Some of the greatest migrations of living creatures on our planet occur in the oceans. Most people are familiar with the "horizontal" migrations of whales, sardines, eels, and seabirds. But other migrations occur "vertically," with creatures moving up to the surface in the daytime and retreating to darkness at night (or vice-versa).
Some of that migrating biomass consists of plankton, and some of those plankton are microplankton, which are single-celled and have no appendages. Not only do they not have fins - they don't even possess cilia to move themselves around. They are just single-cell balloons - and yet they have evolved the ability to move themselves vertically by several hundred meters in coordination with the dark/light cycle. Ask the smartest person you know how a single-celled animal can propel itself in the open ocean and see if they can postulate a mechanism.
I heard the answer to this in an episode of my favorite science podcast - the BBC's Science in Action. They offer a 30-minute summary each week of the newest developments in all of science - virology, space exploration, global warming, quarks, botany, anthropology, etc., via brief interviews with the researchers. The link I've embedded discusses this plankton migration (from the 8- to 18-minute marks).
The key mechanism is that the organism adjusts its buoyancy by inflating itself to six times its deflated size (pix embedded above of a pair of small deep-water bioluminescent Pyrocystis noctiluca, and a pair showing their adjusted size near the ocean surface).
Further investigation showed this expansion happens as a natural part of the plankton’s cell cycle. Once a single-celled plankton divides into two, an internal structure called a vacuole, a kind of flexible water tank, filters in fresh water, causing the two new cells to massively grow in size. These two daughter cells, now swelled with the lighter freshwater, sail upward. “What we realized is that this is a very clever way to essentially slingshot in the ocean during cell division,” Prakash says. “So, what happens during normal time? You’re making a lot of proteins, you have tons of sunlight, and you make a lot of biomass until you get heavier and you sink. Then, you do cell division in the deeper waters and use inflation to get back to the size of the mother.”
Note that the Pyrocystis is inflating itself with fresh water despite living in the ocean. It amazes me that a single-celled organism has basically a desalination method. And they do this without diluting their own cytoplasm. Their cell membrane has aquaporins that transport water molecules without the dissolved ions.
The links above are for the general public; the primary research was published in Current Biology.
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