Cryptomonads: solar-powered armoured battleships

I've been 'scoping around some pond water lately and came across some relatively big cryptomonads (g. Cryptomonas, I think). Cryptos aren't all that rare, but most of them whirl about rather hyperactively, rendering them as troublesome photo subjects. This specimen, on the other hand, had a convenient habit of pausing every once in a while to have its picture taken. Finally, I have my own cryptomonad shots!

Cryptomonas(?) sp. The cell is about ~30µm long, pretty big for a cryptomonad. On its right side the cryptomonad has a furrow – or, in some species, an tube-like gullet – lined with ejectisomes (particularly visible in the top right image). The vesicle at the anterior tip of the cell is its contractile vacuole. Refractile stuff is the starch granules. 40x objective, DIC

Despite their small size and superficially generic algal appearance, cryptomonads do have quite a few awesome bits about them. From an evolutionary standpoint, they have pretty damn awesome plastids – products of secondary endosymbiosis of red algae, complete with a shrunken relict nucleus ("nucleomorph") of the red algal ex-host! The plastids also have four membranes, complicating the delivery of plastid-targetting proteins from the cryptomonad host nucleus. But I'll save that story for some other time, and instead keep it superficial. Literally: it has ejectile things lining its surface, and who doesn't like the idea of a microscopic solar-powered hyperactive battleship?

Prior to embarking on some battle scenes, lets look around the ship's anatomy a little bit mostly as an excuse to show off a diagram. At its fore we have a pair of flagella, lined with little hairs – also a characteristic of many Alveolates and Stramenopiles, with whom Cryptomonads might share the secondary red algal symbiosis event with. Much of the cell is occupied with a single plastid, making the fucker a bit difficult to diagram. In all his/her/its infinite wisdom, the designer apparently failed to take into consideration the future pains of this student attempting to tame the wild beast that is Illustrator while drawing this cell. Asshole. Besides the plastid, there's also a single mitochondrion and a bunch of other small crap that a eukaryote ought to have. The plastid's outermost (fourth) membrane is contiguous with the endoplasmic reticulum system, presumably homologous to the original digestive vesicle that enveloped the 'enslaved'* red alga. The third membrane derives from the red algal cell membrane, whereas the inner pair are the usual plastid membranes. Pop quiz: where would you expect to find the relict endosymbiont's nucleus (the nucleomorph)? (Answer at the bottom of the post, or in the diagram if you're so inclined to 'cheat' ;p)

*Google [scholar] "Cavalier-Smith" and "enslaved". When he likes certain words, he really likes them.

Back to the surface. The cryptomonad surface is quite complex, consisting of an inner and surface periplast layers separated by the cell membrane. Sometimes the surface layer can be be covered in scales, sometimes fibrous matter. This periplast is perforated with pores for ejectisomes, much like battlements on a warship. Ejectisomes themselves consist of coiled proteinaceous ribbons that extend forcefully upon firing.

Cryptomonad periplast. IPC – inner periplast layer, PM – plasma membrane, S – scales (of the surface periplast layer). On the right is a freeze fracture EM of the plasma membrane, which shows imprints of the surface scales (vaguely hexagonal) and pores for ejectisomes (E). In other words, the surface of an armoured warship with battlements. (Brett & Wetherbee 1986 Protoplasma)

Ejectisomes – more generally, extrusomes – are not all that unusual in the protist world. Many ciliates are loaded with menacing trichocysts and green algae like Pyramimonas are not afraid to fire similar structures either. Some bacterial endo- and episymbionts also bear similar coiled structures, but that's a topic for some other day as well. Extrusomes can also be used more locally to glue prey to the organism – if you, upon finding yourself shrunk to microns, accidentally bump into a frail-looking centrohelid heliozoan, be afraid. Be very afraid. It will smother you in adhesive proteins from the extrusomes lining its fragile-looking axopodia and devour you alive and possibly paralysed.

