(Dinophysis acuminata. Photo credit: fjouenne, http://planktonnet.awi.de/)
Scientists reportedly discovered that an organism eats chloroplasts for breakfast…but doesn’t digest them until it has had the chance to use them first. The phenomenon might explain how the ancestor of all plant cells came to be over a billion years ago.
The study, published in the journal PLoS One, looked at single-celled organisms of the genus Dinophysis. Dinophysis species eat other single-celled organisms that contain chloroplasts, which are the green-pigment containing organelles that enable plants to make their own food from sunlight.
Usually, the story would end here; most organisms just digest up what they eat and that is the end of it. But researchers in the past noticed that sometimes Dinophysis does not immediately digest chloroplasts. Instead, it keeps them around.
This was an interesting observation because of an idea that is popular in biology that I will call the “endosymbiotic hypothesis.” The hypothesis is a proposed explanation for why complex cells–like human and plant cells–contain chloroplasts and/or mitochondria, while bacterial cells do not contain them.
The endosymbiotic hypothesis starts from the premise that chloroplasts and mitochondria–which make energy and food, respectively–look very much like bacteria. If you compare them under a microscope, they are around the same size. They grow and divide within our cells independently. They even contain their own DNA genomes that look like bacterial genomes.
The proposed conclusion is that simple cells that lacked chloroplasts and mitochondria could have eaten the bacterial ancestors of chloroplasts and mitochondria. Instead of digesting them, though, they could have just saved them to use as little energy factories.
This is a nice hypothesis, but to really buy it we would want to see single-celled organisms that can eat other organisms and then save their chloroplasts. So back to Dinophysis. Other scientists had already shown that Dinophysis can steal chloroplasts from their prey, but the team from Denmark wanted to track the chloroplasts in the Dinophysis cells over a long period of time. So they designed an experiment that involved growing Dinophysis in sea water and feeding them chloroplast-containing microorganisms and plenty of light. At a certain point, the researchers stopped the flow of microorganism prey but left the lights on. They did this for many days, and tracked the growth of the Dinophysis. Each time the number of Dinophysis in the sea water doubled, the scientists took a sample of them and looked at them under a microscope, taking note of the number and size of stolen chloroplasts remaining.
Keep in mind, the Dinophysis ate these chloroplasts. Based on that information, you would expect them to be digested before long. But we already knew that Dinophysis can keep the chloroplasts around for a while. The researchers were surprised, though: the chloroplasts were not just sticking around, but they were even growing and dividing in the Dinophysis! It was as though the chloroplasts had taken up residence in the Dinophysis and made it look like a plant cell.
Importantly, this is what we would expect to have happened at some point early in the evolution of complex cells. Remember, the endosymbiotic hypothesis predicts that there was once a single-celled organism that gobbled up the ancestors of chloroplasts and kept them around for a while to exploit them for a food source. The fact that we can see something like this happening in Dinophysis–and that the chloroplasts are even able to take up residence in their new host and start growing and dividing–adds evidence in favor of the endosymbiotic hypothesis.
This study does leave some unanswered questions, though. While the researchers showed some convincing evidence that Dinophysis is not digesting the chloroplasts and that the chloroplasts are in fact dividing over the course of weeks within their new hosts, they fail to show how this might be benefiting the Dinophysis. The endosymbiotic hypothesis predicts that the Dinophysis would gain a source of food from the new chloroplasts, but evidence of this in Dinophysis will have to come later.
In the meantime, this study could offer an interesting look back in time. We will never be able to turn the clock back on evolution, but experiments like this allow us to see how it may have proceeded all of those eons ago.