Abstract
A few extremely halophilic Archaea (Halobacterium salinarum, Haloquadratum walsbyi, Haloferax mediterranei, Halorubrum vacuolatum, Halogeometricum borinquense, Haloplanus spp.) possess gas vesicles that bestow buoyancy on the cells. Gas vesicles are also produced by the anaerobic endospore-forming halophilic Bacteria Sporohalobacter lortetii and Orenia sivashensis. We have extensive information on the properties of gas vesicles in Hbt. salinarum and Hfx. mediterranei and the regulation of their formation. Different functions were suggested for gas vesicle synthesis: buoying cells towards oxygen-rich surface layers in hypersaline water bodies to prevent oxygen limitation, reaching higher light intensities for the light-driven proton pump bacteriorhodopsin, positioning the cells optimally for light absorption, light shielding, reducing the cytoplasmic volume leading to a higher surface-area-to-volume ratio (for the Archaea) and dispersal of endospores (for the anaerobic spore-forming Bacteria). Except for Hqr. walsbyi which abounds in saltern crystallizer brines, gas-vacuolate halophiles are not among the dominant life forms in hypersaline environments. There only has been little research on gas vesicles in natural communities of halophilic microorganisms, and the few existing studies failed to provide clear evidence for their possible function. This paper summarizes the current status of the different theories why gas vesicles may provide a selective advantage to some halophilic microorganisms.
Highlights
A small number of species of extremely halophilic Archaea of the family Halobacteriaceae (8 out of the 137 species with names with standing in the nomenclature as of August 2012) [1] are able to produce gas vesicles
Gas vesicle production is by no means restricted to the Halobacteriaceae
Halobacterium became a popular object for the study of gas vesicles, as shown by the admirable electron micrographs2even including early stereopictures of surprisingly high quality2published by Houwink in 1956 [6] and by the early physiological studies by Helge Larsen and his coworkers [7]
Summary
A small number of species of extremely halophilic Archaea of the family Halobacteriaceae (8 out of the 137 species with names with standing in the nomenclature as of August 2012) [1] are able to produce gas vesicles. Halobacterium became a popular object for the study of gas vesicles, as shown by the admirable electron micrographs2even including early stereopictures of surprisingly high quality2published by Houwink in 1956 [6] and by the early physiological studies by Helge Larsen and his coworkers [7] Since those early times many possible functions have been suggested for gas vesicle synthesis in different members of the Halobacteriaceae: buoying the cells towards more oxygen-rich surface layers in hypersaline water bodies to prevent oxygen limitation, reaching higher light intensities for the light-driven proton pump bacteriorhodopsin, positioning the cells in an optimal orientation for light absorption, light shielding, and reducing the cytoplasmic volume leading to a higher surface-areato-volume ratio. Theories proposed to explain why gas vesicles may provide a selective advantage to some halophilic microorganisms
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