
Gas vesicles are intracellular gas-filled protein-shelled nanocompartments. The structures are spindle or cylinder-shaped, and typically 0.1~2 μm in length and 45~250 nm in width. A variety of prokaryotes including photosynthetic bacteria and halophilic archaea form gas vesicles in their cytoplasm. Gas vesicles provide cell buoyancy as flotation devices in aqueous habitats. They are used as nanoscale molecular reporters for ultrasound imaging for biomedical purposes. The structures in halophilic archaea are poorly resolved due to the low signal-to-noise ratio from the high salt concentration in the medium. Such a limitation can be overcome using focused ion beam-thinning or inelastically scattered electrons. As the concentric bodies (~200 nm in diameter) in fungi possess gas-filled cores, it is possible that the concept of gas vesicles could be applied to eukaryotic microbes beyond prokaryotes.
Gas vesicles are intracellular gas-filled protein-shelled nanocompartments. Their function is to provide buoyancy which allows aerophilic bacteria to float into oxygenated surface waters (Walsby, 1994). They also enable cyanobacteria to float up toward the light, stratifying in layers below the water surface (Walsby, 1994). The gas vesicles collapse and disappear when subjected to abrupt pressure increase (Ramsay et al., 2011).
Mature gas vesicles are spindle or cylinder-shaped, and typically 0.1~2 μm in length and 45~250 nm in width (Pfeifer, 2012) (Fig. 1A and B). They begin as a bicone (biconical structure) and later become the mature forms of gas vesicles (Fig. 1C). There are characteristic 4.6-nm striations or ‘ribs’ of the 7~8 kDa gas vesicle protein A that are perpendicular to the long axis of the gas vesicles. The presence of gas vesicles can reduce the volume of the cytoplasm during the later stage of gas vesicle formation, which might help cells to survive under stress conditions (Pfeifer, 2012) (Fig. 1D). Enclosed by a 2 nm-thick hydrophobic protein shell, gas vesicles exclude water by surface tension at the hydrophobic inner surface, but permit gas from outside to freely diffuse in and out of their barrier (Shapiro et al., 2014; Walsby, 1994) (Fig. 1E). Therefore, the gas within the vesicle is in equilibrium with the gas dissolved in the cytoplasm or medium (Pfeifer, 2012). But they are not considered to store gas as pressurized balloons.
Although gas vesicles are intensively studied in aquatic microbes such as the cyanobacterium
Many bacteria have been known to harbor gas vesicles in their cytoplasm. They include cyanobacteria, anoxygenic photosynthetic bacteria, cold-loving heterotrophic bacteria, spore-forming bacteria, and so on (Pfeifer, 2012). A filamentous cyanobacterium
Belonging to the Enterobacteriaceae, a gram-negative enterobacterium
Gas vesicles are frequently found in mesophilic archaea living in high-salt environments. The vesicles help the cells to rise from the bottom of the brine, where oxygen concentrations are low (Pfeifer, 2012). Originally isolated as a contaminant from salted fish in 1919,
An alternative to the cryo-FIB milling is the application of inelastically scattered electron imaging rather than the most commonly used zero-loss imaging (Kim, 2009). The high salt concentration apparently increases the frequencies of inelastic electron scattering, resulting in the position shift of the highest peak of transmitted electrons. For a higher signal-to-noise ratio in image acquisition, the slit position of an installed energy filter should be shifted toward the low or high-loss region. Such a modification in energy filtering for halophilic archaea can unravel structural details of gas vesicles hidden in poorly resolved micrographs when cryo-FIB milling is not available.
Little information is available on the occurrence of gas vesicles in eukaryotes. However, there are some candidates for putative gas vesicles found in the cytoplasm of eukaryotes. Their proposed structure and functions are similar to those of prokaryotic gas vesicles. Once referred to as ellipsoidal bodies, concentric bodies are found in a variety of fungi (Fig. 4A). They are round in shape and ca. 200 nm in diameter (Kim & Kim, 2017). The structures are composed of a gas-filled electron-transparent core and an electron-dense shell with a radiating fibrillar sheath (Fig. 4B). They have been speculated to be involved in cytoplasmic cavitation processes for desiccation tolerance by keeping desiccation-inflicted fungal protoplasts in contact with cell walls (Kim et al., 2004). It is possible that concentric bodies might function as nanobubbles (<1 μm in diameter) or gas vesicles in the fungal cytoplasm (Kim & Kim, 2017). It was conceived by freeze-etching results that concentric bodies have a gas-filled center surrounded by proteinaceous material (Honegger, 2007). However, the chemical compositions of concentric bodies were not elucidated thoroughly.
Concentric bodies appear partially similar to gas-filled liposomes synthesized
Gas vesicles have been intensively studied in the domains of bacteria and archaea. The spindle or cylinder-shaped structures are nanocompartments where the gas in the vesicles is in equilibrium with that in the cytoplasm. Apart from buoyancy-associated archetypal gas vesicles in aquatic cyanobacteria and halophilic archaea, mobility-associated gas vesicles regulated by oxygen availability and quorum-sensing mechanism have been characterized in an enterobacterium. These instances imply the prevalent occurrence of gas vesicles in prokaryotes. However, there is a scarcity of studies on the occurrence of gas vesicles in eukaryotes. Globose structures (ca. 200 nm in diameter) with gas-filled cores, commonly called concentric bodies, have been reported mostly from lichenized fungi. Considering that the gas-filled core is a requisite for gas vesicles, the concentric bodies could be referred to as putative gas vesicles in eukaryotes. Furthermore, concentric bodies are partially similar to gas-filled liposomes synthesized
This research was supported by Kyungpook National University Research Fund, 2016.
![]() | Fig. 1. Gas vesicles. (A) and (B) Transmission electron micrographs of gas vesicles in |
![]() | Fig. 2. Transmission electron micrographs of gas vesicles in |
![]() | Fig. 3. Gas vesicles in |
![]() | Fig. 4. Transmission electron micrographs of sooty molds on crape myrtle leaves. (A) Cluster of concentric bodies. M, mitochondrium. Bar=500 nm. (B) Higher magnification of concentric bodies. Bar=200 nm. (A, B) Adapted from , with permission from the publisher.
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![]() | Fig. 5. Gas-filled liposomes. (A) Transmission electron micrographs at different reaction time points. Bars=100 nm. (B) Schematic diagram of the hypothetical formation. Blue=gas core. Red=phospholipids. (A, B) Adapted from , with permission from the publisher.
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