May 31, 2013 | 3
Sometimes I want to show you something just because it’s wonderful. So today I’m introducing a new feature: Wonderful Things. The name is taken from my blog’s masthead, but is also inspired by Howard Carter. When he broke in to the tomb of Tutankhamun, he stuck a candle in the hole and surveyed the contents. After a pause, his patron Lord Carnarvon asked him if he saw anything. “Yes,” Carter replied. “Wonderful things.”
So here’s Wonderful Thing #1: Desmids. I stumbled on these as I was writing about the Alternation of Generations in my post a few back on mosses. With tiny, sculpted bodies and some distinctly unplantlike behavior, desmids are gorgeous botanical oddballs. The best description for them might be “chiseled” — although not the sort of chiseling that comes from protein shakes and 6-minute abs. This is more the kind that comes from guys with Italian last names and lucrative papal commissions.
Take a look:
Here’s a Micrasterias that resembles a Maltese or Greek cross:
Per Gary Larson, this appears to be yet another way nature says, “Do Not Touch”.
Believe it or not, these are all plants. Desmids are green algae, which most botanists consider to be the earliest-evolved plants. One of the ways that we can tell they are closely related to plants is they share the same photosynthetic pigments — chlorophylls a and b — and so have the same familiar green glow. Like other plants, they also build cell walls made of cellulose and store starch and chlorophyll inside double membrane-bound chloroplasts.
Desmids are dual life forms. Most cells have two pronounced lobes spanned by an isthmus, as you can see above. Their name derives from this feature; “desmos” is Greek for “bond”. Inside the bridge is the nucleus, and in each lobe is a single large chloroplast that may have lobes, plates, or other “processes” that radiate outward. A few species are colonial, occurring in chains instead of pairs. Desmids are creatures of freshwater — like peat bogs or ponds poor in minerals.
Another striking feature of these plants is that they are covered in slime. They secrete mucilaginous sheaths through pores in their walls. These gooey coatings host unusual and possibly symbiotic bacteria. In this picture of the desmid Xanthidium you can see some bacteria (I think) stuck to the transparent mucilage coat.
Not only are they covered in slime, these plants can actually move themselves around. This video of the crescent-shaped desmid Cosmarium (Isn’t it interesting desmids come in both crosses and crescents?) shows this feature. Watch just up until the two-minute mark. It gets pretty slow after that.
“Where does a plant get off doing that!?” Indeed, sir. Especially considering it has no flagella. Filaments of a common cell protein called actin (your body is loaded with this stuff) in the secretions may be responsible, according to Margulis and Schwartz, although it’s not clear to me exactly how.
I have not been able to discover the function of the barium sulfate crystals at either end, and the narrator frustratingly cuts himself off right before he’s about to give us some thoughts on the subject. However, barium sulfate crystals are heavy. I mentioned in a post earlier this year that heavy barium sulfate was used by another green alga — Chara — as a gravity sensing system, so that is possible.
Though they were moving in the last video, the crystals are not being actively propelled; rather, they are displaying “Brownian motion” (remember this from high school chemistry?), the movement of small particles in a liquid caused by their random bombardment by water molecules.
Desmids can reproduce both sexually and asexually, and both processes are fascinating to watch. If you had to guess, how do you think a desmid would go about asexually dividing? Take a few seconds right now and imagine it in your head. Now compare your imagined division to the reality. Would you have guessed this was they way they did it? (Note: video is time lapsed)
Desmids, like all other plants, can also have sex. Unlike the rest of the plants, desmids make no sperm. In fact, they don’t even bother to make gendered gametes. Instead, their cells becomes amoeba-like. The desmid parts and each half crawls out of its shell to fuse with a mate. Sometimes, only the “male” amoeba crawls into the shell of the “female”, where the zygote is formed. Here’s what the process looks like in a closely related alga (not a desmid, though) called Spirogyra.
Did you notice that the fused cells did not simply remain a giant pile of cytoplasm? Instead, they condensed into an orb-like object. This is the zygote, and it forms a spore-like resting structure that is strikingly beautiful itself. In this stunning photograph by Wim von Egmond, you can see how both desmids chucked their shells before fusing into a beautiful zygote. You can also see a zygote (sometimes called zygospore) plopped in the center of this image of desmid diversity by Ernst Haeckel. The evac’ed shells surround it. Note also the many other fantastic desmid forms.
Does the spiny zygote remind you of anything? Those who have studied fungi might pick out the uncanny resemblance to the sexual reproductive structures of fungi called powdery mildews, bane of my herb gardening efforts. Compare the desmid zygote to the pigmented, ornamented houses in which these fungi make their sexual spores — particularly Microsphaera and Podosphaera. In desmids, the zygospore is also the place where sexual cell division takes place, and algal daughter cells will emerge when the time is right.
I want to leave you with this sharp video, which gives a nice overview of the desmid Micrasterias. Watch for mucilage ooze being extruded on the left at :21. The best part, though, is the first few seconds, which shows a bunch of these little microbes in a lifelike habitat. When you look at those first few images, think about how they’re each a tiny, impossibly intricate plant just a few dozen micrometers wide.