The Survival and Resistance of Halobacterium salinarum NRC-1, Halococcus hamelinensis, and Halococcus morrhuae to Simulated Outer Space Solar Radiation

Solar radiation is among the most prominent stress factors organisms face during space travel and possibly on other planets. Our analysis of three different halophilic archaea, namely Halobacterium salinarum NRC-1, Halococcus morrhuae, and Halococcus hamelinensis, which were exposed to simulated solar radiation in either dried or liquid state, showed tremendous differences in tolerance and survivability. We found that Hcc. hamelinensis is not able to withstand high fluences of simulated solar radiation compared to the other tested organisms. These results can be correlated to significant differences in genomic integrity following exposure, as visualized by random amplified polymorphic DNA (RAPD)-PCR. In contrast to the other two tested strains, Hcc. hamelinensis accumulates compatible solutes such as trehalose for osmoprotection. The addition of 100 mM trehalose to the growth medium of Hcc. hamelinensis improved its survivability following exposure. Exposure of cells in liquid at different temperatures suggests that Hbt. salinarum NRC-1 is actively repairing cellular and DNA damage during exposure, whereas Hcc. morrhuae exhibits no difference in survival. For Hcc. morrhuae, the high resistance against simulated solar radiation may be explained with the formation of cell clusters. Our experiments showed that these clusters shield cells on the inside against simulated solar radiation, which results in better survival rates at higher fluences when compared to Hbt. salinarum NRC-1 and Hcc. hamelinensis. Overall, this study shows that some halophilic archaea are highly resistant to simulated solar radiation and that they are of high astrobiological significance.

KEY WORDS

Halophiles-Solar radiation-Stress resistance-Survival.

Halococcus thailandensis sp. nov., from fish sauce in Thailand

Fifteen strains of red-pigmented, strictly aerobic, coccoid, extremely halophilic archaea were isolated from fish sauce (nam-pla) produced in Thailand. They grew optimally at 37 °C, pH 6–8 and in the presence of 20–30 % (w/v) NaCl. The DNA G+C contents of the isolates were 60.0–61.8 mol%. They had MK-8(H2) as a major menaquinone component and C20C20 and C20C25 derivatives of phosphatidylglycerol, phosphatidylglycerol methylphosphate and a sulfated glycolipid, S-DGA-1, as major polar lipid components. 16S rRNA gene sequence comparisons revealed that a representative strain, HDB5-2T, was affiliated with Halococcus dombrowskii JCM 12289T, Halococcus qingdaonensis JCM 13587T and Halococcus morrhuae JCM 8876T (levels of similarity of 98.2–98.7 %). Based on data from DNA–DNA hybridization experiments, the 15 strains represented a single species, showing hybridization values of >78.9 % to representative strain HDB5-2T, but were unrelated to either Halococcus dombrowskii JCM 12289T or Halococcus morrhuae JCM 8876T, with levels of relatedness of <50 %. Moreover, a comparison of phenotypic properties discriminated these new isolates from recognized species of the genus Halococcus. The 15 strains are thus considered to represent a novel species of the genus Halococcus, for which the name Halococcus thailandensis sp. nov. is proposed. The type strain is HDB5-2T (=BCC 20213T =JCM 13552T =PCU 278T).

Halococcus qingdaonensis sp. nov., a halophilic archaeon isolated from a crude sea-salt sample

A Gram-negative, extremely halophilic, coccoid archaeal strain, CM5(T), was isolated from a crude sea-salt sample collected near Qingdao, China. The organism grew optimally at 35-40 degrees C and pH 6.0 in the presence of 20 % (w/v) NaCl. Its colonies were red in colour and it could use glucose as a sole carbon source for growth. The 16S rRNA gene sequence of CM5(T) was most closely related to those of Halococcus species. Its pattern of antibiotic susceptibility was similar to those of other described Halococcus species. Biochemical tests revealed no sign of H(2)S production or gelatin liquefaction. The main polar lipids of strain CM5(T) were phosphatidylglycerol, phosphatidylglycerol methylphosphate and sulfated diglycosyl diether. No phosphatidylglycerol sulfate was present. The DNA G+C content of strain CM5(T) was 61.2 mol% and it gave DNA-DNA reassociation values of 33.7, 57.1 and 29.6 %, respectively, with Halococcus salifodinae DSM 8989(T), Halococcus dombrowskii DSM 14522(T) and Halococcus morrhuae ATCC 17082(T). Based on its morphological and chemotaxonomic properties and phylogenetic analysis of 16S rRNA gene sequence data, we propose that CM5(T) should be classified within a novel species, Halococcus qingdaonensis sp. nov., with strain CM5(T) (=CGMCC 1.4243(T)=JCM 13587(T)) as the type strain.

