Shifts in Microbial Community Structure and Co-occurrence Network along a Wide Soil Salinity Gradient

The response of microbiomes to salinity has been clarified in different geographic scales or ecosystems. However, how soil microbial community structure and interaction respond to salinity across wide salinity range and climatic region is still unclearly resolved. To address this issue, we examined the microbial community's composition in saline soils from two climatic regions (coastal wetland and arid desert). Our research confirms that soil salinity had a negative effect on soil nutrient content. Salinity decreased the relative abundance of bacteria, but increased archaea abundance, leading to the shifts from bacteria dominant community to archaea dominant community. Low-water medium-salinity soil (LWMS) had the most complex archaeal community network, whereas for bacteria, the most complex bacterial community network was observed in low-water high-salinity soils (LWHS). Key microbial taxa differed in three salinity gradients. Salinity, soil water content, pH, total nitrogen (TN), and soil organic carbon (SOC) were the main driving factors for the composition of archaeal and bacterial community. Salinity directly affected archaeal community, but indirectly influenced bacteria community through SOC; pH affected archaeal community indirectly through TN, but directly affected bacterial community. Our study suggests that soil salinity dramatically influences diversity, composition, and interactions within the microbial community.

Salinirarus marinus gen. nov., sp. nov., Haloplanus salilacus sp. nov., Haloplanus pelagicus sp. nov., Haloplanus halophilus sp. nov., and Haloplanus halobius sp. nov., halophilic archaea isolated from commercial coarse salts with potential as starter cultures for salt-fermented foods

Five halophilic archaeal strains, XH8T, CK5-1T, GDY1T, HW8-1T, and XH21T, were isolated from commercial coarse salt produced in different regions of China. Their 16S rRNA and rpoB' gene sequences indicated that four of the strains (CK5-1T, GDY1T, HW8-1T, and XH21T) represent distinct species within the genus Haloplanus (family Haloferacaceae), while strain XH8T represents a novel genus within the same family. These assignments were supported by phylogenetic and phylogenomic analyses, which showed that strains CK5-1T, GDY1T, HW8-1T, and XH21T cluster with the current species of the genus Haloplanus, while strain XH8T forms a separate branch from the genus Haloplanus. The digital DNA-DNA hybridization and average amino acid identity (AAI) values among these four strains and the current members of the genus Haloplanus were 23.1%-35.2% and 75.9%-83.8%, respectively; and those values between strain XH8T and other genera in the family Haloferacaceae were 18.8%-33.6% and 59.8%-66.6%, respectively, much lower than the threshold values for species demarcation. Strain XH8T may represent a novel genus of the family Haloferacaceae according to the cut-off value of AAI (≤72.1%) proposed to differentiate genera within the family Haloferacaceae. These five strains could be distinguished from the related species according to differential phenotypic characteristics. Based on these results, it is proposed that strain XH8T represents a novel genus within the family Haloferacaceae, and strains CK5-1T, GDY1T, HW8-1T, and XH21T represent four novel species of the genus Haloplanus, respectively. Additionally, these five strains possess genes encoding enzymes critical for the fermentation process in salt-fermented foods, indicating their potential as starter cultures for these applications.

Halococcus salsus sp. nov., a novel halophilic archaeon isolated from rock salt

Two pink-pigmented halophilic archaea, designated strains ZJ1^T and J81, were isolated from rock salt of Yunnan Salt Mine, China, and commercial salt imported from Bolivia, respectively. Cells were non-motile, coccoid, approximately 0.8-1.6 µm in diameter, stained Gram-negative and often occurred in pairs. Colonies were wet, opaque and smooth-edged. Strain ZJ1^T grew optimally with 20 % (w/v) NaCl, at pH 7.5 and at 38-40 °C, which was the same as for strain J81. 16S rRNA gene sequence similarity between strains ZJ1^T and J81 was 99.7 %. Sequence similarity searches based on the 16S rRNA gene and cell morphology suggested that strains ZJ1^T and J81 belong to the genus Halococcus in the family Halococcaceae. The major polar lipids of the type strain, ZJ1^T, were phosphatidylglycerol, phosphatidylglycerol phosphate methyl ester and sulfated diglycosyl-diether-1. The profile of polar lipids, cell shape, motility and lack of lysis of cells in distilled water show that strains ZJ1^T and J81 were similar to other members of the genus Halococcus. Strain ZJ1^T shared the highest 16S rRNA gene and rpoB' gene sequence similarities of 99.0 and 95.3 % with Halococcus hamelinensis 100A6^T, respectively, followed by less than 94.6 % with sequences of other species in the genus Halococcus. DNA-DNA relatedness between strains ZJ1^T and J81 was 90.1±0.7 %, while 27±0.5 % was found between strain ZJ1^T and H. hamelinensis JCM 12892^T (=100A6^T), and 29.0±0.5 % between strains J81 and H. hamelinensis JCM 12892^T. The DNA G+C content of strain ZJ1^T was 66.5 mol% (Tm). The stable phylogenetic position, differential physiological and biochemical properties and extensive sequence divergence suggest that strains ZJ1^T and J81 represent a novel species, for which the name Halococcus salsus sp. nov. is proposed. The type strain is ZJ1^T (=CGMCC 1.16025^T=NBRC 112867^T).

Systematics of haloarchaea and biotechnological potential of their hydrolytic enzymes

Halophilic archaea, also referred to as haloarchaea, dominate hypersaline environments. To survive under such extreme conditions, haloarchaea and their enzymes have evolved to function optimally in environments with high salt concentrations and, sometimes, with extreme pH and temperatures. These features make haloarchaea attractive sources of a wide variety of biotechnological products, such as hydrolytic enzymes, with numerous potential applications in biotechnology. The unique trait of haloarchaeal enzymes, haloenzymes, to sustain activity under hypersaline conditions has extended the range of already-available biocatalysts and industrial processes in which high salt concentrations inhibit the activity of regular enzymes. In addition to their halostable properties, haloenzymes can also withstand other conditions such as extreme pH and temperature. In spite of these benefits, the industrial potential of these natural catalysts remains largely unexplored, with only a few characterized extracellular hydrolases. Because of the applied impact of haloarchaea and their specific ability to live in the presence of high salt concentrations, studies on their systematics have intensified in recent years, identifying many new genera and species. This review summarizes the current status of the haloarchaeal genera and species, and discusses the properties of haloenzymes and their potential industrial applications.