What on Earth is Mixotricha paradoxa doing?

In another blog, the termite Mastotermes darwiniensis was introduced and discussed. While interesting in its own right, living inside the gut of Mastotermes is an even more remarkable organism, Mixotricha paradoxa.

Mixotricha is a protozoan, an informal term for any single-celled, free-living or parasitic, non-photosynthetic eukaryote. Mixotricha is a type of parabasalid, one of several lineages of protozoans. Parabasalids mostly live inside multicellular animals, and many have a symbiotic (mutually beneficial) relationship with them.

A Mixotricha paradoxa cell. At the far end are four flagella. The cell is abundantly covered in fine 'hairs' or 'cilia' that it uses to move around in its termite host. Source: https://microbewiki.kenyon.edu/index.php/File:Margulis_3_gross.jpg

Mixotricha is a consummate symbiont – in fact, it's been described as the poster organism for symbiosis. This is because it's in the middle of a web of symbioses. It only lives inside its termite hosts, where it joins a community of other microbes that digest cellulose and lignin from the wood and other plant matter that termites eat. Both partners benefit: the termite can eat wood (digested by the Mixotricha), while the Mixotricha is protected, carried around, and fed by the termite. This is a classic symbiosis.

But inside Mixotricha are more symbionts, and this is where Mixotricha can do your head in. In fact, Mixotricha is a 'superorganism' with at least five genomes.

This in itself is not all that surprising. You are also a superorganism, because inside each of your cells lives symbiotic bacteria, your mitochondria, and inside your gut lives a rich microbiome of symbiotic bacteria and archaea, many of which benefit you in various ways. The world is full of symbioses.

But five genomes? It goes like this.

Genome 1 comprises the genes and other DNA in the nucleus of Mixotricha itself. These control much of its physiology and morphology. Mixotricha is more or less pear-shaped, with four flagella at its narrower end. The Australian microbiologist and taxonomist who discovered and named Mixotricha, Jean Sutherland, also described hair-like cilia covering its body. These were responsible for the name she gave to this paradoxical organism – Mixotricha means 'mixed hairs' – because no other protozoan was known at the time with both flagella and cilia.

Genomes 2 and 3 comprises the genes and DNA in two species of spherical bacteria that live inside the Mixotricha cell. These produce energy for the Mixotricha in much the same way that mitochondria produce energy in other plant and animal cells. Mixotricha have lost their mitochondria – which would be a liability inside the anaerobic gut of a termite, as typical mitochondria require oxygen for metabolism – and have secondarily formed a symbiotic partnership with these anaerobic bacteria to solve this problem.

Genomes 4 and 5 comprise the genes and DNA in two species of bacteria that almost completely cover the surface of the Mixotricha cell. One species is rod-shaped, and forms a close. regularly-arranged, armour-like covering. The second species is an elongate, helical spirochaete. These are the 'cilia' that so surprised Jean Sutherland. Mixotricha swims using its attached spirochaetes (the flagella only steer), a bit like a Viking ship with slave-rowers.

Both the rod-shaped and spirochaete bacteria are attached to bracket-like microstructures on the Mixotricha cell wall. How the Mixotricha coordinates the spirochaetes so they beat in synchrony and allow it to move in one direction rather than going around in circles is a mystery, as is the function of the rod-shaped bacteria.

Its obvious why the famous microbiologist Lynn Margulis, who did much to elucidate and understand symbiosis, spent much time studying Mixotricha.

Of course, while Mixotricha is apparently only found in the guts of Darwin giant termites, many other protozoans and bacteria live inside the guts of the other 3000 species of termites in the world, as well as in the closely related, wood-eating, social cockroaches that are termites' closest relatives. What other remarkable stories will come from these is anyone's guess.