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Among the halophilic microorganisms, the halophilic actinobacteria are of special interest for their amazing metabolic diversity, biological activities and biotechnological applications. On the same regard, there is also evidence that actinobacteria isolated from the extremobiosphere will be a rich source of novel natural products Bull Most are chemoorganotrophs and some of them are halophiles. Recently, members of actinobacteria were raised to the taxonomic rank of a phylum which is one of the major phyla in the domain Bacteria, as inferred from its branching pattern in the 16S rrna gene tree Garrity and Holt ; Ludwig and Klenk The phylum actinobacteria includes phenotypically diverse microorganisms which show diverse morphological properties that range from cocci to highly differentiated mycelia Goodfellow The halophilic actinobacteria are widely distributed in saline, hypersaline terrestrial and aquatic habitats.
Isolation of them does not demand special enrichment techniques Kanekar et al , supplementing the isolation media with salt would be enough. Taxonomically, they belong to different taxa. In this chapter, the biodiversity of halotolerant and halophilic actinobacteria from different aspects will be reviewed. The focus will be on their taxonomy and ecological distribution in different habitats as well as a short description of the halophilic genera and species of the actinobacteria group.
The most widely used classification by most scientists was proposed by Kushner and Kamekura This classification is based on the optimal growth of microorganisms with respect to the concentration of NaCl. In contrast, non-halophilic microorganisms. There are representatives of actinobacteria in all of the above mentioned categories.
An important point that should be mentioned here is that the salt requirement and tolerance of many species may vary according to growth conditions such as medium composition and temperature. Therefore, the growth temperature should be specified in the definition of the salt range for growth. An example for this effect is Marinococcus halophilus which grows in presence of low concentrations starting from 0. The domain Bacteria contains many halophilic and halotolerant organisms spread in different bacterial taxa and true halophiles are well known within the phyla Cyanobacteria, Proteobacteria, Firmicutes, Actinobacteria, Spirochaetes and Bacteroidetes Oren The halophilic and halotolerant actinobacteria belong to phylum Actinobacteria.
This taxon is one of the major phyla in the domain Bacteria, as inferred from its branching position in the 16S rrna phylogenetic tree Garrity and Holt ; Ludwig and Klenk and taxon-specific 16S rrna signatures Zhi et al. Moreover, some conserved indels in 23S rrna and protein sequences support the distinctness of members of actinobacteria from all other bacteria Gao and Gupta In the most recent edition of the Bergey s Manual of Systematic Bacteriology Goodfellow , the phylum Actinobacteria encompasses 5 classes, 19 orders, 50 families, and genera.
The constituent classes are Acidimicrobiia class. However, many new taxa were discovered after the publication of the Manual. To date, the halophilic and halotolerant actinobacteria have been accommodated only in the subclass Actinobacteridae of class Actinobacteria, and of the three orders Actinomycetales, Bifidobacteriales, and Nitriliruptorales of the subclass Actinobacteridae, only the Actinomycetales comprises halophilic and halotolerant actinobacteria.
Nowadays, the order Actinomycetales Buchanan ; Stackebrandt et al ; Zhi et al is composed of 23 suborders.
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It was reported that the halophilic and halotolerant actinobacteria are included in the following suborders of the order Actinomycetales: Actinopolysporineae, Corynebacterineae, Glycomycineae, Jiangellineae, Micrococcineae, Micromonosporineae, Propionibacterineae, Pseudonocardineae, Streptomycineae and Streptoporangineae. Hozzein These taxa that comprises halophilic and halotolerant actinobacteria do not necessarily consist solely of halophiles. The opposite is true: there are only few phylogenetically consistent groups that are composed entirely of halophiles. In most cases halophiles and non-halophilic relatives are found together in the phylogenetic tree, and many genera, families and orders have representatives with greatly different salt requirement and tolerance patterns Oren However, in most cases the halophilic species will be grouped in close clades in the phylogenetic tree and not scattered within their non-halophilic relatives in the tree.
The halophilic and halotolerant actinobacteria as accommodated in different taxa, they show a remarkable range of different morphologies from organisms that form cocci e. Ruania and Serinicoccus , short rods e. Cellulomonas , irregular rods e. Microbacterium and Salinibacterium , rods and cocci e. Arthrobacter , and mycelia that fragment into coccoid and rod-like elements e. Others show more extensive morphological differentiation ranging from those which produce extensively branched substrate hyphae that bear spores e. Micromonospora to those that form a stable branched mycelium that carries aerial hyphae which differentiate into short or long chains of spores e.
