Thrombolites are buildups of carbonate that exhibit a clotted internal structure formed through the interactions of microbial mats and their environment. Despite recent advances, we are only beginning to understand the microbial and molecular processes associated with their formation. In this study, a spatial profile of the microbial and metabolic diversity of thrombolite-forming mats of Highborne Cay, The Bahamas, was generated by using 16S rRNA gene sequencing and predictive metagenomic analyses. These molecular-based approaches were complemented with microelectrode profiling and in situ stable isotope analysis to examine the dominant taxa and metabolic activities within the thrombolite-forming communities. Analyses revealed three distinctive zones within the thrombolite-forming mats that exhibited stratified populations of bacteria and archaea. Predictive metagenomics also revealed vertical profiles of metabolic capabilities, such as photosynthesis and carboxylic and fatty acid synthesis within the mats that had not been previously observed. The carbonate precipitates within the thrombolite-forming mats exhibited isotopic geochemical signatures suggesting that the precipitation within the Bahamian thrombolites is photosynthetically induced. Together, this study provides the first look at the spatial organization of the microbial populations within Bahamian thrombolites and enables the distribution of microbes to be correlated with their activities within modern thrombolite systems. Key Words: Thrombolites-Microbial diversity-Metagenome-Stable isotopes-Microbialites. Astrobiology 17, 413-430.
One of the largest assemblages of living marine microbialites, with shapes and sizes analogous to ancient structures, is found along the margins of Hamelin Pool, Shark Bay, Western Australia. An investigation of microbial mats on the surfaces of these structures using petrographic analysis, light, and scanning electron microscopy identified the in situ precipitation of micrite as an important accretion mechanism in all major mat types (pustular, smooth, and colloform). Within each mat type, peloidal micrite, composed of nano-bulbous spheres to tabular and rod-shaped crystals, was closely linked with cells of the coccoid cyanobacterium Entophysalis, and microtextures of the micrite reflected the size and distribution of Entophysalis colonies. In pustular surface mats, where large colonies of Entophysalis were common, large clots of micrite were distributed randomly throughout the mat. In contrast, in smooth and colloform mats, where smaller colonies of Entophysalis were distributed along horizons, micrite formed fine laminae. In all surface mat types, micrite associated with Entophysalis had a characteristic honeycomb appearance, resulting from cell and/or colony entombment. These findings redefine our understanding of microbialite accretion in Hamelin Pool, recognizing the importance of microbial micrite in microbialite growth and showing that coccoid cyanobacteria are capable of building laminated structures. Moreover, Entophysalis, the dominant visible microbe associated with the precipitation of micrite in Hamelin Pool, has a lineage to Eoentophysalis, found throughout early and middle Proterozoic microbialites assemblages. These findings reinforce the importance of Hamelin Pool as a window to the past.
Microbialites and peloids are commonly associated throughout the geologic record. Proterozoic carbonate megafacies are composed predominantly of micritic and peloidal limestones often interbedded with stromatolitic textures. The association is also common throughout carbonate ramps and platforms during the Phanerozoic. Recent investigations reveal that Hamelin Pool, located in Shark Bay, Western Australia, is a microbial carbonate factory that provides a modern analog for the microbialite-micritic sediment facies associations that are so prevalent in the geologic record. Hamelin Pool contains the largest known living marine stromatolite system in the world. Although best known for the constructive microbial processes that lead to formation of these stromatolites, our comprehensive mapping has revealed that erosion and degradation of weakly lithified microbial mats in Hamelin Pool leads to the extensive production and accumulation of sand-sized micritic grains. Over 40 km2 of upper intertidal shoreline in the pool contain unlithified to weakly lithified microbial pustular sheet mats, which erode to release irregular peloidal grains. In addition, over 20 km2 of gelatinous microbial mats, with thin brittle layers of micrite, colonize subtidal pavements. When these gelatinous mats erode, the micritic layers break down to form platey, micritic intraclasts with irregular boundaries. Together, the irregular micritic grains from pustular sheet mats and gelatinous pavement mats make up nearly 26% of the total sediment in the pool, plausibly producing ~ 24,000 metric tons of microbial sediment per year. As such, Hamelin Pool can be seen as a microbial carbonate factory, with construction by lithifying microbial mats forming microbialites, and erosion and degradation of weakly lithified microbial mats resulting in extensive production of sand-sized micritic sediments. Insight from these modern examples may have direct applicability for recognition of sedimentary deposits of microbial origin in the geologic record.
