Abstract
The halophilic archaea (haloarchaea) live in hyersaline environments such as salt lakes, salt ponds and marine salterns. To cope with the salt stress conditions, haloarchaea have developed two fundamentally different strategies: the "salt-in" strategy and the "compatible-solute" strategy. Although investigation of the molecular mechanisms underlying the tolerance to high salt concentrations has made outstanding achievements, experimental study from the aspect of transcription is rare. In the present study, we monitored cellular physiology of Natrinema sp. J7-2 cells incubated in different salinity media (15%, 25% and 30% NaCl) from several aspects, such as cellular morphology, growth, global transcriptome and the content of intracellular free amino acids. The results showed that the cells were polymorphic and fragile at a low salt concentration (15% NaCl) but had a long, slender rod shape at high salt concentrations (25% and 30% NaCl). The cells grew best in 25% NaCl, mediocre in 30% NaCl and struggled in 15% NaCl. An RNA-seq analysis revealed differentially expressed genes (DEGs) in various salinity media. A total of 1,148 genes were differentially expressed, consisting of 719 DEGs (348 up-regulated and 371 down-regulated genes) between cells in 15% vs 25% NaCl, and 733 DEGs (521 up-regulated and 212 down-regulated genes) between cells in 25% vs 30% NaCl. Moreover, 304 genes were commonly differentially expressed in both 15% vs 25% and 25% vs30% NaCl. The DEGs were enriched in different KEGG metabolic pathways, such as amino acids, glycerolipid, ribosome, nitrogen, protoporphyrin, porphyrin and porhiniods. The intracellular predominant free amino acids consisted of the glutamate family (Glu, Arg and Pro), aspartate family (Asp) and aromatic amino acids (Phe and Trp), especially Glu and Asp.
Highlights
The discovery of Archaea as a distinct domain of life occurred in the late 1970s [1,2,3], since the research about the unique community has flourished, in the ecology of extremophiles [4]
Physiological study of haloarchaea has revealed two fundamentally different strategies by which these microorganisms achieve osmotic equilibrium: (a) the "salt-in" strategy which involves accumulation of equimolar concentrations of inorganic ions in the cytoplasm and (b) the "compatiblesolute" strategy which involves the accumulation of highly organic compatible solutes
The results showed that growth under salt stress conditions resulted in modulation of genes coding for ion transporters [19] and accumulation of compatible solutes [16]
Summary
The discovery of Archaea as a distinct domain of life occurred in the late 1970s [1,2,3], since the research about the unique community has flourished, in the ecology of extremophiles [4]. To identify the active biochemical pathways of the DEGs, a KEGG pathway analysis is useful for understanding the biological function in media of different salinities.
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