Lives within lives

Life on this world takes many forms, and science sometimes discovers ways of living which are as strange and fascinating as anything one might expect to encounter on an alien planet.  Consider the case of Mixotricha paradoxa.

Mixotricha is a protozoan -- a microscopic one-celled organism (there are many species of protozoa, amoebas being another example).  Under the microscope, it appears vaguely pear-shaped, covered with about 250,000 "cilia" -- tiny hair-like growths which wave in a synchronized way to propel it through its environment.  (That environment itself is of some interest, but I'll get to that in a moment).  Many protozoa have cilia, but Mixotricha is different.  Its cilia are not really cilia.  They are separate organisms, bacteria of the "spirochete" type, long and thin and active.  These spirochete bacteria are attached to the surface of the Mixotricha by brackets and are symbiotic with it.  They have been compared to rowers propelling a ship.

You might be surprised that 250,000 bacteria could be attached to one protozoan; however, even though bacteria and protozoa are both microscopic one-celled organisms, there is a tremendous difference in size between them.

There exist on Earth two types of cells.  The "prokaryotic" type is tiny and simple, without much internal structure; the "eukaryotic" type is far larger, with very complex internal structure including a distinct nucleus.  Bacteria, and a class of similar organisms called "archaea", are prokaryotic cells.  Protozoa are eukaryotic cells.  All multi-cellular organisms -- animals (including humans), plants, fungi, etc. -- are made of eukaryotic cells.

Aside from the spirochetes, three other species of bacteria are symbiotic with Mixotricha, living on or even inside it, performing a variety of functions without which it could not survive, such as extracting energy from the nutrients which it absorbs from its environment.

(It's now believed, by the way, that eukaryotic cells first arose as symbiotic combinations of the original, simpler prokaryotic cells.  Modern animal cells contain small fuel-processing bodies called mitochondria, which have their own DNA and whose "ancestors" must have been bacteria which became symbiotic with larger cells billions of years ago and ended up being absorbed by them.  The same is true of the chloroplasts (photosynthesizing bodies) within plant cells.  Mixotricha's symbiotic relationships may resemble the arrangements which gave rise to eukaryotic cells in the first place.)

Mixotricha are not solitary creatures; they swarm through their environment in great numbers.  And each individual one of them is, as we have seen, host to a whole community of hundreds of thousands of bacteria.

And what is that environment in which these Mixotricha live?  It is the digestive tract of a termite -- specifically, a termite of a species native to northern Australia.  You probably know that termites cannot, on their own, digest the wood they eat; they depend on micro-organisms inside their digestive systems to do it for them.  Mixotricha is one such micro-organism.  (Different species of termites use different species of microscopic helpers.)

Termites, of course, are social insects, living in colonies of millions.  Most of the termites in such a colony are sterile, with the few "queen" termites functioning as egg-laying machines.  Rather than viewing each termite as an individual, it's probably more correct to think of an entire colony as a super-organism, with the "queens" being analogous to stem cells which replenish the colony's numbers to replace worker termites as they die off; and the flying termites which occasionally leave to start new colonies are the super-organism's reproductive organs, or spores.

So that super-organism consists of millions of individual termites, each of which contains countless Mixotricha in its gut, each of which in turn is host to hundreds of thousands of symbiotic bacteria.

Lives within lives within lives within lives.....