Survival Strategies of Halophilic Oligotrophic and Desiccation Resistant Prokaryotes

Abstract

Viable halophilic and halotolerant Archaea and Bacteria have been found in ancient salt deposits around the world. The first cultivations of halophilic microorganisms from Permian salt sediments (about 250 million years old) were reported in the 1960s (Reiser and Tasch, 1960; Dombrowski, 1963) and met with considerable skepticism. Some 30 years later, detailed taxonomic descriptions of halophilic Bacteria and Archaea (haloarchaea) obtained from ancient evaporites began to be published (Norton et al., 1993; Denner et al., 1994; Stan-Lotter et al., 2002; Mormile et al., 2003; Gruber et al., 2004; Vreeland et al., 2007). Sequences of small ribosomal RNA (16S rRNA) genes and other molecules allowed more meaningful comparisons of isolates with known strains than was possible before. In many cases, no exact matches of sequences from subsurface isolates with those of known strains from surface waters were found (McGenity et al., 2000). This does not necessarily mean that they do not exist in surface environments, merely that they have not been isolated yet from there. In one other case, three strains of Halococcus salifodinae with identical 16S rRNA sequences were found in three geographically separated subsurface regions, all of Permo-Triassic age: strain BIp from Permian Zechstein rock salt, mined at Bad Ischl, Austria; strain Br3 from solution-mined Triassic Northwich halite; and strain BG2/2 from a core of Permian Zechstein salt, Berchtesgaden, Germany (Stan-Lotter et al., 1999; McGenity et al., 2000). A detailed characterization of the three independently isolated halococci revealed that they were very similar and should be considered as strains of the same species (Stan-Lotter et al., 1999). During the Permian period, the large hypersaline Zechstein Sea covered an area of about 250,000 km² over much of northern Europe. This sea would have provided a connection between the areas from which the three strains of Hcc. salifodinae were isolated (Stan-Lotter et al., 1999; McGenity et al., 2000; Radax et al., 2001). It is conceivable that Hcc. salifodinae was present in the Zechstein Sea and became trapped in the evaporating salts.