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Caldiplasma sukawensis gen. nov., sp. nov., and Cuniculiplasma thermophilum sp. nov., mesophilic, acidophilic, cell–wall–less, archaea of the order Thermoplasmatales, isolated from extremely acidic hot spring
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DOI：10.1016/j.syapm.2025.126686.png)
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Abstract
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Four mesophilic, acidophilic, cell wall–less archaea, strains—SKW1, SKW2T, SKW3, and SKW4T—were isolated from a green microbial mat in an acidic hot spring. SKW1 and SKW2T cells were predominantly coccoid and rarely pleomorphic, whereas SKW3 and SKW4T cells were mostly pleomorphic. The cell sizes ranged from 0.3 to 2.0 μm. SKW1 and SKW2T grew at temperatures between 15 and 55 °C (optimally at 37–40 °C) and pH 1.5–5.0 (optimally 3.0–4.0). SKW3 and SKW4T also grew at temperatures between 15 and 55 °C (optimally at 30 °C) and pH 1.5–5.0 (optimally 1.8–2.0). All the strains were facultatively anaerobic and heterotrophic, requiring tryptone for growth. The dominant membrane lipid in both SKW2T and SKW3 was glycerol dibiphytanyl glycerol tetraether (GDGT). The genomic G + C mol% of SKW1 to SKW4T ranged from 35.8 to 38.1 mol%. In the 16S rRNA gene–based phylogenetic tree, all the strains were placed within the family Cuniculiplasmataceae and the order Thermoplasmatales. Sequence identities to the closest strain, Cuniculiplasma divulgatum strain S5T, were 94.28 % (SKW1), 94.35 % (SKW2T), 95.81 % (SKW3), and 95.88 % (SKW4T). Phylogenetic analyses showed that SKW 1 and SKW2T were distantly related to C. divulgatum strain S5T, whereas SKW3 and SKW4T were more closely related. Based on phylogenetic analyses and physiological properties of the four isolates, a novel genus, Caldiplasma gen. Nov., and two novel species Caldiplasma sukawensis sp. nov. and Cuniculiplasma thermophilum sp. nov. are proposed: Caldiplasma sukawensis SKW1 (=JCM 39523) and SKW2T (=JCM 39524T), and Cuniculiplasma thermophilum SKW3 (=JCM 39525) and SKW4T (=JCM 39526T).
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