This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Perhaps you've heard of -- or even read -- the children's book "Everyone Poops". This illustrative tome explains that because everyone eats, everyone poops. Truer words, as they say, were never spoken. But this is no less true for you than for single-celled organisms like Paramecium.
The creatures in the following film are members of Paramecium caudatum, one of the species of microbial predators I blogged about last week that was capable of eating the spores of the frog-killing fungus Batrachochytrium dendrobatidis -- and one of the two species actually collected from the lakes under study. I came across this video while researching that post and thought it was interesting enough to share with you.
In the video, you can see Paramecium both pooping and peeing, although the peeing analogy is perhaps less apt because the primary function of what you will see is not nitrogen excretion. More on that in a minute. First, filmstrip time.
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Here's a quick rundown of what you just saw: at :12 is a great closeup shot; note the flickering cilia -- short beating hairs that power the cell -- at the edges of the cells, particularly at the left tip of the cell in the center.
:35 is the beginning of dramatic pooping scene. If we are lucky, this is the ultimate fate of many a nasty, frog-seeking chytrid spore. Watch the upper right corner of the Paramecium for slow movement toward the exterior. The most exciting action happens at about :45.
Finally, at :51 you see a special star-burst shaped organelle called the contractile vacuole in action. Notice how the radiating collecting tubes dump their contents into the central vacuole one by one. I wonder, do they have pressure-sensitive doors that pop open once the water inside reaches a certain volume? You and I have nothing like this structure in our bodies. What is it doing for this microbe?
As you'll recall from high school biology, single-celled organisms have a fundamental problem created by freshwater: the inside of their body is saltier than the outside, so osmosis will tend to try and stuff water inside. Unchecked, they would explode like balloons. So they need the microbial equivalent of a sump pump to constantly shove that water back outside.
In Paramecium, the solution(!) to this problem is the contractile vacuole, a starburst-shaped organ that collects water and periodically pumps it outside. Recent experiments on Parameciumsuggest it does this by actively pumping ions into the vacuole to make the inside saltier still than the rest of the cell's contents. Water flows in the same way it flows into the cell, then gets squeezed mechanically out.
This isn't quite the same thing as urination, though -- it's more like wateration. True urination is about getting rid of nitrogen (your nitrogen waste is excreted in the form of the urea in your urine; fish excrete ammonia, which requires plenty of water to safely dispose, and reptiles and birds excrete uric acid -- that white pasty stuff that lands on your car -- which requires least water of all), and in the process of doing their job, contractile vacuoles no doubt do excrete some nitrogen waste.
But, since Paramecium is a single celled-organism, nitrogen waste can also simply leave by diffusing across the cell membrane, conveniently located not too far from any given point in its body. So this Paramecium is peeing ... sort of. On the other hand, you could also argue that for these organisms, excess water is just as toxic as excess nitrogen is to us, and so the analogy is perfectly apt. Whether you are a human, a whale, or a Paramecium, we all must heed nature's call.