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From the surface of stinky mud: diatoms, their parasites, and other comrades-in-silt

The views expressed are those of the author and are not necessarily those of Scientific American.

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An advantage of having a, let’s say, ‘volatile’ memory is that you can sit back and do microscopy… on your own hard drive. When sampling is good, you have only so much time to deal with everything, so images accumulate faster than you can process them. The idea is that then soon after you take a break and go back to look over what you’ve been doing. But when sampling is good, why would you chain yourself to some stupid computer when you could easily be outside gather stuff, and looking at it? Especially at an awesome field station? (I’ve already complained that there’s simply too much great stuff to do at field stations…) Anyway, now it’s winter, and all our sampling is rather sad and frozen. Probably until May, because Spring isn’t in much of a hurry to remember this remote northeastern corner of the western hemisphere. Actually, I think Spring might have taken this year off, maybe for a sabbatical. In any case, the few protists who didn’t have the foresight to huddle inside a cyst, or go deep underground, are shivering too much to be sampled. Though I’m sure we could still find some, but we’re also shivering too much to sample. You caught me — we’re just blaming protists for our own aversion to the climate ;-)

Anyway, let’s look at some more cool stuff from the other coast. First off, a swimming cell of a peritrich ciliate — notable for appearing like flowers with a spring-like stalk. When one of those ‘flowers’ divides, the daughter cell detaches and swims around until it finds a nice surface to call home. This is one of those young swarmers. You can see the mouth, a large contractile vacuole, and bits of the large nucleus. Just beneath the cell membrane are mitochondria, visible in the 4th section.

These were sampled at low tide in a place called False Bay, which is aptly named: at the lowest tide, water recedes from the entire bay and only delicious silt remains behind. Given the treachery of the Pacific Northwest to seafarers past and present, one can imagine that a bay that turned to silt at low tide was not something seamen admired quite as much as protistologists do.

At some point on this job, you become a connoisseur of stinky muds. Like fine wine, muds come in many shapes, sizes, and smells. Each bay has its own unique regional concoction, but just as Burgundy and Alsace each boast a vast variety of different wines, the mud of each bay varies from spot to spot. You can smell it. You can feel it with your boots as you question the wisdom of venturing out so far on your own. Remember: turn the foot, don’t try to yank it out by force. Too much fine wine can lead to a hangover, too much fine mud gets you stuck.

Much depends on the size of granules, the depth, speed of current, and what gets mixed in with the mud. You get dramatically different compositions of microbes, as well as inverts, just metres away. Generally, only the top few millimetres are oxygenated — the rest is anoxic and black. On the very surface you can find blooms of photosynthetic organisms, and not just protists: purple sulphur bacteria leave a purplish tinge on the mud! The rest of the images come from the very surface of brown-coloured mud — an area teeming with countless diatoms, who then support the rest of the local ecosystem. Like this one:

Of course, one would be hard-pressed to find an all-you-can-eat buffet left untouched. Something fishy is going on inside this diatom: (as an aside, note the bacteria attached to the surface of the diatom seen in the middle — many diatoms have symbiotic relationships with bacteria)

The appearance of two nuclei suggests one of them does not particularly belong there. Plastids generally divide first (as you’ll see in a moment), and the cytoplasm is very strange-looking. This suggests something else is nibbling on the cell from within. I’ve no idea what it might be — many parasites look very similar when going about their pillaging business. In cases like this, only DNA can tell. There was another diatom with a more severe case of being-eaten-alive-from-within — maybe a later stage of the same parasite, maybe something else entirely. Note how the plastid has completely decayed away into tiny granules. This diatom is no more (and not just because I came along with my glass slides of doom).

This is the inverse of what we’ve seen before, with an amoeboid cell completely engulfing a diatom (scroll to the bottom)! If anyone has any ideas on what this might be, please let us know!

When not being nibbled on from outside or within, diatoms go about their business of multiplying. These guys usually have 2 plastids; the cell below has 4! The nucleus is presumably getting ready to split itself too.

And here’s another cell that has almost finished dividing: two pairs of plastids and a nucleus each. The glass wall (frustule) has already formed between the two cells. It’s pretty neat to catch multiple stages of a cell’s life cycle: division is usually short compared to the rest of the lifespan, so you need a fairly dense clump of cells to find some!

Enough with the diatoms. Peranema Heteronema (two emergent flagella, not one, ‘duh’…brain fart) is a non-photosynthetic relative of Euglena, the latter being notable for often being the first critter one encounters that messes with the ingrained folk biology of flora vs. fauna, as it’s both green and hyperactive. Peranema Heteronema is also impressively twisty, but has never seen colour in its entire evolutionary history. Photosynthetic pigment, that is — you don’t have to care much about the sun to be colourful, but it certainly helps. One lineage of euglenids once stole an entire green alga and kept its chloroplast and membranes, through a process called secondary endosymbiosis (A helpful diagram at the beginning of this post).

The rest live happily as voracious predators of eukaryotes and prokaryotes alike. Some even sport a rather ambitious gullet or ingestion apparatus. You can see a section of one here in the first and last images. The middle view shows the surface — faint lines of the pellicle strips and small granules on them — as well as some spent extrusomes (cellular artillery) in the bottom left corner. The rightmost view shows some unfired extrusomes as little rods that look almost bacterial, inside the cytoplasm. The large clear-ish thing is the nucleus.

And finally, just for fun: the guts of a flattened ciliate (a Nassulinid, for those who care). You can see its characteristic mouth structure in the bottom left quadrant of the cell. In principle, it looks a bit similar to Heteronema‘s, eh? These two critters are almost as distant as two eukaryotes can be, and yet again and again, convergent solutions to common problems arise. With a healthy dose of contingency, of course!

And who could have objections towards an amoeba?

Psi Wavefunction About the Author: Psi Wavefunction is a graduate of the University of British Columbia working as a protist researcher (soon to be graduate student) at Dalhousie University in Halifax, Nova Scotia, and blogs about protists and evolution at The Ocelloid as well as at Skeptic Wonder. Follow on Twitter @Ocelloid.

The views expressed are those of the author and are not necessarily those of Scientific American.

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