A range of bacteria and archaea produce intracellular gas-filled proteinaceous structures that function as flotation devices in order to maintain a suitable depth in the aqueous environment. The wall of these gas vesicles is freely permeable to gas molecules and is composed of a small hydrophobic protein, GvpA, which forms a single-layer wall. In addition, several minor structural, accessory or regulatory proteins are required for gas vesicle formation. In different organisms, 8-14 genes encoding gas vesicle proteins have been identified, and their expression has been shown to be regulated by environmental factors. In this Review, I describe the basic properties of gas vesicles, the genes that encode them and how their production is regulated. I also discuss the function of these vesicles and the initial attempts to exploit them for biotechnological purposes.

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A fluorescence-based live-cell adhesion assay was used to examine biofilm formation by 20 different haloarchaea, including species of Halobacterium, Haloferax and Halorubrum, as well as novel natural isolates from an Antarctic salt lake. Thirteen of the 20 tested strains significantly adhered (P-value < 0.05) to a plastic surface. Examination of adherent cell layers on glass surfaces by differential interference contrast, fluorescence and confocal microscopy showed two types of biofilm structures. Carpet-like, multi-layered biofilms containing micro- and macrocolonies (up to 50 μm in height) were formed by strains of Halobacterium salinarum and the Antarctic isolate t-ADL strain DL24. The second type of biofilm, characterized by large aggregates of cells adhering to surfaces, was formed by Haloferax volcanii DSM 3757T and Halorubrum lacusprofundi DL28. Staining of the biofilms formed by the strongly adhesive haloarchaeal strains revealed the presence of extracellular polymers, such as eDNA and glycoconjugates, substances previously shown to stabilize bacterial biofilms. For Hbt. salinarum DSM 3754T and Hfx. volcanii DSM 3757T , cells adhered within 1 day of culture and remained viable for at least 2 months in mature biofilms. Adherent cells of Hbt. salinarum DSM 3754T showed several types of cellular appendages that could be involved in the initial attachment. Our results show that biofilm formation occurs in a surprisingly wide variety of haloarchaeal species.

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A substantial number of spontaneous bacterio-opsin mutants of Halobacterium halobium are the result of insertion elements up to 1400 bp upstream of the bacterio-opsin (bop) gene. The nucleotide sequence of 1800 bp upstream of the bop gene has been determined. There is a 1118 bp open reading frame (ORF) located within this region which is transcribed and which coincides with the distribution of insertion elements upstream of the bop gene in Bop mutants. Therefore, we propose that there is a gene (brp gene) 526 bp upstream of the bop gene. This putative gene is transcribed in the opposite direction as the bop gene and could encode a protein of 37,500 D (359 amino acids) with a codon usage similar to bacterio-opsin. The 5' terminus of the brp transcript has been determined. The brp transcript and the bop mRNA are complementary for 13 residues near their 5' termini and both transcripts start at or near the initiating codon of the gene. Both transcripts could form similar hairpin loop structures at their 5' termini which contain possible ribosomal binding sites. The DNA sequences immediately upstream of the bop and the brp genes have significant homologies and there is a short complementary sequence. The role of the brp gene in bacterio-opsin gene expression is unclear.