Study Reveals Important Step in Photosynthesis Evolution

This transmission electron micrograph shows bacteria-feeding in the green alga Cymbomonas. In the cross-section features a large vacuole (v) containing bacterial cells. It also shows the tubular duct (d) that transports food into the vacuole. Other structures pictured are plastids/chloroplasts (p), a mitochondrion (m), and Golgi bodies (g). The scale bar represents 2 micrometers. Credit: Shinichiro Mauyama

New research out of the American Museum of Natural History in New York is the first to provide definitive proof that green algae eat bacteria.

The finding, captured with electron microscope images, offers a glimpse at how scientists think early organisms acquired free-living chloroplasts, the structures responsible for converting light into food. This event is thought to be a critical first step in the evolution of photosynthetic algae and land plants, which helped raise oxygen levels in Earth’s atmosphere and paved the way for the rise of animals.

In a paper that appears in the June 17 issue of Current Biology and is available online, Eunsoo Kim, an assistant curator in the Museum’s Division of Invertebrate Zoology, and her colleague Shinichiro Mauyama, a postdoctoral researcher at Japan’s National Institute for Basic Biology, identify a mechanism by which a green alga that resembles early ancestors of the group engulfs bacteria. Their work provides conclusive evidence for a process that had been proposed but not definitely shown.

“This behavior had previously been suggested but we had not had clear microscopic evidence until this study,” Kim said. “These results offer important clues to an evolutionary event that fundamentally changed the trajectory of the evolution of not just photosynthetic algae and land plants, but also animals.”

In green algae and land plants, photosynthesis, or the conversion of light into food, is carried out by a specialized cell structure known as a chloroplast. The origin of chloroplast is linked to endosymbiosis, a process in which a single-celled eukaryote—an organism whose cells contain a nucleus—captures a free-living photosynthetic cyanobacterium but does not digest it, allowing the photosynthetic cell to eventually evolve into a chloroplast. The specific feeding mechanisms for this process, however, have remained largely unknown until now.

In this study, researchers used transmission electron microscopy to take conclusive images showing how a basic green alga from the genusCymbomonas feeds on bacteria.

The alga draws bacterial cells into a tubular duct through a mouth-like opening and then transports these food particles into a large, acidic vacuole where digestion takes place. The complexity of this feeding system in photosynthetic modern alga suggests that this bacteria-feeding behavior, and the unique feeding apparatus to support it, descend from colorless ancestors of green algae and land plants and may have played important roles in the evolution of early photosynthetic eukaryotes, the precursors to plants like trees and shrubs that cover the Earth today.


The green alga used in this study and shown here is from the genus Cymbomonas, which presumably resembles early ancestors of the group. The scale bar represents 10 micrometers. Credit: AMNH/E. Kim
This illustration demonstrates the bacteria-eating behavior of green alga and the eventual evolutionary path it created for photosynthetic algae and land plants. Credit: AMNH/E. Kim

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