If there’s one thing we all know about bacteria, it’s that they’re tiny. So tiny they can only be seen under a microscope. Living creatures in rainwater, ten thousand times smaller than a water-flea!
And if there’s one other thing we all know, it’s that “Bacteria, of course, have no nucleus.” Of course they don’t! Only prime-time organisms like us sophisticated human beings can be nucleus-havers!
Well, don’t look now, but bacteria are making their move.
It’s just been reported in bioRxiv (with a blurb in Science) that a type of bacterium found in Guadeloupe has a maximum single-cell length of at least 2 centimeters, is about as wide as a human hair (so it’s easily visible), and has membrane-bound nuclear structures to hold its DNA. This changes the limits we had put on bacteria, and it suggests they’re capable of much more than we’d thought.
Bacteria are pretty much our overlords already anyway, so no need for additional worry on that score. Their combined weight on Earth is around 1,000 times that of humans. (Check this out to be momentarily distracted from their dominance over us by some fun and colorful graphics.)
Despite this, we know surprisingly little about them as a whole. We still don’t know how to grow most of them in a laboratory, and that implies that there are plenty of metabolic and mechanical features of life yet to be discovered (at least by us).
The proposed new species, Thiomargarita magnifica, is about 50 times larger than any other known bacterium, and it’s also the only bacterium we know of to keep its DNA inside a membrane-bound structure. Either of those discoveries by itself would be very significant, so this double whammy really is a rare find. Game changer! Paradigm shift! And all that jazz! Microbiologists sure seem impressed:
“When it comes to bacteria, I never say never, but this one for sure is pushing what we thought was the upper limit [of size] by 10-fold,” says Verena Carvalho, a microbiologist at the University of Massachusetts, Amherst.
“Separating genetic material from everything else allows more sophisticated control and greater complexity,” says Chris Greening, a microbiologist at Monash University, Clayton.
“All too often, bacteria are thought of as small, simple, ‘unevolved’ life forms — so-called ‘bags of proteins’ — but this bacterium shows this couldn’t be much further from the truth.”
OK, so let’s take a look at them already!
There are other bacteria that can form chains of cells, but each of the filaments you see above is a single cell. Found attached to sunken leaves of the red mangrove tree, they can be up to at least 2 centimeters (nearly an inch) in length, and they only divide into daughter cells at their unanchored ends.
To illustrate just how comparatively gigantic these cells are, suppose a typical bacterium — an E. coli cell — made the acquaintance of a T. magnifica cell. It would be like you or I meeting a person who was a hundred feet wide and ten miles tall.
The reason these cells get so long is that they have lost their penchant for dividing. In fact, when we look at their genome we notice that they are not only missing some key cell-division genes that bacteria related to them have (like ftsQ and ftsA), but they also have extra copies of genes that assist in cell elongation (mreD, mrdA, and rodZ):
Some of us just don’t like to divide….
We can see what happens in other bacteria when, say, we delete the ftsQ cell-division gene:
That single change gives much longer bacteria, though not nearly as large as T. magnifica. We can also generate a bacterial cell that makes too much of the elongation protein RodZ and see what happens then:
Start adding up a few tweaks like that, and some major changes can happen. This is a good illustration of how subtle changes in genetic elements can end up having a big impact. You use the pieces you have and alter them a bit or move them around, and you can do a lot. So when you hear that chimpanzees and humans share 99 percent of their DNA, you see how much that 1 percent can mean.
Now we know how these bacteria achieve their great lengths, but they’re also much wider than typical bacteria. That’s really important because conventional wisdom says rod-shaped bacteria can’t get any wider than a micrometer or two. All of the contents of a bacterial cell have to be near the surface so it can quickly exchange nutrients and waste with its environment. Everything inside the bacterial cell can only diffuse around randomly, so if you’re a very wide bacterial cell, it takes nutrients so long to diffuse to your center that the svelte bacteria with higher surface-to-volume ratios are going to kick your butt out in the wild.
Diffusion is just soooo slow. If you put a drop of food coloring into a glass, how long is it going to take for it to diffuse all around to make the water uniformly green?
If you guessed “bloody ages”, you are correct!
So how do these tree-stump bacteria get around this problem? They use two VERY cool tricks:
1) They crowd all their living matter near the surface of the cell, and at the center is a giant reservoir of liquid
2) This giant reservoir of liquid contains a water-soluble oxygen alternative (nitrate) that they can use to breathe when there’s no oxygen around
So with 1) they essentially create a new dimension of space to use — who says you can only exchange things with the outside? And with 2) they get an advantage over their competitors by storing up boatloads of nitrate they can use to “breathe” when the oxygen runs out and everybody else suffocates.
These behemoths get energy by pulling in sulfide from their environment, ripping off its electrons, and handing them over to the nitrate stored at their centers, much as humans donate electrons to oxygen in respiration. This sets up an electric current they can use to fuel the synthesis of molecules they need to survive. They don’t need no stinkin’ sunlight, and they don’t need no stinkin’ oxygen. I mean, it’s totally brilliant.
The sulfide gets oxidized to sulfur and is sequestered in granules that give the bacteria their opalescent white glow:
If these cells haven’t impressed you already, consider also that they are the only bacteria we know of to store their DNA inside a membrane-bound organelle. This is the classic thing that prokaryotes like bacteria are NOT supposed to do. But these guys do it.
And with that, this one species just demolished its 2nd central tenet of biology.
Below you see a picture that topples this entire thing. The fluorescent yellow dye stains membranes, and the fluorescent blue dye stains DNA. What you see is localized packets of DNA, surrounded by membranes, in a bacterium. A “prokaryote” that has nuclei. So what’s a “prokaryote” now? Hmmmm…….
One other cool DNA fact about Thiomargarita magnifica is that a full-grown two-centimeter cell has about 700,000 copies of its genome! (You and I only get 2 copies of our genome per cell.) This is once again because of the huge cell size and diffusion. If T. margarita only had one copy of its genome on one side of the cell, imagine trying to use that to make RNA and protein and get it all the way around to the other side by diffusion. Can’t happen because it would take wayyyyyy too long. So they disperse their DNA everywhere throughout the cell, with franchisees all over! Kind of reminds me of an Onion article from several years ago: “New Starbucks Opens In Rest Room Of Existing Starbucks”.
Back in 2014, when the first bacteria that could be seen with the naked eye were discovered, a prescient statement appeared in Nature, and even after the discovery of Thiomargarita magnifica, it still holds true:
Bacteria so big that you can see them may be surprising. But given that most bacteria on the planet have still not been well characterized, there are probably many more surprises yet in store.
Well, we certainly have one now. An iconoclastic bacterium for the ages that may be at the forefront of a new branch of evolution for bacteria. You can love it (I do!) or be grossed out by it — but just don’t call it a prokaryote…...