Martian lava tube caves resulting from a time when the planet was still volcanically active are proposed to contain deposits of water ice, a feature that may increase microbial habitability. In this study, we taxonomically characterized and directly measured metabolic activity of the microbial communities that inhabit lava tube ice from Lava Beds National Monument, an analogue environment to martian lava tubes. We investigated whether this environment was habitable to microorganisms by determining their taxonomic diversity, metabolic activity, and viability using both culture-dependent and culture-independent techniques. With 16S rRNA gene sequencing, we recovered 27 distinct phyla from both ice and ice–rock interface samples, primarily consisting of Actinobacteria, Proteobacteria, Bacteroidetes, Firmicutes, and Chloroflexi. Radiorespiration and Biolog EcoPlate assays found these microbial communities to be metabolically active at both 5°C and −5°C and able to metabolize diverse sets of heterotrophic carbon substrates at each temperature. Viable cells were predominantly cold adapted and capable of growth at 5°C (1.3 × 10⁴ to 2.9 × 10⁷ cells/mL), and 24 of 38 cultured isolates were capable of growth at −5°C. Furthermore, 14 of these cultured isolates, and 16 of the 20 most numerous amplicon sequences we recovered were most closely related to isolates and sequences obtained from other cryophilic environments. Given these results, lava tube ice appears to be a habitable environment, and considering the protections martian lava tubes offer to microbial communities from harsh surface conditions, similar martian caves containing ice may be capable of supporting extant, active microbial communities.

中文翻译：

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熔岩床国家纪念碑熔岩管冰洞中冷适应微生物群落的分类学特征和微生物活动测定，高保真火星模拟环境

火星火山仍处于火山活动时期形成的熔岩管洞穴被认为含有水冰沉积物，这一特征可能会增加微生物的宜居性。在这项研究中，我们对居住在熔岩床国家保护区熔岩管冰中的微生物群落进行分类表征并直接测量其代谢活动，熔岩床国家纪念碑是火星熔岩管的类似环境。我们通过使用依赖培养和不依赖培养的技术确定微生物的分类多样性、代谢活动和生存能力来研究这种环境是否适合微生物居住。通过 16S rRNA 基因测序，我们从冰和冰-岩界面样品中回收了 27 个不同的门，主要由放线菌、变形菌、拟杆菌、厚壁菌门和绿屈曲菌组成。放射呼吸和 Biolog EcoPlate 测定发现这些微生物群落在 5°C 和 -5°C 下都具有代谢活性，并且能够在每个温度下代谢不同组的异养碳底物。活细胞主要是冷适应的，能够在 5°C (1.3 × 10⁴至 2.9 × 10 ^{7 个}细胞/mL)，38 个培养分离株中的 24 个能够在 -5°C 下生长。此外，这些培养的​​分离株中有 14 个，以及我们回收的 20 个最多的扩增子序列中的 16 个与从其他低温环境中获得的分离株和序列最密切相关。鉴于这些结果，熔岩管冰似乎是一个宜居的环境，考虑到火星熔岩管为微生物群落提供免受恶劣表面条件的保护，类似的含冰的火星洞穴可能能够支持现存的、活跃的微生物群落。