Actinopolyspora and Streptomyces. In general, spores are nonmotile. Currently, the number of species names that have been validly published as halophilic and halotolerant actinobacteria is very large and is growing exponentially due to the recent efforts to characterize microorganisms from hypersaline environments. In the following section, the halophilic and halotolerant actinobacterial species will be reviewed with their classification and main characteristics. The focus will be only on those species names that have been validly described.
More detailed data about their characteristics can be found on the original descriptions or in the most recent edition of the Bergey s Manual of Systematic Bacteriology. It should be clear here that besides the halophilic and halotolerant actinobacterial species included here whose names have been validly published, there are a large number of halophilic and halotolerant actinobacterial organisms that have been isolated and studied from other points of view, such as for their biotechnological applications or other reasons, which have not been properly characterized.
Another point of interest is that some species have been described as halophilic or halotolerant without a scientific basis as their response to NaCl has not been studied in detail or their NaCl concentration for optimal growth has not been reported on their taxonomic descriptions Suborder Actinopolysporineae Family Actinopolysporaceae and Genus Actinopolyspora Family Actinopolysporaceae is the only family of order Actinopolysporales. It is also a monogeneric family that contains the genus Actinopolyspora Gochnauer et al ; Zhi et al.
Genus Actinopolyspora is very interesting as all the validly described species in the genus are halophilic actinobacteria and some of them can be considered as extreme halophiles.
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Fragmentation of the substrate mycelium is occasionally observed, but substrate hyphae are mostly non-fragmented. Sporophores containing ten or more smooth-walled spores which are produced basipetally on aerial hyphae are formed. Spores are not observed on the substrate mycelium Gochnauer et al ; Tang et al.
The genus Actinopolyspora was originally established by Gochnauer et al. The genus currently contains 11 species with validly published names: A. The strain A. A thermophilic species, A. Also, the species A. Strains of genus Actinopolyspora require salt for growth and most are extreme halophiles. The genus Corynebacterium represents a large group of Gram-positive, asporogenous, rodshaped bacteria Liebl et al.
Hozzein Genus Glycomyces Members of the genus Glycomyces Labeda et al ; Labeda and Kroppenstedt are aerobic with branching vegetative mycelia and aerial mycelium that may be produced on certain growth media. Spores may be formed on the vegetative hyphae in some species and they are oval, spherical or rod-like. Chains of square-ended conidia may be produced on aerial hyphae. All the previously described species were not reported to be halophilic or halotolerant except the recently described Glycomyces halotolerans Guan et al.
It was described as an aerobic actinobacterium with well-developed aerial mycelium on most media and branched substrate mycelium that fragments into short or elongated rods Suborder Jiangellineae Family Jiangellaceae and Genus Haloactinopolyspora The family Jiangellaceae comprises two genera, Jiangella and Haloactinopolyspora. The genus Haloactinopolyspora contains the type species H.
Both have substrate mycelium that fragments into short or elongated rods, aerial mycelium that differentiates well into long spore chains and described as moderately halophiles. Only Georgenia halophila Tang et al. Cells of the species are facultatively anaerobic, nonmotile, non-endospore-forming and short rods. Cells of the species are facultatively anaerobic and motile rods. They are strictly aerobic, non-sporeforming and non-motile cocci. Genus Microbacterium Genus Microbacterium currently includes 86 species and only M.
Cells of the organisms are nonmotile and irregular short rods that may form V shapes but no branching or rod-coccus cycle are observed. Genus Salinibacterium Salinibacterium is currently contains two species, S.
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Both species have nonmotile, aerobic, non-sporeforming, irregular rods. Genus Arthrobacter The majority of species in the genus Arthrobacter Conn and Dimmick ; Koch et al exhibit a marked rod-coccus growth cycle when grown in complex media. The stationary-phase cultures generally after 2 7 days are composed entirely or largely of coccoid cells and some species are showing only. Hozzein spherical cells throughout the growth cycle.
Arthrobacter halodurans Chen et al. Genus Kocuria Genus Kocuria contains only two halotolerant actinobacteria; Kocuria halotolerans Tang et al. Cells of the species are coccoid, non-motile and non-endospore-forming. Genus Nesterenkonia The genus Nesterenkonia was first proposed by Stackebrandt et al.