Anaerobic halophilic alkalithermophiles
Mesbah, Noha Mostafa
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Halophilic alkalithermophiles are poly-extremophiles adapted to grow at high salt concentrations, alkaline pH values and temperatures greater than 50ºC. Halophilic alkalithermophiles are of interest from physiological perspectives as they combine unique adaptive mechanisms and cellular features that enable them to grow under extreme conditions. The alkaline, hypersaline lakes of the Wadi An Natrun, Egypt were chosen as sources for isolation of novel halophilic alkalithermophiles. These lakes are characterized by saturating concentrations of NaCl (5.6 M), alkaline pH (8.5-11) and temperatures of 50ºC due to intense solar irradiation. The prokaryotic communities of three large lakes of the Wadi An Natrun were assessed using 16S rRNA clone libraries. The Wadi An Natrun lakes are dominated by three phylogenetic groups of Bacteria (Firmicutes, Bacteroidetes, α- and γ-proteobacteria) and two groups of Archaea (Halobacteriales and Methanosarcinales). Extensive diversity exists within each phylogenetic group; half of the clones analyzed did not have close cultured or uncultured relatives. Three novel halophilic alkalithermophiles were isolated from the Wadi An Natrun. A novel order, Natranaerobiales, was proposed to encompass these novel isolates. Natranaerobius thermophilus was chosen as a model for more detailed physiological studies. Analysis of the bioenergetic characteristics of N.thermophilus revealed the absence of cytoplasmic pH homeostasis. Rather, N.thermophilus has the novel feature of maintaining the cytoplasmic pH at a constant 1 unit below that of the extracellular pH, the cytoplasmic pH continuously changed with the extracellular pH. To investigate the mechanism of this dynamic cytoplasmic pH regulation, genes encoding putative cation/proton antiporter proteins were functionally characterized. The physiological characteristics of these cation/proton antiporters were well suited to the intracellular conditions of N.thermophilus. Collectively, antiporter proteins function over the whole pH range for growth of N.thermophilus and display antiport activity over a wide range of Na+ ion concentrations. Thus, they are capable of providing active cytoplasmic acidification even if the intracellular environment of N.thermophilus changes. Most of the antiporter proteins belong to the NhaC family of cation/proton antiporters; a group that is primarily involved in expulsion of intracellular Na+. These results indicate that these antiporters provide N.thermophilus tolerance to both alkaline pH and high salt concentrations.