Scientists Prove Green Algae's Appetite for Bacteria

Research from the Museum is the first to provide definitive proof that a green alga eats 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 appeared in the June 17, 2013, issue of Current Biology, Eunsoo Kim, an assistant curator in the Museum’s Division of Invertebrate Zoology, and her colleague Shinichiro Mauyama, a post doctoral 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.

 


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Chloroplast is a specialized cell structure that converts light into food in green algae and land plants.


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The green algae used in Dr. Kim's study is from the genus Cymbomonas, which resembles early ancestors of the group.


“This behavior had previously been suggested but we had not had clear microscopic evidence until this study,” Dr. 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.

 

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This transmission electron micrograph shows bacteria-feeding behavior in the green alga Cymbomonas, with the tubular duct that transmits food highlighted.


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An illustration shows an alga with key internal structures, including the feeding channel and large, acidic vacuole (bottom right) where digestion takes place .


 

In this study, researchers used transmission electron microscopy to take conclusive images showing how a basic green alga from the genus Cymbomonas feeds on bacteria. The alga draws bacterial cells into a tubular duct through a mouthlike 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 algae 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 Earth today. 

Read more about this work here.