The halobacteriaceae are the most extreme of the world’s halophilic (Greek: salt-loving) organisms: in environments above 20% salinity, halobacteria abound in numbers above all other organisms.Although this family has the term “bacteria” in its name and its members are prokaryotic, as are bacteria, the halobacteria are organisms belonging to the domain Archaea, an evolutionary lineage recognized in the 1990s as fundamentally and basally distinct from other two domains of life, Bacteria and Eukaryota (Woese et al. 1990).Relationships among and nomenclature involving the three domains remain highly controversial, for e.g. see Cavalier-Smith 2002.
Halobacteria were first isolated about 100 years ago but like other Archaea they were not recognized as separate from bacterial prokaryotes until the late 1970s (Oren 2012 and references within).Scientists sometimes use the “unofficial” term ‘haloarchaea’ to designate the halophilic Archaea as distinct from bacteria, since the nomenclature term holobacteria is a confusing holdover from the time when all prokaryotic organisms were classified together in kingdom Monera (Oren 2012).
Molecular and chemical (especially lipid analysis) methods have brought significant advances in distinguishing species of Archaea, and the number of recognized species in the Halobacteriaceae has grown dramatically in the last 30 years (for example, see Ghai et al. 2011); in 2011 there were 36 recognized genera and 129 species (Arahal 2011; Oren 2012) and the "List of Prokaryotic Names with Standing (LPNS)" on clearinghouse site bacterio.net reports a currently accepted 40 genera and 144 species (Parte 2013). The rapid discovery of species is facilitating a new understanding of the diversity within this family.
Species of Halobacteria have been found to inhabit diverse extreme conditions: alkaline, acidic, warm- and cold temperatures, and show a range of different morphologies: rods, coccoid (spherical), flat, square; some can change shape with environmental conditions.There is a non-pigmented aerobic species (Natrialba asiatica; see below for more on halobacteria pigmentation) and a non-pigmented species with an anaerobic life style (Halorhabdus tiamatea).Unlike bacterial and eukaryotic halophiles, halobacteria don’t just survive in high salt concentrations, instead most require high salt solutions for growth and stability and many grow best in 3-4M solutions (Gutierrez et al. 2002).However, as it turns out, not all halobacteria thrive at saturated salt concentrations, some appear to have lower salt requirements (Oren 2012).
Most halobacteriaceans are tinted red or purple from carotinoid pigments bound up in bacteriorhodopsin proteins, which they use in low-nutrient enviornments for capturing energy from sunlight in a manner similar to photosynthesis.Lake Owens in California, the Great Salt Lake in Utah, Lake Eyre in Australia, and the upper Volga region of the Caspian Sea are just a few of the thousands of salt lakes with salinities >30% where halobacteria populations exist in enormous numbers (106-107 cells/ml), and are visible by their pink hue (Dyall-Smith 2013).Archaea are the only organisms to make bacteriorhodopsins (although the proteins have been cloned and expressed in E. coli) and the scientific community has become increasingly interested in these proteins for medical and technological uses, for example computer screens (NASA news 1998); a range of uses is reviewed in Trivedi et al. 2011.
Complete genome sequences of members of the family Halobacteriaceae are becoming available rapidly. The first, Halobacterium NRC-1 (a strain of Hbt. salinarum), was published in 2000 and at least fourteen more have been sequenced subsequently (Oren 2012).These are available in the HaloWeb database http://halo4.umbi.umd.edu (DasSarma et al. 2010).
