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A green alga with throat- and stomach-like structures can swallow and digest bacteria when deprived of light, further bolstering Lynn Margulis's widely accepted idea that the origin of the plant-powering chloroplast was a fortuitous bout of indigestion.
Termed "Endosymbiotic Theory", the idea is that early nucleated cells called eukaryotes ate bacteria that managed to escape digestion but also couldn't escape their captors. Instead, they partnered with them. Today's mitochondria -- eukaryotic cellular engines -- and chloroplasts -- the light-harvesting machinery that turns sunlight into food for photosynthetic plants -- are believed to be the products of two early such events. Here's a great video from the American Museum of Natural History that helps explain how the new finding supports the idea:
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Here's the image of the alga caught with bacteria right in its gullet:
Caught in the act. Bacteria in the digestive vacuole of Cymbomonas. Arrowheads point to bacteria. d, digestive duct; v, digestive vacuole. p, plastid (cholorplast); m, mitochondrion; g, Golgi body. Scale bar represents 2 μm. Maruyama and Kim, 2013, Current Biology. Click image for link.
And here's a cute little image that sums it all up:
Some of you may be saying: Wait. I've heard of photosynthetic protists that can eat bacteria on the side before. How is this news? Great question.
Although it's been well known for some time that some protists -- microorganisms with a cell nuclei and specialized compartments called organelles -- can both harvest energy from light and eat bacteria depending on ambient conditions, the well-known types of these protists got their chloroplasts by subsequent endosymbioses of the descendants of the original chloroplast-capturing organism (primary endosymbionts). The primary endosymbiosis is believed to have taken place in the protist that would be the ancestor of the green algae and land plants, the red algae, and a quirky little group called the glaucophytes, a group some scientists collectively call the Archaeplastids.
These organisms, in turn became "seeds" for further endosymbioses, events called secondary or tertiary endosymbiosis. The cryptophytes (protists), the haptophytes (protists), stramenopiles/heterokonts (assorted protists, diatoms, various algae, and the organism that caused the Irish Potato Famine), and euglenophytes (protists) are believed to be beneficiaries of such higher-order engulfments. You can even see this if you look at their chloroplasts. Instead of the usual chloroplast double membrane (the inner membrane is the descendant of the captured cell's membrane, while the outer membrane is the descendant of the vacuole that originally engulfed the invader), these secondary or even tertiary endosymbionts have multiple membranes around their chloroplasts derived from the stacked ingestions.
According to Shinichiro Maruyama and Eunsoo Kim, it's been harder to spot photosynthetic algae with primary chloroplasts that also still capture and eat bacteria -- an ability that presumably would greatly improved the odds of one of any dinner items breaking out of digestive prison and cutting a deal with its captor.
As Kim says in the video, it's been suspected that Cymbomonas might have this capability based on its suspicious-looking esophagous- and stomach-like structures. Now, they have photographic proof that somehow -- and Cymbomonas wouldn't have any idea how *whistle* -- bacteria and digestive enyzmes ended up in those structures.
About that digestive system: unlike amoebas, Cymbomonas doesn't just ingest food on any old part of its body. It has a permanent, dedicated digestive system with fixed mouth, throat, and stomach equivalents. The food vacuole seems to be acidic, just like our stomach, based on its reaction to the various dies and labels the scientists used. And amazingly, the esophagus-like duct even has a "striated, muscle-like root" which Maruyama and Kim suggested could be contractile and involved in a swallowing-like movement of bacteria through the channel. Wow.
Several closely related protists in different genera also seem to possess similar digestive structures. The appearance of the structure in a wide range of early-diverging green algae, Maruyama and Kim suggest, supports the idea it was present in the last common ancestor of Archaeplastids or at least of the green algae and land plants, and that it descends from the digestive system of that organism's non-photosynthetic ancestor. Interestingly, this lineage of green algae also lacks a rigid cell wall, which means that the ancestor of today's land plants was not only sometimes carnivorous, but likely wall-less -- a universal feature of land plant cells today.
Reference
Maruyama S. & Kim E. (2013). A Modern Descendant of Early Green Algal Phagotrophs, Current Biology, 23 (12) 1081-1084. DOI: 10.1016/j.cub.2013.04.063
Further Reading
Spiegel F.W. (2012). Contemplating the First Plantae, Science, 335 (6070) 809-810. DOI: 10.1126/science.1218515