How Ocean Warming Endangers the Essential Microbe Behind a Third of Earth’s Oxygen

Earth thrives thanks to photosynthesis, which converts sunlight into energy for plants, algae, and tiny organisms like Prochlorococcus. This marine microbe is the most abundant photosynthetic organism on the planet. It contributes to nearly a third of the oxygen we breathe and serves as a crucial part of underwater food webs.

Recent studies highlight a worrying trend: Prochlorococcus may be more sensitive to rising ocean temperatures than scientists previously believed. Found in over 75% of sunlit ocean waters, these microbes thrive primarily in tropical regions. There, the water’s bright blue color indicates its low nutrient content, making Prochlorococcus one of the few life forms that flourish.

Lead researcher François Ribalet from the University of Washington noted that while these microbes are well-adapted to warm conditions, their ideal temperature range is 19 to 28 degrees Celsius (66 to 82 degrees Fahrenheit). With ocean temperatures projected to rise significantly over the next 75 years, many areas may soon exceed this range.

Past studies led many to believe that warming would benefit Prochlorococcus. However, Ribalet’s team found that when temperatures rise above 30 degrees Celsius, the microbes struggle—cell division rates drop sharply. This finding suggests that warmer isn’t always better for them, potentially affecting the entire marine food web. Less Prochlorococcus means less food for species that rely on it.

The researchers analyzed data collected from 800 billion Prochlorococcus cells during 90 research voyages over 13 years. Using advanced techniques, they explored how these marine microbes respond to temperature variations in their natural habitat rather than in controlled laboratory settings.

Interestingly, while Prochlorococcus has adapted to thrive in nutrient-poor, warm waters, this simplicity may come at a cost. Their “bare-bones” genome, stripped of extra genes, limits their ability to handle stress, raising concerns about their resilience in a warming world.

This change could create opportunities for Synechococcus, another type of cyanobacteria that can tolerate warmer conditions but requires more nutrients. If Synechococcus begins to take over, we face uncertainty about how the delicate balance of ocean life will shift.

The potential decline in Prochlorococcus is alarming. By the end of the century, their productivity could decrease by 17% in tropical regions under moderate warming scenarios and even higher in extreme cases. This means fundamental changes in ocean ecosystems.

Despite these concerns, Ribalet is cautious. The study’s methodology might overlook rare strains that could withstand heat better than their peers. He welcomes any new findings that could bring hope for these vital organisms.

This research was published in Nature Microbiology. As the climate continues to change, observing how Prochlorococcus and their competitors adapt will be crucial for understanding the future of our oceans.

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