Search Results (33)

The discovery of cyanobacteria fossils in microbialite prompts the investigation of carbonate biomineralization using cyanobacteria. However, the impact of coexisting magnesium and iron in microbialite on carbonate biomineralization has been overlooked. Here, Synechocystis sp. PCC 6803 was used to induce calcium carbonate in the presence of coexisting magnesium and ferric ions. The findings demonstrate that cell concentration, pH, carbonic anhydrase activity, and carbonate and bicarbonate concentrations decreased with increasing concentrations of magnesium and calcium ions. Ferric ions yielded a contrasting effect. The levels of deoxyribonucleic acid, protein, polysaccharides, and humic substances in extracellular polymeric substances increased in the presence of separated or coexisting calcium, magnesium, and ferric ions. Magnesium ions inhibited calcium ion precipitation, whereas ferric ions exhibited the opposite effect. Protein secondary structures became more abundant and O-C=O and N-C=O contents increased with increasing ion concentrations by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses. Scanning electron microscopy revealed that ferric ions lead to rougher surfaces and incomplete rhombohedral structures of calcite, whereas magnesium ions promoted greater diversity in morphology. Magnesium ions enhanced the incorporation of ferric ions. This work aims to further understand the effect of magnesium and ferric ions on calcium carbonate biomineralization induced by cyanobacteria. Full article

The Yacoraite Fm. (Salta rift basin, Argentina) consists of a mixed carbonate–siliciclastic lacustrine succession, interbedded with volcanic ash layers and organised in four third-order stratigraphic sequences. It is one of the few sites in South America that encompass the Cretaceous–Paleogene (K–Pg) transition, the position of which remains debated. Here, samples were collected along a depocentral stratigraphic section that was previously dated by zircon and carbonate U-Pb geochronology. The consistency between zircon and carbonate U-Pb ages, together with an accurate petrographic analysis, allowed the selection of carbonates potentially preserving the original geochemical signature. Accordingly, C-O stable isotopes were analysed from microbialites, oncoids, ooids and lacustrine cements. The available depositional age model from zircon geochronology defined the stratigraphic interval, potentially including the K–Pg transition. Within this interval, carbonates provided negative δ¹³C values consistent with the negative C anomaly recorded in various K–Pg sites elsewhere. Additionally, spherical particles resembling spherulites related to meteorite impacts were found in two samples. Accordingly, the K–Pg transition could be placed at the top of the second stratigraphic sequence. These findings encourage further investigation of the Yacoraite Fm. to gain insights into the response of South American terrestrial settings to the K–Pg palaeoenvironmental crisis. Full article

The Chigbrak Formation in the Upper Ediacaran is one of the important exploration targets in the Tarim Basin, NW China. However, no significant discoveries have been made in this field, and unclear reservoir genesis is one of the important factors restricting exploration breakthrough. This study examined the outcrops of the Aksu area in northwestern Tarim Basin by using systematic descriptions of petrologic features in the Upper Ediacaran Chigbrak Formation. Samples were selected for tests of stable carbon and oxygen isotopic compositions, strontium isotopic compositions, rare earth elements, LA–ICP–MS element mapping and U–Pb dating. It was found that (1) the Chigbrak Formation is mainly composed of dolomitic microbialite, with average values of δ¹³C (PDB), δ¹⁸O (PDB) and ⁸⁷Sr/⁸⁶Sr of 3.50‰, 2.95‰ and 0.709457, and has similar geochemical characteristics to the coeval seawater. The dolomites have also been characterized by a medium degree of cation ordering (avg. 0.68), a low content of ΣREEs (avg. 9.03 ppm) and a chondrite standardized curve of REEs showing enrichment of LREE and depletion of HREE. The U–Pb ages range from 538 to 618 Ma, corresponding to the age of Ediacaran period. (2) Dolomitization occurred in a marine diagenetic environment during the penecontemporaneous period, with seawater as the dolomitization fluid. (3) Vugs are the dominant pore type of Chigbrak Formation, and they are the products of the dissolution of meteoric water in penecontemporaneous period. (4) The main controlling factors of reservoir were lithofacies, meteoric water dissolution controlled by fourth– or fifth–order sequences and tectonic movement, and early dolomitization. The research results are of great significance to the dolomite reservoir prediction of the Upper Ediacaran Chigbrak Formation of the Tarim Basin. Full article

Microbialite textures, such as microbial mats and biofilms, were observed in the Middle Triassic dolomite in the Sichuan Basin, western China, using core examination, thin section petrography, scanning electron microscopy (SEM), and geochemical analyses. The dolomite texture, consisting of fibrous and spherulitic structures, is similar in morphology and size distribution to those observed in microbial culture experiments. Extracellular polymeric substances (EPS) were identified based on the occurrence of fibers forming a reticular pattern and nanometer-sized spheroids. The rare earth element (REE) and stable isotope (C, O, and Sr) compositions of the Middle Triassic dolomite were measured to determine their geochemical characteristics. Using seawater as a standard, the dolomitic microbialites (MD) exhibited significantly positive La and Eu anomalies and higher REE concentrations and (Nd/Yb)_sn values than associated limestones, and these patterns are inferred to be related to initial complexation on organic ligands in the biofilm, as proposed by previous researchers. The ambient temperature during dolomite precipitation was estimated to be within the 23 °C to 50 °C range, as indicated by the δ¹⁸O values of the dolomite. This study suggests that various microbial effects can significantly affect diagenetic processes in the Middle Triassic dolomite. Full article