<p>Organo-sedimentary structures built by benthic microbial communities, known as microbialites, dominated the fossil record for the first 3 billion years of Earth&#8217;s history. Various microbial metabolisms contribute to microbialite lithification, each of which can be based on biogeochemical cycling of elements capable of supporting life. Arsenic (As), a common element on the surface of Precambrian Earth, has been proposed to have supported the development of early life associated with the construction of primitive microbial carbonates. These As-based metabolisms have left evidence of their existence within the 2.7 Ga old Tumbiana stromatolites, showing the potential of this metalloid to serve as an archive of the dynamic interplay between microbes, minerals, and their environment of deposition throughout Earth&#8217;s history. However, significant changes in the geochemical composition of microbialites likely occur during early taphonomic modification and later diagenetic alteration. Therefore, establishing the mechanisms driving the arsenic geochemistry of ancient microbialites can be challenging.</p> <p>Motivated by these challenges, our objective was to evaluate the mechanisms controlling the initial incorporation of arsenic into actively accreting microbialites, as well as the preservation of the [As] signal during early taphonomic alteration of the structure. Hamelin Pool (Western Australia) is one of the few modern systems that host As-based metabolisms in the microbial communities involved in microbialite accretion. Conventional terminology recognizes four types of microbial mats that produce recognizable internal microfabrics in Hamelin Pool microbialites: pustular, smooth, colloform, and transitional mat types. Over time, these initial microfabrics all follow a similar evolution subdivided into two successive stages: (1) precipitation of micrite along laminations and around clots and; (2) precipitation of aragonitic marine cement. Therefore, Hamelin Pool microbialite fabrics provide a unique and step-wise window into the processes that form ancient microfabrics, particularly highlighting the importance of their early taphonomic evolution in the fate of the As biosignal originally incorporated during initial accretion of the structure.</p> <p>Based on microbialites collected from Hamelin Pool that have been characterized petrographically, we evaluated the evolution of [As] recorded in the Hamelin Pool microbialites at all stages of deposition and early taphonomic modifications. Results were interpreted in relation to the distinct microbial mats and their metabolisms, as well as the physicochemical and geological variability of the depositional environment. To accomplish this, we conducted a sequential leaching experiment to chemically isolate the organic matter and carbonate fractions, and measured As concentrations on a triple-quadrupole inductively coupled mass spectrometer (Agilent 8900 ICP-QQQ). Preliminary results show that elevated As concentrations are initially incorporated into microbial organic matter before being transferred to the carbonate fraction through successive stages of early taphonomic alteration. Because the carbonate fraction is diagenetically more resistant than the organic matter, this discovery could have major implications for the preservation of geochemical biosignatures in the geological record of microbialites. Our results serve as a first step towards improving the utility of [As] as an indicator of biogenicity in the fossil record of early Earth and, possibly, other planets such as Mars.</p>
Hamelin Pool, Shark Bay, Western Australia hosts the world's largest and most extensive assemblages of living marine microbialites, comparable in size and shape to ancient structures found throughout the fossil record. Here, we document the internal fabrics of modern microbialites collected throughout Hamelin Pool. Meso‐ and microscale observations of microbialite polished slabs and thin section scans, optical microscopy, and scanning electron microscopy coupled with energy‐dispersive X‐ray spectroscopy formed the basis for a fabric classification system that combines accretionary mat type with microfabric. Accretionary mat types included pustular, smooth, colloform, as well as ‘transitional’ mats that are a cross between pustular and smooth mats. Mapping of fabrics in 45 microbialite heads indicated bi‐directional evolution. An upward progression of fabrics corresponded to changes in mat type as the head grew upward into shallower water. A downward evolution of microfabrics occurred as surface mats transitioned into the subsurface of the microbialite structure. Downward microfabric evolution occurred as a result of early taphonomic processes, and involved a progression from the original depositional architecture to subsequent stages of micritic thickening, and finally, cement infilling. The observed bi‐directional evolution of microbialite microfabrics within Hamelin Pool offers a conceptual framework for the study of modern microbialites, not simply as the sole product of accretionary mat types but rather as the combined result of the activity of surface mats and their taphonomic evolution. Early taphonomic processes induce further lithification of the microbialites which may enhance preservation potential in the geological record.
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