Gas vesicles are a type of cellular organelle contained in some prokaryotic organisms. They are not true membrane bound organelles since prokaryotes are characterized by the absence of membrane bound organelles. Gas vesicles are composed of entirely of protein instead (no lipids or carbohydrates have been detected).
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Function
Gas vesicles occur primarily in aquatic organism as they are used to modulate the cell's buoyancy and modify the cell's position in the water column so it can be optimally located for photosynthesis or move to locations with more or less oxygen.
Structure
Gas vesicles are generally lemon-shaped or cylindrical, hollow tubes of protein with conical caps on both ends. The vesicles vary most in their diameter. Larger vesicles can hold more air and use less protein making them the most economic in terms of resource use, however, the larger a vesicle is the structurally weaker it is under pressure and the less pressure required before the vesicle would collapse. Organisms have evolved to be the most efficient with protein use and use the largest maximum vesicle diameter that will withstand the pressure the organism could be exposed to. In order for natural selection to have affected gas vesicles, the vesicles's diameter must be controlled by genetics.
Although genes encoding gas vesicles are found in many species of haloarchaea, only a few species produce them. The first Haloarchaeal gas vesicle gene, gvpA was cloned from Halobacterium sp. NRC-1. 14 genes are involved in forming gas vesicles in haloarchaea.
The first gas vesicle gene, gvpA was identified in Calothrix. There are at least 2 proteins that compose a cyanobacteria's gas vesicle: GvpA, and GvpC. GvpA forms ribs and much of the mass (up to 90%) of the main structure. GvpA is strongly hydrophobic and may be one of the most hydrophobic proteins known. GvpC is hydrophyllic helps to stabilize the structure by periodic inclusions into the GvpA ribs. GvpC is capable of being washed out of the vesicle and a consequential decreases in the vesicle's strength. The thickness of the vesicle's wall may range from 1.8- 2.8 nm. The ribbed structure of the vesicle is evident on both inner and outer surfaces with a spacing of 4-5 nm between ribs. Vesicles may be 100-1400 nm long and 45-120 nm in diameter.
Within a species gas vesicle sizes are relatively uniform with a standard deviation of ±4%.
Growth
It appears that gas vesicles begin their existence as small biconical (two cones with the flat bases joined together) structures which enlarge to the specific diameter than grow and expand their length. It is unknown exactly what controls the diameter but it may be a molecule that interferes with GvpA or the shape of GvpA may change.
Role in ultrasonic algaecide
Ultrasonic algae killing devices mode of action in blue-green algae is likely rupturing the cell's gas vesicles and causing the cells to lose buoyancy and sink to the bottom where they can no longer photosynthesize.