Ejectisomes in cryptomonads and their non-photosynthetic close relatives, katablepharids. Pyramimonas is only distantly related, and probably evolved its ejectisomes completely independently. (Kugrens et al. 1994 Protoplasma: nice review on protist ejectisomes in general, excluding ciliates)

One of the poor cryptomonads got stuck as my slide was drying out, and in its agony, released an explosion of ejectisomes. As any other biologist excessively attached to their subjects, I hate seeing protists die; at least this one didn't die in vain but gave us a nice demonstration. Extrusome firing often accompanies stress in protists that have them, drying out definitely qualifying. The following images are quite graphic, and not for the faint of heart. At least because the image quality is seriously compromised by a random layer of air between the coverslip and the specimen covered with remnants of water – a total chaos of refraction indexes.

Lysed cryptomonad on a dried out slide, surrounded fired ejectisomes. The fibrils around the cryptomonad remains are the uncoiled ribbons propelling the ejectisomes (refractile granules seen well in phase contrast, bottom images). 40x obj, DIC and PC.

While the cryptomonad may use its ejectisomes for hunting (most photosynthetic unicellular protists tend to be predators as well), perhaps they play a larger role in defense. Partly in stabbing its own predators, but additionally in a way that's quite counterintuitive to large creatures like us – sudden movement.

You might notice there isn't really much projectile action per se happening at the microbial scale. The firing is closer to an extrusion of a structure rather than freely propelling it a far distance. Furthermore, unlike an actual battleship, the cryptomonad can stop and turn almost instantaneously, and doesn't have much inertia. There is a reason for that, and it lies in the physics of fluid dynamics, a topic few of us outside biophysical biology concern ourselves with. Luckily, Purcell took care of that for us in his rather interesting 1977 paper, "Life at low Reynold's Number*" – turns out, the effect of viscosity on the behaviour of an object depends on its size, and water from a microorganism's perspective is a very different substance than what it is to us. In fact, it helps to imagine that microbial creatures live in honey or molasses – while water's viscosity doesn't actually change, it acts on µm-size things in a manner somewhat similar to how highly viscous fluids would act on things of our scale. Biophysics is quite a bit different at that scale, and different strategies are required in dealing with it.

*Reynold's number = proportion between object's velocity*size*[fluid density] and the fluid's viscosity)

In highly viscous fluids, coasting is not really an option. Things stop as soon as the driving force ceases to be applied, as anyone who's paddled a canoe across a lake of molasses would know (Bostonians from the early 1900's, perhaps?). This is why you don't really see stiff fins on bacteria or single-celled eukaryotes, at least not for motility itself. There are many ways to use a flagellum – a topic deserving of its own post – the beating strategy requiring it to be flexible at the right times. More importantly to our topic, you can't realistically give something enough force for it to keep moving like a bullet, so shooting things is out of question. Instead, the projectile must keep being pushed, usually by something unfolding or unraveling – in the case of the cryptomonad, a coiled protein ribbon. Cryptomonad artillery is perhaps more similar to harpoons than cannons.

This means a fired ejectisome can be used to essentially "push off" in the opposite direction, providing the organism with a sudden, drastic movement it wouldn't be able to obtain by flapping its flagella. The armoury of a threatened cryptomonad may be more important in providing it with rapid escape than damaging its pursuers. The microbial art of war is seldom discussed in non-enzymatic terms, but it is too a diverse and fascinating area, peppered with counterintuitive surprises. Life, and war, are indeed very different at low Reynold's numbers.

References
Brett, S., & Wetherbee, R. (1986). A comparative study of periplast structure inCryptomonas cryophila andC. ovata (Cryptophyceae) Protoplasma, 131 (1), 23-31 DOI: 10.1007/BF01281684

Kugrens, P., Lee, R., & Corliss, J. (1994). Ultrastructure, biogenesis, and functions of extrusive organelles in selected non-ciliate protists Protoplasma, 181 (1-4), 164-190 DOI: 10.1007/BF01666394

Purcell, E. (1977). Life at low Reynolds number American Journal of Physics, 45 (1) DOI: 10.1119/1.10903

Answer to the nucleomorph scavenger hunt: between the third (red algal) and second (plastid outer) membranes. The nucleus was originally in the cytoplasm, within the red algal cell membrane and outside the plastid. Oh, and if you want real topological clusterfuck, may I recommend the tertiary endosymbiosis in Kryptoperidinium – also try to count the genomes!