Lava caves, formed through basaltic volcanism, are accessible conduits into the shallow subsurface and the microbial life residing there. While evidence for this life is widespread, the level of dependence of these microbial communities on surface inputs, especially that of organic carbon (OC), is a persistent knowledge gap, with relevance to both terrestrial biogeochemistry and the characterization of lava caves as Mars analog environments. Here, we explore carbon cycling processes within lava caves at Lava Beds National Monument, CA. We interrogate a range of cave features and surface soils, characterizing the isotopic composition (δ¹³C) of bulk organic and inorganic phases, followed by organic geochemical analysis of the distribution and δ¹³C signatures of fatty acids derived from intact polar lipids (IPLs). From these data, we estimate the carbon sources of different sample types, finding that surface soils and mineral-rich speleothems incorporate plant-derived biomass (δ¹³C_VPDB ∼ −30‰), whereas biofilms are dominated by strongly ¹³C-depleted lipids (minimum δ¹³C_VPDB −45.4‰) specific to bacteria, requiring a significant proportion of their biomass to derive from in situ fixation of inorganic carbon from previously respired OC. Based on the prevalence and abundance of these ¹³C-depleted lipids, we conclude that biofilms here are fueled by in situ chemolithoautotrophy, despite relatively high concentrations of dissolved OC in colocated cave waters. This unexpected metabolic potential mirrors that found in other deep subsurface biospheres and has significant positive implications for the potential microbial habitability of the Martian subsurface.

中文翻译：

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脂质生物标志物中的稳定碳同位素消耗表明熔岩洞穴中的地下碳固定

通过玄武岩火山作用形成的熔岩洞穴是进入浅层地下和居住在那里的微生物生命的通道。虽然这种生命的证据很普遍，但这些微生物群落对地表输入的依赖程度，尤其是有机碳 (OC)，是一个持续存在的知识差距，与陆地生物地球化学和熔岩洞穴的特征有关，如火星类似物环境。在这里，我们探索了加利福尼亚州熔岩床国家纪念碑的熔岩洞穴内的碳循环过程。我们询问了一系列洞穴特征和表层土壤，表征了大量有机相和无机相的同位素组成 ( δ ¹³ C)，然后对分布和δ ¹³进行了有机地球化学分析来自完整极性脂质 (IPL) 的脂肪酸的 C 特征。从这些数据中，我们估计了不同样本类型的碳源，发现表层土壤和富含矿物质的洞穴植物包含植物来源的生物量 ( δ ¹³ C _VPDB  ∼ -30‰)，而生物膜主要由¹³ C 耗尽的脂质主导(最小δ ¹³ C _VPDB -45.4‰) 特定于细菌，需要很大比例的生物量来自先前呼吸的 OC 的无机碳的原位固定。基于这¹³C 耗尽的脂质，我们得出结论，尽管共存洞穴水中溶解的 OC 浓度相对较高，但这里的生物膜是由原位化学自养促进的。这种意想不到的代谢潜力反映了在其他深层地下生物圈中发现的情况，并对火星地下的潜在微生物宜居性具有重要的积极意义。