The question of why a cell would host a cyanobacterium centers on one of the most transformative events in the history of life on Earth. Cyanobacteria are ancient, single-celled organisms classified as photosynthetic prokaryotes, meaning they lack a nucleus and convert light into energy. This capability became the foundation for a profound symbiotic relationship where two different life forms lived together to their mutual benefit. The successful merger between an ancestral host cell and a cyanobacterium fundamentally changed how life on our planet acquired energy, enabling the eventual rise of all plants and algae.
The Endosymbiotic Origin of the Relationship
The establishment of this internal partnership, known as primary endosymbiosis, is hypothesized to have occurred over a billion years ago. The process began when a large ancestral eukaryotic cell, which fed by engulfing microbes, encountered a free-living cyanobacterium. Instead of digesting the cyanobacterium for nutrients, the host cell failed to break it down, allowing the smaller cell to remain alive inside a protective vacuole within the host’s cytoplasm.
This initial incorporation set the stage for an evolutionary bond. The host cell, previously a consumer, gained the ability to produce its own food internally, while the cyanobacterium received protection and a stable environment. Over geological time, this mutual arrangement transitioned to an obligate state where neither partner could survive independently. The cyanobacterium became an integrated organelle, the chloroplast, and the host cell became the ancestor of all photosynthetic eukaryotes, including green algae and land plants.
The Central Advantage: Photosynthetic Energy Production
The core benefit of the internalized cyanobacterium is the acquisition of a self-sustaining, internal energy factory using light as its power source. This ability is photosynthesis, the metabolic process that converts light energy, water, and carbon dioxide into chemical energy, primarily sugars. This output provides the host cell with a steady supply of complex organic molecules that fuel its growth and metabolism, giving the host a direct, internal food source.
Before this event, the host cell relied entirely on consuming other organisms or organic matter, a method that was energy-intensive and subject to external availability. By contrast, the chloroplast offers an efficient, sustainable, and relatively limitless energy source, dependent only on sunlight and atmospheric carbon dioxide. The splitting of water molecules during this process also releases oxygen, which fundamentally changed Earth’s atmosphere and continues to sustain aerobic life. This shift from a purely heterotrophic consumer to a photoautotrophic producer allowed these cells to colonize new environments and diversify.
Structural and Genetic Signatures of the Internalized Cell
The original advantage of this relationship is recorded in the physical and genetic characteristics of the chloroplast. One compelling piece of evidence is the chloroplast’s surrounding double membrane. The inner membrane represents the original cell membrane of the engulfed cyanobacterium, while the outer membrane is a remnant of the host cell’s vacuole membrane from the initial engulfment.
Chloroplasts retain their own distinct genetic material, organized as a single, circular chromosome characteristic of bacteria. This independent DNA encodes proteins required for photosynthesis and replication, providing a direct link to the organelle’s prokaryotic ancestry. Furthermore, chloroplasts reproduce within the host cell by binary fission, a simple division method used by free-living bacteria. Their ribosomes are also structurally similar to those in bacteria, confirming the chloroplast’s origin from a once free-living cyanobacterium.