3.7 Square Atlas of Bacterial and Archaeal Cell Structure Home
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Gas vesicles

Some species of archaea and bacteria use gas vesicles to control their buoyancy. This can allow them to rise or fall in a water column, which can be a great advantage. Halobacterium salinarum like this one produce gas vesicles in response to cues from the environment, lifting themselves out of the sediment and into more favorable conditions of oxygen or sunlight for photosynthesis. This cell has just started producing gas vesicles, so they are small and isolated. Later, vesicles elongate into larger spindles or cylinders with conical ends, as you saw in Haloquadratum walsbyi. Each cell might contain dozens of vesicles, and they often cluster together.

Gas vesicles are microcompartments enclosed by a hydrophobic shell made of a single layer of protein. (Sometimes some additional proteins reinforce the shell.) You will see more examples of microcompartments in Chapter 4. Gas vesicles do not actively store gas; they simply allow gas dissolved in the cytoplasm to diffuse in, while forming a tight barrier against anything else, like water. They are fragile and prone to collapse with even a slight increase in the surrounding pressure.

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Square

So far we have focused on bacteria, but archaea hold their own in the specialized shape competition. In fact, one of the most extreme examples of maximizing surface area relative to volume comes from this archaeon, Haloquadratum walsbyi, which grows as thin, square tiles. Very thin, square tiles. This property helps keep them oriented with their broad sides to the sun, whose light they rely on for photosynthesis. To float at the surface of the super-salty lakes where they live, they use gas vesicles (⇩).

We still do not know exactly how this shape is determined, but at least part of the mechanism seems to involve glycoproteins on the cell’s surface layer.

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