School of Earth Sciences, The University of Western Australia, Perth, WA, Australia.
Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Perth, WA, Australia.
Geobiology. 2020 Nov;18(6):725-741. doi: 10.1111/gbi.12400. Epub 2020 May 28.
Fibrous-radiating carbonate spherulites spatially associated with poorly crystalline Mg-Si substances have formed within conical microbialites in modern hypersaline lakes on Rottnest Island, Western Australia. Two spherulitic fabrics can be distinguished based on compositional and textural differences. The oldest (lowermost) fabric comprises variably intergrown aragonitic spherulites 100-500 μm wide, containing micritic nuclei with coccoid cell molds in various stages of cell division. Spherulite matrices contain aggregates of individual nanospheres 150-200 nm wide, composed of a poorly crystalline Mg-Si phase, locally containing cell molds with similar dimensions to those within spherulite nuclei. The younger (upper) fabric comprises sub-polyhedral networks of mineralized EPS composed of an Mg-Si substance. The polyhedrons contain aragonite-replaced coccoid cells, voids, and polyhedral spherulites 8-12 μm wide with a morphology determined by fossil EPS, interpreted to have been produced by coccoid cyanobacteria. These spherulites are composed of high-Mg calcite, inferred to have formed in association with heterotrophic bacteria. Stable isotope data, textural relationships, and geochemical modeling are consistent with cyanobacterial oxygenic photosynthesis influencing the precipitation of Mg-Si substances and aragonitic spherulites by locally increasing the pH. The morphology of the polyhedral spherulites suggests the former presence of EPS and that faceted spherulites with similar dimensions in the geological record may represent biosignatures. The Rottnest Island conical microbialites demonstrate an intimate association between microbial features and processes and spherulitic fabrics, potentially providing insights into texturally and compositionally similar features in the geological record.
在澳大利亚西部罗特尼斯岛的现代高盐湖泊中，与非晶质 Mg-Si 物质空间相关的纤维状放射状碳酸盐球晶已在锥形微生物岩中形成。根据成分和结构的差异，可以区分出两种球晶结构。最古老（最底层）的结构由宽窄变化的混生文石球晶组成，球晶宽 100-500μm，包含微纹石核，其中有处于不同细胞分裂阶段的球菌细胞模具。球晶基质包含由不晶质 Mg-Si 相组成的单个纳米球的聚集体，宽 150-200nm，局部含有与球晶核内相似尺寸的细胞模具。较年轻（上层）的结构由矿化 EPS 的亚多面体网络组成，由 Mg-Si 物质组成。多面体包含被文石取代的球菌细胞、空隙和 8-12μm 宽的多面体形球晶，其形态由化石 EPS 决定，被解释为由球菌蓝细菌产生。这些球晶由高镁方解石组成，据推断是与异养细菌共生形成的。稳定同位素数据、结构关系和地球化学模拟都表明，蓝细菌的需氧光合作用通过局部增加 pH 值，影响了 Mg-Si 物质和文石球晶的沉淀。多面体形球晶的形态表明了 EPS 的存在，并且地质记录中具有相似尺寸的有面形球晶可能代表生物特征。罗特尼斯岛锥形微生物岩展示了微生物特征和过程与球晶结构之间的密切联系，这可能为地质记录中具有相似结构和成分的特征提供了一些见解。
