Abstract The composition of dissolved organic matter (DOM) in extremely deep groundwaters has rarely been investigated. Focusing on deep fracture waters in billion year old rocks in South Africa, we hypothesized the DOM would be predominantly derived from in situ microbial sources, consistent with H 2 -driven lithoautotrophic ecosystems previously documented in these fluids. We collected groundwater from 0.6 to 3.4 km depths via boreholes in mines, and characterized the DOM by a variety of analytical methods. Dissolved organic carbon concentrations ranged from 0.25 to 4.9 mg C/L. Low molecular weight organic acids (formate, acetate, lactate, propanoate) and amino acids (after hydrolysis) accounted for varying fractions of the total dissolved organic carbon. UV spectrophotometry, scanning fluorimetry, and nuclear magnetic resonance (NMR) spectra indicated lower concentrations of aromatic compounds than are commonly found in surface waters and shallow groundwaters. Molecular weights of aromatic compounds measured by size-exclusion chromatography were dominantly 500–1000 Da. Scanning fluorimetry revealed humic substances in two shallow (578 and 1300 m) samples; deeper samples showed peaks indicating aromatic amino acids. Intensive study of one sample from a previously sampled borehole in Beatrix mine resulted in large NMR peaks for aliphatics and carboxyls with lesser aromatics peaks; negative electrospray ion Fourier transform ion cyclotron resonance-mass spectrometry detected protein-, lipid-, and lignin-like compounds. These results are consistent with DOM in the form of microbial metabolites, and modified cell components (membrane lipids and proteins). These findings support previous geomicrobiological reports of chemolithoautotrophic microbial ecosystems functioning in isolation from photosynthetic primary production.

中文翻译：

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南非 Kaapvaal Craton 0.6-3.4 km 深裂隙水中溶解的有机物质成分

摘要 极深地下水中溶解有机物（DOM）的组成很少被研究。关注南非 10 亿年前岩石中的深裂缝水，我们假设 DOM 将主要来自原位微生物来源，这与先前在这些流体中记录的 H 2 驱动的岩石自养生态系统一致。我们通过矿井中的钻孔收集了 0.6 至 3.4 公里深度的地下水，并通过各种分析方法对 DOM 进行了表征。溶解的有机碳浓度范围为 0.25 至 4.9 mg C/L。低分子量有机酸（甲酸盐、乙酸盐、乳酸盐、丙酸盐）和氨基酸（水解后）占总溶解有机碳的不同部分。紫外分光光度法、扫描荧光法、和核磁共振 (NMR) 光谱表明，芳香族化合物的浓度低于地表水和浅层地下水中常见的浓度。通过尺寸排阻色谱法测量的芳香族化合物的分子量主要为 500-1000 Da。扫描荧光法显示两个浅层（578 米和 1300 米）样品中含有腐殖质；更深的样品显示出表明芳香族氨基酸的峰。对来自 Beatrix 矿井先前采样的钻孔中的一个样品进行深入研究，发现脂肪族和羧基的 NMR 峰较大，芳香族峰较小；负电喷雾离子傅立叶变换离子回旋共振质谱检测蛋白质、脂质和木质素类化合物。这些结果与微生物代谢物形式的 DOM 一致，和修饰的细胞成分（膜脂质和蛋白质）。这些发现支持了先前关于化学自养微生物生态系统与光合初级生产分离的地球微生物学报告。