Halococcus hamelinensis sp. nov., a novel halophilic archaeon isolated from stromatolites in Shark Bay, Australia

Several halophilic archaea belonging to the genus Halococcus were isolated from stromatolites from Hamelin Pool, Shark Bay, Western Australia, collected during field trips in 1996 and 2002. This is the first incidence of halophilic archaea being isolated from this environment. Stromatolites are biosedimentary structures that have been formed throughout the earth's evolutionary history and have been preserved in the geological record for over 3 billion years. The stromatolites from Hamelin Pool, Western Australia, are the only known example of extant stromatolites forming in hypersaline coastal environments. Based on their 16S rRNA gene sequences and morphology, the isolates belong to the genus Halococcus. Strain 100NA1, isolated from stromatolites collected in 2002, was closely related to strain 100A6(T) that was isolated from the stromatolites collected in 1996, with a DNA-DNA hybridization value of 94 +/- 8 %. DNA-DNA hybridization values of strain 100A6(T) with Halococcus morrhuae NRC 16008 and Halococcus saccharolyticus ATCC 49257(T) were 17 +/- 6 and 11 +/-7 %, respectively. The DNA G + C content of strain 100A6(T) was 60.5 mol% (T(m)). The main polar lipid was S-DGA-1, a sulphated glycolipid that has been detected in all strains of the genus Halococcus. Whole-cell protein profiles, enzyme composition and utilization of various carbon sources were distinct from those of all previously characterized Halococcus species. The recognition of this strain as representing a novel species within the genus Halococcus is justified, and the name Halococcus hamelinensis sp. nov. is proposed. The type strain is 100A6(T) (=JCM 12892(T) = ACM 5227(T)).

Halococcus dombrowskii sp. nov., an archaeal isolate from a Permian alpine salt deposit

Several extremely halophilic coccoid archaeal strains were isolated from pieces of dry rock salt that were obtained three days after blasting operations in an Austrian salt mine. The deposition of the salt is thought to have occurred during the Permian period (225-280 million years ago). On the basis of their polar-lipid composition, 16S rRNA gene sequences, cell shape and growth characteristics, the isolates were assigned to the genus Halococcus. The DNA-DNA reassociation values of one isolate, strain H4T, were 35 and 38% with Halococcus salifodinae and Halococcus saccharolyticus, respectively, and 65.8-67.8% with Halococcus morrhuae. The polar lipids of strain H4T were C20-C25 derivatives of phosphatidylglycerol and phosphatidylglycerol phosphate. Whole-cell protein patterns, menaquinone content, enzyme composition, arrangements of cells, usage of carbon and energy sources, and antibiotic susceptibility were sufficiently different between strain H4T and H. morrhuae to warrant designation of strain H4T as a new species within the genus Halococcus. It is proposed that the isolate be named Halococcus dombrowskii, and the type strain is H4T (= DSM 14522T = NCIMB 13803T = ATCC BAA-364T).

Very similar strains of Halococcus salifodinae are found in geographically separated permo-triassic salt deposits

The authors have previously isolated a novel extremely halophilic archaeon, Halococcus salifodinae Blp, from Austrian rock salt deposited about 250 million years ago. In this study they compared strain Blp with two other halococci isolated independently from geographically distant salt deposits of similar age, and with two recent isolates (N1 and H2) from the same site as strain Blp. Strain BG2/2 was from a salt mine in Germany and strain Br3 from a halite deposit in England; both resembled Hc. salifodinae Blp in cellular and colonial morphology. Strains Blp, BG2/2 and Br3 had identical 16S rRNA sequences, very similar whole-cell protein patterns, which were different from those of other halococci, similar G+C contents and identical sequences in a 108-base insertion in their 5S rRNA gene. Other similarities included composition and relative abundances of polar lipids, antibiotic susceptibility, enzymic activities and Fourier-transform infrared spectra. Strains N1 and H2 showed similar morphology, whole-cell protein patterns and biochemical characteristics as strains Blp, Br3 and BG2/2. Their partial 16S rRNA sequences (682 and 641 bases, respectively) were indistinguishable from those of strains Blp, Br3 and BG2/2. Therefore strains N1 and H2 can be considered as reisolates of Hc. salifodinae which were obtained 8 years after the first samples were taken from that mine. The results presented suggest that viable halophilic archaea, which belong to the same species, occur in widely separated evaporite locations of similar geological age, and support the notion that these halophilic isolates from subterranean salt deposits may be the remnants of populations which inhabited ancient hypersaline seas.

Proposal to transfer Halococcus turkmenicus, Halobacterium trapanicum JCM 9743 and strain GSL-11 to Haloterrigena turkmenica gen. nov., comb. nov

The 16S rRNA gene sequences of Halococcus saccharolyticus and Halococcus salifodinae were closely related (94.5-94.7% similarity) to that of Halococcus morrhuae, the type species of the genus Halococcus. However, Halococcus turkmenicus was distinct from the other members of this genus, with low 16S rRNA similarities when compared to Halococcus morrhuae (88.7%). On the basis of phylogenetic tree reconstruction, detection of signature bases and DNA-DNA hybridization data, it is proposed to transfer Halococcus turkmenicus to a novel genus, Haloterrigena, as Haloterrigena turkmenica gen. nov., comb. nov., and to accommodate Halobacterium trapanicum JCM 9743 and strain GSL-11 in the same species. On the basis of morphological, cultural and 16S rRNA sequence data, it is also proposed that the culture collection strains of Halobacterium trapanicum NCIMB 767, ATCC 43102 and JCM 8979 should be renamed as Halococcus sp.