The phytoplankton of the oceans forms the base of marine food chains and earthly oxygen production. But one of the most important representatives of these marine oxygen producers, the cyanobacterium Prochlorococcus, could massively impair the climate change, as researchers have discovered. In more than 100 measurements through different areas of the Pacific, they found that Prochlorococcus grows and thrives at water temperatures of up to 28 degrees rapidly. With even greater heating, its increase breaks down drastically. This critical temperature threshold could be exceeded in tropical and subtropical seas by climate change by 2100. As a result, the dominant oxygen producer of these octopuses could decrease by 17 to 51 percent depending on the extent of future warming, as the team reports.
The cyanobacterium Prochlorococcus Marinus is the smallest and most common photosynthetic plankton organism on earth. The only 0.5 to a micrometer small unicella occurs in 75 percent of the sea water that is close to the surface worldwide and is responsible for around a fifth of all earthly oxygen production. This cyanobacterium dominates especially in subtropical and tropical sea areas. “It contributes to almost half of the entire phytoplankton biomass there,” explain François Ribalet from the University of Washington in Seattle and his colleagues. The tiny blue maiden benefits from the fact that it thrives well in warm water, but at the same time needs only a few nutrients for their growth. Because especially in warmer sea areas, sea water is often very stable. When the plankton then defines the nutrients of the upper layer, little fulfills from cooler, deeper layers of water. “The sea in the tropics is so beautifully blue away from the coasts because there is little alive there except Prochlorococcus,” explains Ribalet. The cyanobacterium has adapted to the nutrient -poor conditions, in which it has reduced its size for millions of years and reduced its genome except for the bare essentials.
Measurement trips over 200,000 kilometers
However, despite the great importance of Prochlorococcus for the subtropical and tropical oceans and their photosynthetic oxygen production, it has so far been unclear how this cyanobacterium will react to climate change. “Our knowledge of how small phytoplankton reacts to future sea temperatures are largely based on laboratory tests with model organisms,” the researchers explain. “But this cannot recap the complexity of the interactions and its genetic drives.” That is why they have carried out 90 measurements over a total of 200,000 kilometers in the Pacific for around 14 years since 2003, on which they examined the micro-phytoplankton and in special Prochlorococcus in different sea areas and at different water temperatures. Ribalet and his team took water samples from three to eight meters of water depth and thus from the light -flooded water layers that are crucial for phytoplankton and photosynthesis. With the help of an flow cytometer, they automated how many Prochlorococcus cells were included in the water samples. In total, the team covered more than 800 billion cells of this cyanobacteria.
“My basic question was: Prochlorococcus is doing well when it is warm or not?” Explains Ribalet. The comparative analyzes showed that the density of the cyanobacterium and the propagation rate determined from the data with the help of a model depends primarily on the water temperature. However, unlike assumed, Prochlorococcus can quickly become too warm: “The analyzes revealed that the maximum division rates initially increase exponentially with increasing temperature until a threshold of around 28 degrees is reached,” report Ribalet and his colleagues. However, there is a drastic slump above this temperature. “The increase in the cyanobacterium reduces up to 31 degrees to only a third of the value at 19 degrees,” said the team. Even in relatively nutrient -rich regions of the Indian ocean, where Prochlorococcus reaches particularly high propagation rates, this rapid waste was shown as soon as the sea temperatures rose over 28. The population density also decreased with the propagation rates. The researchers observed similar things in additional laboratory tests with 13 different Prochlorococcus tribes: they also showed a clear upper limit of their temperature tolerance area. “This temperature threshold is therefore far lower than we thought,” says Ribalet.
Acceptance of productivity by 17 to 51 percent
This data suggests that the important marine oxygen producer Prochlorococcus is more sensitive to heat than long assumed. “Unlike coastal -close and higher widths of phytoplankton, which is used to substantial temperature fluctuations and has a broad temperature tolerance, Prochlorococcus shows a specific susceptibility to high temperatures,” explain Ribalet and his colleagues. They also attribute this to the fact that this cyanobacterium has reduced many genes important for adaptation in the course of its evolution. According to the new findings, the warming of the oceans could also have significant consequences for oxygen production in the sea and the marine food chains. According to climate forecasts, many tropical and subtropical sea areas could exceed the 28-degree threshold of the water temperature in the near future, at least in the near future. Ribalet and his colleagues have examined how this will have a specific impact on Prochlorococcus using a three -dimensional numerical ecosystem model. In this, they simulated the multiplication and population density of the cyanobacterium up to 2100 under the moderate climate change scenario RCP4.5 and will react with largely unchecked climate change (RCP8.5). In the temperate scenario, the sea surface temperatures increase by 1.6 degrees, and with average with strong climate change.
The simulation showed: “Until 2100, the productivity of Prochlorococcus in tropical regions will drop by 17 to 51 percent depending on the heat scenario,” report Ribalet and his team. “This will have a particularly strong effect in regions such as the West Pacific Heat Pool, for which our model predicts an almost complete collapse of the Prochlorococcus populations.” However, the model also predicts that the distribution areas of the cyanobacterium are also significantly shifted. This phytoplankton will increasingly shift into cooler subtropical and moderate sea regions of the tropics that have become too hot. “Despite the expansion of the habitat, the global productivity of Prochlorococcus will decrease by 17 percent in the moderate climate scenario, with 38 percent warming,” the researchers write.
“Caskading effects on the marine food chains”
But what does this mean specifically for the sea environment? In theory, it would be conceivable that other phytoplankton types compensate for these losses at least in part. As the team determined, for example, the second most common cyanobacteria in tropical and subtropical sea areas, Synechococcus, increases with rising sea temperatures – without the break -in at 28 degrees water temperature. However, Synechococcus is larger and requires more nutrients, which means that some marine areas would be unsuitable for him. “And if Synechococcus takes over, it is not yet said that other organisms can interact with him in the same way as they have been doing with Prochlorococcus for millions of years,” says Ribalet. The loss of Prochlorococcus could have cascading effects on the marine food chains because this cyanobacterium takes up a special ecological niche that no other organism can occupy in the same way.
However, the research team also admits that many aspects of the future development of this cyanobacterium and the rest of the marine phytoplankton have not yet been adequately examined. It is currently unclear whether there is a tribe of this cyanobacterium somewhere in the tropics, which has developed higher heat tolerance. “If this should be shown, it would be very welcome, because then there would be hope for this crucial organism group,” says Ribalet.
Source: François Ribalet (University of Washington, Seattle) et al., Nature Microbiology, DOI: 10.1038/S41564-025-106-4
