A second dawn for photosynthesis
In freshwater pools and rivers in Germany and around the world lives a single-celled shelled amoeba with a deep evolutionary story. The organism, Paulinella chromatophora, is the only known case of a eukaryote acquiring photosynthesis through primary endosymbiosis, aside from the ancient event that created all plants and algae over 1.5 billion years ago. Sometime between 90 and 140 million years ago, an ancestor of this amoeba engulfed a cyanobacterium related to the modern genera Prochlorococcus or Synechococcus. Instead of being digested, the bacterium became a permanent resident.
This event initiated the transformation of a free-living bacterium into an integrated organelle. The cyanobacterium, now called a chromatophore, provides the amoeba with energy from photosynthesis. In return, the amoeba provides shelter and essential resources. This relationship became so absolute that the amoeba lost its ability to hunt other microbes, and the chromatophore can no longer survive on its own. This recent evolutionary leap provides scientists a living laboratory to study organellogenesis—the process of an organism becoming an organelle—which was previously only studied by looking at the ancient genetic remnants within modern plant cells.
An organelle in the making
The chromatophore is a nascent organelle, caught in the middle of its evolution. Its genome shows clear signs of this transition. While its free-living ancestor likely had a genome of around 3 million base pairs, the chromatophore's genome has shrunk to about 1 million base pairs, encoding just 867 proteins. This is a massive reduction, but still ten times larger than the genome of a typical plant chloroplast.
An important step in organelle evolution is Endosymbiotic Gene Transfer (EGT), where the endosymbiont's genes move to the host's nucleus. This has happened in Paulinella. At least 30 essential genes for photosynthesis and other functions have migrated from the chromatophore to the amoeba's nucleus. The proteins made from these genes are then imported back into the chromatophore, a complex process that allows the host cell to control its new photosynthetic machinery. The presence of some genes, like psaI, in both the nuclear and chromatophore genomes suggests this gene transfer is still happening. Unlike plant chloroplasts, the chromatophore is still surrounded by a remnant of the cyanobacterium's peptidoglycan cell wall, another sign of its more recent origin.