Amoeba Invents Photosynthesis, 60 Million Years Ago
An amoeba that enslaved a cyanobacterium only 60-90 million years ago, creating a second independent evolution of photosynthesis. We can watch chloroplast evolution happening in real time, billions of years after plants.
A second dawn for photosynthesis
All the green life on Earth—from the tallest redwood to the smallest speck of algae—owes its existence to a single event over 1.6 billion years ago. An ancient eukaryotic cell engulfed a photosynthetic cyanobacterium, and instead of digesting it, put it to work. This was the birth of the chloroplast, the engine of nearly all photosynthesis on the planet. For more than a billion years, it was thought to be one event, a biological revolution that happened only once.
Then, scientists looked closer at Paulinella chromatophora, a tiny shelled amoeba. Inside this protist, they found two sausage-shaped photosynthetic bodies called chromatophores. Genetic analysis confirmed the unbelievable: this amoeba had repeated history. Around 90 to 140 million years ago, a Paulinella ancestor captured its own cyanobacterium, from a lineage related to the modern Synechococcus and Prochlorococcus genera. This was a completely separate and independent primary endosymbiosis, creating a new photosynthetic lineage from scratch. The research clarifying this unique evolutionary position is centered at institutions like the University of Cologne, right here in North Rhine-Westphalia.
An organelle in the making
The chromatophore inside Paulinella shows organellogenesis—the process of an endosymbiont becoming a true organelle. It is a work in progress. The chromatophore's genome is about 1.02 million base pairs long, encoding around 867 genes. This is dramatically smaller than its free-living cyanobacterial relatives, which have genomes around 3 million base pairs, but it is still ten times larger than a typical plant chloroplast genome of 100-200 thousand base pairs.
This size difference shows the chromatophore is mid-transition. It has shed many genes but has not yet fully surrendered control to its host. The process of endosymbiotic gene transfer (EGT) is underway. Dozens of genes have migrated from the chromatophore's DNA to the amoeba's nucleus. The host cell now produces these proteins, such as subunits for photosystem I, and has even developed a novel protein-targeting system to send them back into the chromatophore. This transfer of genetic control is the step in making the relationship permanent and turning a captive into a part of the cellular machinery. The amoeba can no longer capture food and is entirely dependent on its new internal power plants.
💡Fun Facts
*Paulinella chromatophora* was first discovered on Christmas Eve of 1894 in the Rhine River by German biologist Robert Lauterborn.
The amoeba's nuclear genome contains genes acquired not just from its chromatophore, but also from other bacteria it likely used to eat, a process called horizontal gene transfer.
Unlike the robust photosynthetic systems in plants, *Paulinella* is very sensitive to high light levels and divides slowly, only about once every six or seven days.
Researchers have identified at least two distinct species of photosynthetic *Paulinella*, suggesting this new lineage began diversifying after the initial endosymbiotic event.
This POI refers to the scientific research conducted at the University of Cologne's biological institutes, which are not open to the public for general visits.
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The University of Cologne campus is generally flat and accessible.