals, from blue whales to giant squid, but its most important residents are some of the smallest. Microbes are the foundations of many of the food webs and nutrient cycles in the sea, making them crucial for life on Earth.
Living across 75% of the ocean surface, Prochlorococcus has one of the biggest impacts. It thrives in warm but nutrient poor waters, having lost many of its genes for tolerating environmental stress as it adapted to this habitat.
The loss of genes means the bacteria uses less energy but has a reduced ability to adapt. Measurements taken over the past decade in the Pacific Ocean have shown how Prochlorococcus’ growth is linked to the water temperature, revealing steep declines when the temperature passed 28°C.
The team then modelled what impact this would have on the microbe on a global scale as temperatures rise. While it’s likely that Prochlorococcus will spread into cooler waters closer to the poles, other areas will see the bacteria’s populations collapse almost entirely.
Overall, it’s estimated that Prochlorococcus’ productivity will decline by between 10-37% depending on how much the earth warms. However, as the team’s data comes from strains living in the Pacific, they speculate that more heat-resistant forms might exist elsewhere.
“We haven’t found heat-adapted strains of Prochlorococcus yet, but we are actively looking,” adds François.
“However, we did test how hypothetical warm-adapted strains might respond in our models. Even with a 2ºC increase in thermal tolerance, significant declines may still occur in the hottest regions.”
How is other ocean life affected?
For a lot of microbial life, declines in Prochlorococcus will have a significant impact on their own survival. For other bacteria, however, it presents an opportunity.
The microbe Synechococcus, for example, still has the genes that allow it to better respond to environmental stress. This means it can carry out photosynthesis at much higher temperatures. This should help address the shortfall caused by Prochlorococcus’s decline, but it’s not quite that straightforward.
“Our models predict Synechococcus production could increase 5-20% globally as it expands into niches vacated by Prochlorococcus,” François says. “This provides partial compensation, but Synechococcus cannot fully replace Prochlorococcus’s unique ecological functions.”
“For example, Synechococcus cells are larger with different nutritional content, so the ecosystem will function differently. It’s like replacing wheat with rice in a diet – both are grains, but they’re not the same.”
This change will have knock-on effects right the way through the ocean’s ecosystems. One group of organisms that are likely to be particularly affected are specific bacteria known as SAR11.
These are free-living marine bacteria which make up roughly a quarter of all plankton. They have a mutually beneficial relationship with Prochlorococcus, where each organism produces waste products that the other needs.
This means that if Prochlorococcus populations fall, SAR11 could decline as well. This would impact all kinds of animals that feed on this plankton, from whale sharks to oysters.
As scientists work to understand the impacts climate-driven Prochlorococcus declines, it’s more important than ever to slash greenhouse gas emissions.