Exobiology Branch, Space Science & Astrobiology Division, NASA-Ames Research Center, Moffett Field, CA, USA.
Geobiology. 2019 Nov;17(6):643-659. doi: 10.1111/gbi.12355. Epub 2019 Jul 30.
Microbial mats that inhabit gypsum deposits in ponds at Guerrero Negro, Baja California Sur, Mexico, developed distinct pigmented horizons that provided an opportunity to examine the fixation and flow of carbon through a trophic structure and, in conjunction with previous phylogenetic analyses, to assess the diagenetic fates of molecular δ C biosignatures. The δ C values of individual biomarker lipids, total carbon, and total organic carbon (TOC) were determined for each of the following horizons: tan-orange (TO) at the surface, green (G), purple (P), and olive-black (OB) at the bottom. δ C of individual fatty acids from intact polar lipids (IPFA) in TO were similar to δ C of dissolved inorganic carbon (DIC) in the overlying water column, indicating limited discrimination by cyanobacteria during CO fixation. δ C of the underlying G was 3‰ greater than that of TO. The most δ C-depleted acetogenic lipids in the upper horizons were the cyanobacterial biomarkers C n-alkanes and polyunsaturated fatty acids. Bishomohopanol was 4 to 7‰ enriched, relative to alkanes and intact polar fatty acids (IPFA), respectively. Acyclic C isoprenoids were depleted by 14‰ relative to bishomohopanol. Significantly, ∆[δ C - δ C ] increased from 6.9‰ in TO to 14.7‰ in OB. This major trend might indicate that C-enriched residual organic matter accumulated at depth. The permanently anoxic P horizon was dominated by anoxygenic phototrophs and sulfate-reducing bacteria. P hosted an active sulfur-dependent microbial community. IPFA and bishomohopanol were C-depleted relative to upper crust by 7 and 4‰, respectively, and C isoprenoids were somewhat C-enriched. Synthesis of alkanes in P was evidenced only by C-depleted n-octadecane and 8-methylhexadecane. In OB, the marked increase of total inorganic carbon δ C (δ C ) of >6‰ perhaps indicated terminal mineralization. This δ C increase is consistent with degradation of the osmolyte glycine betaine by methylotrophic methanogens and loss of C-depleted methane from the mat.
墨西哥南下加利福尼亚州巴哈加利福尼亚湾内格罗雷阿诺湖中的石膏矿床中存在微生物垫，这些微生物垫形成了明显的有颜色的层，为研究碳通过营养结构的固定和流动提供了机会，并且结合先前的系统发育分析，评估了分子 δ¹³C 生物标志物的成岩命运。对以下各个层的单个生物标志物脂质、总碳和总有机碳（TOC）的 δ¹³C 值进行了测定：表面的黄褐色（TO）、绿色（G）、紫色（P）和橄榄黑（OB）。TO 中完整极性脂质（IPFA）中单个脂肪酸的 δ¹³C 值与上覆水柱中溶解无机碳（DIC）的 δ¹³C 值相似，表明在 CO 固定过程中蓝藻的辨别能力有限。下面的 G 的 δ¹³C 值比 TO 高 3‰。在较高的层中，最 δ¹³C 耗尽的产乙酸脂质是蓝藻生物标志物 Cn-烷烃和多不饱和脂肪酸。与烷烃和完整极性脂肪酸（IPFA）相比，bishomohopanol 分别富集了 4 到 7‰。无环异戊二烯萜烯的 δ¹³C 值相对于 bishomohopanol 降低了 14‰。显著的是，从 TO 的 6.9‰到 OB 的 14.7‰，∆[δ¹³C-δ¹³C]显著增加。这一主要趋势可能表明，富含 ¹³C 的残余有机质在深处积累。永久缺氧的 P 层主要由兼性光养生物和硫酸盐还原菌组成。P 层存在活跃的硫依赖型微生物群落。与上层地壳相比，IPFA 和 bishomohopanol 分别减少了 7‰和 4‰的 δ¹³C，而异戊二烯萜烯的 δ¹³C 值略有增加。只有 δ¹³C 耗尽的正十八烷和 8-甲基十六烷证明 P 层中烷烃的合成。在 OB 中，总无机碳 δ¹³C（δ¹³C）的显著增加（>6‰）可能表明是末端矿化作用。这种 δ¹³C 的增加与嗜甲基甲烷菌对渗透压调节剂甘氨酸甜菜碱的降解以及微生物垫中富含 ¹³C 的甲烷的损失一致。
