New study asks: what is the likely origin of chloroplasts?

Written by Shreya Ghosh

The chloroplast carries out photosynthesis in all multicellular plants and algae. A new study sheds light on the evolutionary origins of this organelle, potentially answering the question: what is the likely origin of chloroplasts?

Life as we know it would have been impossible but for the evolution of a single important metabolic process—photosynthesis. Billions of years ago, a small group of prokaryotes, the cyanobacteria, first started using solar energy to convert carbon dioxide to carbohydrates, and in the process, they created the oxygen atmosphere that fundamentally changed the redox biochemistry of living organisms. Photosynthetic products are the source of nearly all of the chemical energy that we and other living organisms sustain ourselves with today.

Unlike cyanobacteria, where photosynthesis takes place in extensions of the plasma membrane, land plants and algae use a specialized organelle, the chloroplast, which serves as a molecular factory that carries out photosynthesis. While scientists believe that the chloroplasts from all extant species of plants and algae (with a single exception) share a common ancestor, how and when the first photosynthetic eukaryotes arose remains unknown.

“We know that the chloroplast at some point was a free-living cyanobacteria,” said Patricia Sánchez-Baracaldo from the University of Bristol, who led a study that comprehensively explored the origins of this mysterious organelle. She and her team performed sophisticated molecular clock analyses on sequence data from 119 different taxonomic groups of photosynthetic plants as well as cyanobacteria.

The chloroplast has an evolutionary history similar to that of mitochondria—endosymbiosis and adaptation. A long time ago, a eukaryotic cell engulfed a cyanobacterium, and the two established a mutualistic symbiotic relationship. Over time, as the cyanobacteria adapted to living inside its host, it shed most of its genetic material and became the chloroplast. However, many questions about this evolutionary process remain , including when this endosymbiotic event occurred, the environmental conditions under which it occurred, and the bacterial lineage of the cyanobacterium that became the chloroplast.

Sánchez-Baracaldo and colleagues’ analyses showed that the closest relative of the chloroplast was a freshwater cyanobacterium called Gloeomargarita. The ancestor of the chloroplast and Gloeomargarita diverged about 2.1 billion years ago, which is relatively early in the evolutionary timeline of the cyanobacterial lineage. The researchers also found that land plants and algae share a common ancestor that lived around 1.9 billion years ago. Interestingly, the environment that this ancestor evolved in was possibly a low-salinity environment—suggesting freshwater or terrestrial habitats.

This study provides a new and detailed perspective on the origins of photosynthesis in eukaryotes. “It puts in the picture how biological and genomic studies can contribute to our understanding of the carbon cycle,” said Sánchez-Baracaldo.