Wednesday, November 4, 2015

Breaking the Size Barrier – Giant Bacteria, part 1

If you double the size of a cell in each direction, the volume 
increases eight fold. This makes take eight times longer 
for a molecule to diffuse through the whole cell.
Last week we talked about how the reactions that must take place inside cells often limit the maximum size of bacteria. Because important molecules can reach every part of the bacterial cell only by diffusion, the organism can’t have too large a volume. At the same time, the bacterium needs as much surface area as possible for important molecules to diffuse into the cell. This means that they need a high surface area: volume ratio. We showed last time that if you double (2x) the length of a bacterium in three directions, then the volume is increased eight fold (8x). This would result in cubing (23=8) the mixing rate and traffic time as well. If the size of a bacterium was increased from a typical size of 1 µm to a theoretical 100 µm bacteria, it could take almost a day for two molecules to find one another (traffic time). It wouldn’t seem plausible that bacteria this size could remain alive.

HOWEVER, I want to show you two bacteria that have found ways around this size limitation. Even more impressive (and a sign of how inventive nature can be), each of these organisms has found a different way to beat the system. Our two examples are the two largest prokaryotes known, and can be seen by the naked eye. This is really something considering that we can’t see our own cells without a microscope.

Our first size offender is called Thiomargarita namibiensis (T. namibiensis). The thio- part of the name means that this is a sulfur oxidizing bacterium, while the last part of its name records that it was first found on the ocean floor just off the coast of the African country, Namibia. Sulfur bacteria change elemental sulfur (S0) into sulfur oxides (SO2-4). These reactions release enough energy to make ATP (the chemical energy of the cell). In order to carry out these oxidation reactions, some sulfur bacteria use nitrate as an electron acceptor during ATP production. This works out just fine when there is a lot of nitrogen present in the immediate environment, but at the bottom of the ocean this is not always the case. Most of the nitrogen comes within reach of the bacterium only after a storm disturbs the ocean floor.

Our “sulfur pearl of Namibia” bacterium (arrow) is as big 
as the head of the fruit fly. To compare, each 
eye of the fruit fly contains over 16,000 cells!

Therefore, T. namibiensis must scavenge as much nitrogen as possible and store it within a large central vacuole (a membrane bound sac) for the lean times. It also stores sulfur in smaller granules, leading to a speckled pearl-like appearance over the clear nitrogen vacuole (which explains the middle part of name, margarita = pearl. Often, these bacteria stick together in a line and look like a string of pearls).

T. namibiensis is a spherical bacterium. Round cells are least well equipped for good mixing and traffic times; the center is far from any cell surface. But if the cell was flattened out or narrow in one dimension the traffic times could be reduced, even if the organism was larger. For this reason, many bacteria are not round, but perhaps rod-shaped or flattened rhomboids. Here we see that T. namibiensis is huge (up to 750 µm) while still spherical. That size makes it just about the size of the period at the end of this sentence; not much compared to a beach ball, but 3 million times the volume of a typical spherical bacterium.

T. namibiensis usually occurs in chains of ten or so bacteria, with pearlescent sulfur granules as shown in the left image. In cross-section on the left, you can see both the thin band of cytoplasm and the large nitrogen-containing vacuole.

The first key to Thiomargarita’s size is that large central vacuole of nitrogen. As shown in the righthand photomicrograph (courtesy Woods Hole Oceanographic Institute), there is only a thin layer of cytoplasm (the essential, viscous, water-based medium that fills the cell) between the vacuole and the cell membrane. The vacuole itself consumes almost 98% of the total cell volume. This small layer of cytoplasm means that all the important molecules are close to the surface through which they diffuse; therefore, the large size of the cell does not violate any limitations placed on its mixing rates or traffic times. While the size of the bacterium is huge, the distance any one molecule has to travel is still small. In fact, the amount of cytoplasm in T. namibiensis is just about the same as in a normal sized bacterium.

The large diameter of T. namibiensis also helps it survive in two ways that are less evident. One advantage has to do with the diffusive boundary layer. Because of the natural friction between all molecules, there is always an area next to any surface where the flow of liquid is reduced to near zero. Reduced flow means reduced numbers of important molecules can be picked and carried; therefore, the concentration of important molecules is reduced, a bad thing for bacteria trying to survive. However, because of the huge size of T. namibiensis, much of the cell sticks up above the sea floor’s diffusive boundary layer, into the area where diffusion can be more productive.

The second survival advantage is slightly more straightforward. T. namibienisis and other megabacteria are just too big to be bothered by predators. T. namibiensis doesn’t have to worry about being eaten, because no bacterial predator is big enough to “swallow” it. This is similar to the ancient sauropod species, like Brachiosuarus or Diplodicus, which had no predators once they grew to adult

Just like a T. Rex couldn’t bring down or swallow
a brachiosaur, a normal bacterium (the white dot
in the top right hand corner) can’t eat T. namibiensis.
size – a healthy sense of self-preservation would keep any T. Rex from trying to eat an adult brachiosaur.

We have seen that limitations on bacterial size imposed by diffusion can be overcome if natural selection results in some advantageous characteristic and if there is a reproductive advantage to be being big. The development of a central vacuole permitted T. namibiensis to become bigger, and being bigger provided an advantage for survival on the sea floor. It seemed designed to end up just so, but remember that evolution is not purposeful. It is merely a series of random changes and random environmental changes that render some characteristic advantageous, disadvantageous, or moot.

Next time we will look at another giant bacterium. This second rule-breaker has a completely different solution to the diffusion/size limitation. Just as we highlighted with the nylon metabolizing bacteria a few weeks ago, nature can find an infinite number of ways to overcome a single problem. It just takes random mutation (a change), environmental pressure (a need for the change) and time (for the reproductive advantage afforded by the change to have an effect on the population).

Schulz, H., & Jørgensen, B. (2001). Big Bacteria Annual Review of Microbiology, 55 (1), 105-137 DOI: 10.1146/annurev.micro.55.1.105

Girnth, A., Grünke, S., Lichtschlag, A., Felden, J., Knittel, K., Wenzhöfer, F., de Beer, D., & Boetius, A. (2011). A novel, mat-forming Thiomargarita population associated with a sulfidic fluid flow from a deep-sea mud volcano Environmental Microbiology, 13 (2), 495-505 DOI: 10.1111/j.1462-2920.2010.02353.x

For more information on surface area: volume, sulfur bacteria, and T. namibiensis, please see below:

Cell surface:volume laboratories:

sulfur bacteria:


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