Sanz-Montero M Esther, Cabestrero Óscar, Sánchez-Román Mónica
Department of Mineralogy and Petrology, Faculty of Geological Science, Complutense University of Madrid, Madrid, Spain.
Department of Geology and Geochemistry, Faculty of Science, Vrije Universiteit Amsterdam, Amsterdam, Netherlands.
Front Microbiol. 2019 Feb 7;10:148. doi: 10.3389/fmicb.2019.00148. eCollection 2019.
This paper provides strong evidence for the contribution of the phylum Firmicutes in mediating the primary precipitation of Mg-rich carbonates (hydromagnesite, dolomite, magnesite, and nesquehonite) in recent microbialites from a highly alkaline and ephemeral inland lake (Las Eras, Central Spain). The carbonate mineral precipitation occurs sequentially as the microbial mats decay. Scanning electron microscopy (SEM) provided solid proof that hydromagnesite nucleation is initiated on the exopolymeric substances (EPS) and the microbial cells associated to the microbial mat degradation areas. The progressive mineralization of the EPS and bacterial cells by hydromagnesite plate-like crystals on their surface, results in the entombment of the bacteria and formation of radiating aggregates of hydromagnesite crystals. The hydrous phases, mostly hydromagnesite, were produced at a high percentage in the first stages of the microbial degradation of organic matter. When the availability of organic substrates declines, the heterotrophs tend to reduce their number and metabolic activity, remain dormant. At this stage, the anhydrous phases, dolomite and magnesite, nucleate on bacterial nanoglobules and/or collapsed cells. Evidence for the sequential formation of the Mg-rich carbonates trough the decay of organic matter by a fermentative EPS-forming bacterium isolated from the microbialites, , is drawn through a comparative analysis of carbonate formation in both natural and experimental settings. This study will help to constrain potential mechanisms of carbonate formation in natural systems, which are of fundamental importance not only for understanding modern environments but also as a window into the geologic past of Earth and potentially Mars.
本文提供了有力证据，证明厚壁菌门在介导富含镁的碳酸盐（水菱镁矿、白云石、菱镁矿和镍菱镁矿）在西班牙中部一个高度碱性且短暂的内陆湖（拉斯埃拉斯）近期微生物岩中的初级沉淀过程中发挥了作用。随着微生物垫的腐烂，碳酸盐矿物沉淀依次发生。扫描电子显微镜（SEM）提供了确凿证据，表明水菱镁矿的成核起始于胞外聚合物（EPS）以及与微生物垫降解区域相关的微生物细胞上。EPS和细菌细胞表面被水菱镁矿板状晶体逐渐矿化，导致细菌被包裹，并形成水菱镁矿晶体的放射状聚集体。在有机物微生物降解的第一阶段，含水相（主要是水菱镁矿）大量生成。当有机底物的可用性下降时，异养生物倾向于减少其数量和代谢活性，进入休眠状态。在此阶段，无水相白云石和菱镁矿在细菌纳米球和/或塌陷细胞上成核。通过对从微生物岩中分离出的一种形成EPS的发酵细菌在自然和实验环境中碳酸盐形成的比较分析，得出了通过有机物腐烂依次形成富含镁的碳酸盐的证据。这项研究将有助于限制自然系统中碳酸盐形成的潜在机制，这不仅对于理解现代环境至关重要，而且作为了解地球乃至火星地质历史的一个窗口也具有重要意义。
