A quiet corner of the Baltic Sea, 240 meters below the surface, is like a zombie cemetery. Within the sediments, zillions of tiny, thread-like alga called Skeletonema marinoi entered an inactive state—alive but not growing or metabolizing. The oldest one is 7,000 years old, from when people had just started growing maize, cats had just been domesticated, and the first pyramids were still a few thousand years away.
Yet, despite thousands of years of inactivity without any light or oxygen, a team of scientists revived the algae. They regained their full viability and were as good as new.
Frozen in time (and sediments)
The story begins with a scientific expedition aboard the RV Elisabeth Mann Borgese. Researchers extracted long sediment cores from the bottom of the Eastern Gotland Basin, a deep, anoxic part of the Baltic Sea. The anoxic sea is an area of the ocean where there is little to no oxygen in the water, often due to poor circulation and high organic matter decay. This lack of oxygen helped preserve the algae cells in a dormant state for nearly 7,000 years without decomposition.
In other words, these sediments hold layer upon layer of ecological history, compressed like the pages of an ancient book.
The researchers carefully selected undisturbed sediment layers. Within these layers, they isolated dormant diatom spores dating back as far as 7,000 years ago. These spores belonged to Skeletonema marinoi, a microscopic diatom alga known for blooming each spring in coastal waters. They decided to resurrect it.
The team applied a technique known as “resurrection ecology“—attempting to germinate ancient dormant cells under lab conditions.
First, they checked the algae’s fitness. The youngest group had been dormant for just three years. The oldest had slept for millennia. Yet they all seemed fit.
“It is remarkable that the resurrected algae have not only survived ‘just so,’ but apparently have not lost any of their ‘fitness’ (i.e. their biological performance ability). They grow, divide and photosynthesize like their modern descendants,” said Bolius.
Zombie algae
In tightly controlled lab settings—cold temperatures, dim light, and sterilized Baltic Sea water—the team revived nine distinct cohorts of S. marinoi.
Remarkably, almost as soon as they were exposed to oxygen and light, they began to grow. They formed chains, photosynthesized, and reproduced just like their modern counterparts. Under the microscope, they looked nearly identical to modern ones. It was like they had just taken a nap and were now waking up.
“The fact that we were actually able to successfully reactivate such old algae from dormancy is an important first step in the further development of the ‘Resurrection Ecology’ tool in the Baltic Sea. This means that it is now possible to conduct ‘time-jump experiments’ into various stages of Baltic Sea development in the lab,” says Bolius.
Resurrection ecology is a scientific approach that involves reviving long-dormant organisms—like algae or plankton—from ancient sediment layers to study how life forms functioned and adapted in past environments. By “resurrecting” these microscopic time travelers in the lab, researchers can directly compare ancient and modern organisms, effectively running experiments across thousands of years of ecological history.
Yet resurrecting these creatures is not always straightforward.
How did they survive?
Dormancy is a known survival strategy across the tree of life—from plant seeds and bacterial spores to brine shrimp eggs. But the duration matters. Older creatures had been resurrected, but mostly from land. Until now, the oldest verified resurrection of a viable, growing aquatic organism was 700 years, involving freshwater crustaceans.
“For aquatic habitats, the oldest record of a resurrected and subsequently growing species stems from the crustacean Daphnia pulicaria from a 700-year-old sediment layer of South Center Lake in Minnesota,” the researchers write in the study.
“Viable cells germinated from a initially dormant cells of diatoms (spores) and cyanobacteria (akinetes) were reported from 6600-years old anoxic Baltic Sea (Landsort Deep) and nearly 2000-years-old French lake sediments, respectively.”
While the exact survival mechanisms remain unclear, there’s evidence that S. marinoi spores aren’t entirely inactive. Studies suggest they can assimilate nitrogen and organic compounds even in the dark, anoxic conditions—barely alive, but metabolically ticking.
This study pushes the boundaries of what we thought possible. But it’s also just the beginning.
Could even older microbes be revived? Possibly. The deepest sediment layers sampled in this study date back 7,500 years, but earlier life could still be lying dormant. Other species might harbor similar powers of survival. Could we one day find even more ancient life on Earth, or maybe even on frozen moons like Europa? It’s a tantalizing idea.
For now, the quiet waters of the Baltic Sea hold a remarkable secret: life, it turns out, can press pause for thousands of years.
And with a little light, it can begin again.
