A photosynthetic bacterium that lives inside sea squirts, providing them with sugar. It lacks a cell wall and can't survive outside its host, representing an intermediate stage in the evolution of chloroplasts from free-living bacteria.
Nhobgood Nick Hobgood, CC BY-SA 3.0, via Wikimedia Commons
A partnership in the sun
In the warm, shallow coral reefs of Palau, a peculiar partnership is on display. The sea squirt Lissoclinum patella, a type of tunicate, forms sprawling colonies that get their green color from a resident bacterium. This bacterium, Prochloron didemni, is a cyanobacterium that lives as a symbiont within the tunicate's tissues. The relationship is obligatory; Prochloron has never been successfully grown in a lab away from its host. The bacterium provides its host with a reliable food source, in some cases accounting for 100% of the sea squirt's required carbon through photosynthesis.
The cells of Prochloron didemni are unusually large for a cyanobacterium, measuring 7 to 25 micrometers in diameter. Unlike most other cyanobacteria, they contain both chlorophyll a and chlorophyll b, a trait they share with green plants and algae. This allows them to absorb a broader spectrum of light, a useful adaptation for life in the shallow, sun-drenched waters their hosts prefer. This entire arrangement allows the sea squirt to thrive in nutrient-poor tropical waters.
Ancient evolution
This symbiotic relationship shows a process that transformed life on Earth: endosymbiosis. The endosymbiotic theory suggests that certain organelles within eukaryotic cells, including the chloroplasts that power plants and the mitochondria that power animals, were once free-living bacteria. Billions of years ago, these bacteria were engulfed by larger cells, and over evolutionary time, they became permanent, integrated components of their hosts.
Prochloron shows some of the hallmarks of this transition from free-living organism to internal organelle. It is entirely dependent on its host for survival. While it was once thought that its genome was significantly reduced—a common feature in obligate symbionts—more recent analysis shows this is not the case. Its genome is around 5 to 6 megabases in size, which is not unusually small for a cyanobacterium. Its inability to be cultured outside its host points to a deep metabolic dependency. This makes the Prochloron-Lissoclinum partnership a valuable living model for understanding the early stages of the evolution that led to the complex cells of all plants on Earth.
💡Fun Facts
Sea squirts, or tunicates, are chordates, meaning they are more closely related to humans than to corals or jellyfish.
The endosymbiotic theory was first formally proposed by Russian botanist Konstantin Mereschkowski in 1905.
The cyanobacterial ancestors of chloroplasts were responsible for the "Great Oxidation Event" over 2 billion years ago, which filled Earth's atmosphere with oxygen.
Some compounds produced by *Prochloron didemni*, called patellamides, are being studied for their potential use in cancer treatments.
The Palau International Coral Reef Center is open Monday to Friday, 8:30 am to 5:00 pm. Marine environments are accessible year-round, subject to local diving regulations.
Admission
Admission fees apply for the Palau International Coral Reef Center aquarium. Access to natural reef sites typically requires a permit and may involve tour operator fees.
Accessibility
The Palau International Coral Reef Center is an accessible facility. Accessing the natural reef habitat of *Lissoclinum patella* requires boat travel and snorkeling or diving in open water.
