Halophilic archaea of the class
are the most salt-requiring prokaryotes within the domain
. In 1997, minimal standards for the description of new taxa in the order
were proposed. From then on, the ...taxonomy of the class
provides an excellent example of how changing concepts on prokaryote taxonomy and the development of new methods were implemented. The last decades have witnessed a rapid expansion of the number of described taxa within the class
coinciding with the era of genome sequencing development. The current members of the International Committee on Systematics of Prokaryotes Subcommittee on the Taxonomy of
propose these revisions to the recommended minimal standards and encourage the use of advanced technologies in the taxonomic description of members of the
. Most previously required and some recommended minimal standards for the description of new taxa in the class
were retained in the present revision, but changes have been proposed in line with the new methodologies. In addition to the 16S rRNA gene, the
gene is an important molecular marker for the identification of members of the
. Phylogenomic analysis based on concatenated conserved, single-copy marker genes is required to infer the taxonomic status of new taxa. The overall genome relatedness indexes have proven to be determinative in the classification of the taxa within the class
. Average nucleotide identity, digital DNA-DNA hybridization, and average amino acid identity values should be calculated for rigorous comparison among close relatives.
Haloquadratum walsbyi commonly dominates the microbial flora of hypersaline waters. Its cells are extremely fragile squares requiring >14%(w/v) salt for growth, properties that should limit its ...dispersal and promote geographical isolation and divergence. To assess this, the genome sequences of two isolates recovered from sites at near maximum distance on Earth, were compared.
Both chromosomes are 3.1 MB in size, and 84% of each sequence was highly similar to the other (98.6% identity), comprising the core sequence. ORFs of this shared sequence were completely synteneic (conserved in genomic orientation and order), without inversion or rearrangement. Strain-specific insertions/deletions could be precisely mapped, often allowing the genetic events to be inferred. Many inferred deletions were associated with short direct repeats (4-20 bp). Deletion-coupled insertions are frequent, producing different sequences at identical positions. In cases where the inserted and deleted sequences are homologous, this leads to variant genes in a common synteneic background (as already described by others). Cas/CRISPR systems are present in C23(T) but have been lost in HBSQ001 except for a few spacer remnants. Numerous types of mobile genetic elements occur in both strains, most of which appear to be active, and with some specifically targetting others. Strain C23(T) carries two ∼6 kb plasmids that show similarity to halovirus His1 and to sequences nearby halovirus/plasmid gene clusters commonly found in haloarchaea.
Deletion-coupled insertions show that Hqr. walsbyi evolves by uptake and precise integration of foreign DNA, probably originating from close relatives. Change is also driven by mobile genetic elements but these do not by themselves explain the atypically low gene coding density found in this species. The remarkable genome conservation despite the presence of active systems for genome rearrangement implies both an efficient global dispersal system, and a high selective fitness for this species.
As a group, the halophilic archaea (class
Halobacteria
) are the most salt-requiring and salt-resistant microorganisms within the domain
Archaea
. Halophilic archaea flourish in thalassohaline and ...athalassohaline environments and require over 100–150 g/L NaCl for growth and structural stability. Natural hypersaline environments vary in salt concentration, chemical composition and pH, and occur in climates ranging from tropical to polar and even under-sea. Accordingly, their resident haloarchaeal species vary enormously, as do their individual population compositions and community structures. These diverse halophilic archaeal strains are precious resources for theoretical and applied research but assessing their taxonomic and metabolic novelty and diversity in natural environments has been technically difficult up until recently. Environmental DNA-based high-throughput sequencing technology has now matured sufficiently to allow inexpensive recovery of massive amounts of sequence data, revealing the distribution and community composition of halophilic archaea in different hypersaline environments. While cultivation of haloarchaea is slow and tedious, and only recovers a fraction of the natural diversity, it is the conventional means of describing new species, and provides strains for detailed study. As of the end of May 2020, the class
Halobacteria
contains 71 genera and 275 species, 49.8% of which were first isolated from the marine salt environment and 50.2% from the inland salt environment, indicating that both thalassohaline and athalassohaline environments contain diverse halophilic archaea. However, there remain taxa that have not yet been isolated in pure culture, such as the nanohaloarchaea, which are widespread in the salt environment and may be one of the hot spots in the field of halophilic archaea research in the future. In this review, we focus on the cultivation strategies that have been used to isolate extremely halophilic archaea and point out some of the pitfalls and challenges.
NaCl-saturated brines such as saltern crystalliser ponds, inland salt lakes, deep-sea brines and liquids-of-deliquescence on halite are commonly regarded as a paradigm for the limit of life on Earth. ...There are, however, other habitats that are thermodynamically more extreme. Typically, NaCl-saturated environments contain all domains of life and perform complete biogeochemical cycling. Despite their reduced water activity, ∼0.755 at 5 M NaCl, some halophiles belonging to the Archaea and Bacteria exhibit optimum growth/metabolism in these brines. Furthermore, the recognised water-activity limit for microbial function, ∼0.585 for some strains of fungi, lies far below 0.755. Other biophysical constraints on the microbial biosphere (temperatures of >121°C; pH > 12; and high chaotropicity; e.g. ethanol at >18.9% w/v (24% v/v) and MgCl2 at >3.03 M) can prevent any cellular metabolism or ecosystem function. By contrast, NaCl-saturated environments contain biomass-dense, metabolically diverse, highly active and complex microbial ecosystems; and this underscores their moderate character. Here, we survey the evidence that NaCl-saturated brines are biologically permissive, fertile habitats that are thermodynamically mid-range rather than extreme. Indeed, were NaCl sufficiently soluble, some halophiles might grow at concentrations of up to 8 M. It may be that the finite solubility of NaCl has stabilised the genetic composition of halophile populations and limited the action of natural selection in driving halophile evolution towards greater xerophilicity. Further implications are considered for the origin(s) of life and other aspects of astrobiology.
High-coverage long-read sequencing of the
type strain (91-R6) revealed a 2.17-Mb chromosome and two large plasmids (148 and 102 kb). Population heterogeneity and long repeats were observed. Strain ...91-R6 and laboratory strain R1 showed 99.63% sequence identity in common chromosomal regions and only 38 strain-specific segments. This information resolves the previously uncertain relationship between type and laboratory strains.
We report the genome sequence of a monophasic
subsp
Typhimurium strain (TW-Stm6) isolated in Australia that is similar to epidemic multidrug-resistant strains from Europe and elsewhere. This strain ...carries additional antibiotic and heavy-metal resistance genes on a large (275-kb) IncHI2 plasmid.
Strains C23T and HBSQ001 were isolated from solar salterns and are novel square-shaped, aerobic, extremely halophilic members of the domain Archaea and family Halobacteriaceae. Cells stained ...Gram-negative and grew optimally in media containing 18 % salts at around neutral pH. Mg2+ is not required. The DNA G+C content of both isolates was 46.9 mol% and DNA-DNA cross-hybridization showed a relatedness of 80 %. Their 16S rRNA gene sequences showed only 2 nucleotide differences (99.9 % identity) and phylogenetic tree reconstructions with other recognized members of the Halobacteriaceae indicated that they formed a distinct clade, with the closest relative being Halogeometricum borinquense PR 3T (91.2 % sequence identity). The major polar glycolipid of both isolates was the sulfated diglycosyl diether lipid S-DGD-1. Electron cryomicrosopy of whole cells revealed similar internal structures, such as gas vesicles and polyhydroxyalkanoate granules, but the cell wall of isolate HBSQ001 displayed a more complex S-layer compared with that of isolate C23T. The phenotypic characterization and phylogenetic data support the placement of isolates C23T and HBSQ001 in a novel species in a new genus within the Halobacteriaceae, for which we propose the name Haloquadratum walsbyi gen. nov., sp. nov. The type strain of Haloquadratum walsbyi is C23T (=JCM 12705T=DSM 16854T).
Cultivation of Walsby’s square haloarchaeon Burns, David G.; Camakaris, Helen M.; Janssen, Peter H. ...
FEMS microbiology letters,
09/2004, Letnik:
238, Številka:
2
Journal Article
Recenzirano
The
square
haloarchaea
of
Walsby (SHOW group) dominate hypersaline microbial communities but have not been cultured since their discovery 25 years ago. We show that natural water dilution cultures ...can be used to isolate members of this group and, once in pure culture, they can be grown in standard halobacterial media. Cells display a square morphology and contain gas vesicles and poly-β-hydroxybutyrate (PHB) granules. The 16S rRNA gene sequence was >99% identical to other SHOW group sequences. They prefer high salinities (23–30%), and can grow with a doubling time of 1–2 days in rich media. The ability to culture SHOW group organisms makes it possible to study, in a comprehensive way, the microbial ecology of salt lakes.
A novel halovirus, SH1, with a spherical morphology is described. Isolated from a hypersaline lake, SH1 is divalent, producing clear plaques on
Haloarcula hispanica and a natural
Halorubrum isolate. ...Single-step growth curves gave a latent period of 5–6 h and a burst size of around 200 PFU/cell. The host can differentiate to form tight clusters of thick cell-walled forms, and these were shown to be resistant to infection. Purified virions had no visible tail, were about 70 nm in diameter, and displayed a fragile outer capsid layer, possibly with an underlying membrane component. The structural proteins of the virion were analyzed by SDS–PAGE and several were found to be cross-linked, forming protein complexes. The genome was linear, dsDNA, of approximately 30 kb in length. This morphology and linear genome are features not observed in any other euryarchaeal viruses, but have properties similar to the bacterial virus PRD1.
Two halophilic archaeal strains, R30
T
and tADL
T
, were isolated from an aquaculture farm in Dailing, China, and from Deep Lake, Antarctica, respectively. Both have rod-shaped cells that lyse in ...distilled water, stain Gram-negative and form red-pigmented colonies. They are neutrophilic, require >120 g/l NaCl and 48–67 g/l MgCl
2
for growth but differ in their optimum growth temperatures (30 °C, tADL
T
vs. 40 °C, R30
T
). The major polar lipids were typical for members of the
Archaea
but also included a major glycolipid chromatographically identical to sulfated mannosyl glucosyl diether (S-DGD-1). The 16S rRNA gene sequences of the two strains are 97.4 % identical, show most similarity to genes of the family
Halobacteriaceae
, and cluster together as a distinct clade in phylogenetic tree reconstructions. The
rpoB′
gene similarity between strains R30
T
and tADL
T
is 92.9 % and less to other halobacteria. Their DNA G + C contents are 62.4–62.9 mol % but DNA–DNA hybridization gives a relatedness of only 44 %. Based on phenotypic, chemotaxonomic and phylogenetic properties, we describe two new species of a novel genus, represented by strain R30
T
(= CGMCC 1.10593
T
= JCM 17270
T
) and strain tADL
T
(= JCM 15066
T
= DSMZ 22187
T
) for which we propose the names
Halohasta litorea
gen. nov., sp. nov. and
Halohasta litchfieldiae
sp. nov., respectively.
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