The role of biogenicity in the mineral world is larger than many might assume. Biological processes and physical and chemical processes interact both at the Earth’s surface and far underground, leading to the formation of banded iron and manganese deposits, among others. Microbial mats can form giant sedimentary ore deposits, which include enrichment of further elements. This article reviews the ways in which microbially-mediated processes contribute to mineralization, the importance of mineralized microbial textural features, and the methods that must be used to obtain high-resolution datasets. If the chosen methodology and/or the size dimension of investigation is not appropriate, then it is not possible to recognize that a system is microbially mediated, and the conclusion will be incomplete. We call attention to variable authigenic mineralization as the result of complex mineralization of cells and extracellular polymeric substances in the starving basins, which form giant ore deposits together with ore-forming minerals. Microbial mats and other biosignatures can serve as indicators of environmental reconstruction in ore formations. We suggest tests and analyses that will allow the potential role of biomineralization to be properly investigated for a more comprehensive view of formation processes and their implications. Full article

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. Full article

Upper Triassic carbonate platforms from the Panthalassa Ocean remain less-understood and less-studied than their Tethyan equivalents. This imbalance is largely due to the poorer preservation state of Panthalassan carbonate rock successions in terms of rock quality and depositional geometries, which prevents good appreciation of depositional systems. In this context, carbonate exposures from Lime Peak (Yukon, Canada) represent an outstanding exception. There, the remains of an Upper Norian Panthalassan carbonate platform are well-exposed, show remarkably preserved depositional geometries and overall superior rock preservation. In this work, we analyse the carbonates from the Lime Peak area with particular attention to the vertical and lateral distribution of biotic assemblages and microfacies at the platform scale. Results demonstrate that the Lime Peak platform was surrounded by a basin with an aphotic sea bottom. The carbonate complex developed in warm waters characterized by high carbonate saturation. The area was also defined by moderate to high nutrient levels: this influenced the type of carbonate factory by favouring microbialites and sponges over corals. During its growth, Lime Peak was influenced by tectono-eustatism, which controlled the accommodation space at the platform top, primarily impacting the internal platform environments and the stability of the slope. Gaining better knowledge of the spatial distribution and dynamics of Upper Triassic organisms and sedimentary facies of Panthalassa in relation to tectono-eustatism lays the first foundations for reconstructing more robust platform models and understanding the evolution of other, more dismantled Upper Triassic Panthalassan carbonate systems through time. Full article

The Altiplano-Puna region is a high-altitude plateau in South America characterized by extreme conditions, including the highest UV incidence on Earth. The Laguna Negra is a hypersaline lake located in the Catamarca Province, northwestern Argentina, where stromatolites and other microbialites are found, and where life is mostly restricted to microbial mats. In this study, a particular microbial mat that covers the shore of the lake was explored, to unravel its layer-by-layer vertical structure in response to the environmental stressors therein. Microbial community composition was assessed by high-throughput 16S rRNA gene sequencing and pigment content analyses, complemented with microscopy tools to characterize its spatial arrangement within the mat. The top layer of the mat has a remarkable UV-tolerance feature, characterized by the presence of Deinococcus-Thermus and deinoxanthin, which might reflect a shielding strategy to cope with high UV radiation. Chloroflexi and Deltaproteobacteria were abundant in the second and third underlying layers, respectively. The bottom layer harbors copious Halanaerobiaeota. Subspherical aggregates composed of calcite, extracellular polymeric substances, abundant diatoms, and other microorganisms were observed all along the mat as the main structural component. This detailed study provides insights into the strategies of microbial communities to thrive under high UV radiation and hypersalinity in high-altitude lakes in the Altiplano-Puna region. Full article

Here we review the application of molecular biological approaches to mineral precipitation in modern marine microbialites. The review focuses on the nearly two decades of nucleotide sequencing studies of the microbialites of Shark Bay, Australia; and The Bahamas. Molecular methods have successfully characterized the overall community composition of mats, pinpointed microbes involved in key metabolisms, and revealed patterns in the distributions of microbial groups and functional genes. Molecular tools have become widely accessible, and we can now aim to establish firmer links between microbes and mineralization. Two promising future directions include “zooming in” to assess the roles of specific organisms, microbial groups, and surfaces in carbonate biomineralization and “zooming out” to consider broader spans of space and time. A middle ground between the two can include model systems that contain representatives of important microbial groups, processes, and metabolisms in mats and simplify hypothesis testing. These directions will benefit from expanding reference datasets of marine microbes and enzymes and enrichments of representative microbes from mats. Such applications of molecular tools should improve our ability to interpret ancient and modern microbialites and increase the utility of these rocks as long-term recorders of microbial processes and environmental chemistry. Full article

Following the end-Permian crisis, microbialites were ubiquitous worldwide. For instance, Triassic deposits in the Germanic Basin provide a rich record of stromatolites as well as of microbe-metazoan build-ups with nonspicular demosponges. Despite their palaeoecological significance, however, all of these microbialites have only rarely been studied. This study aims to fill this gap by examining and comparing microbialites from the Upper Buntsandstein (Olenekian, Lower Triassic) and the lower Middle Muschelkalk (Anisian, Middle Triassic) in Germany. By combining analytical petrography (optical microscopy, micro X-ray fluorescence, and Raman spectroscopy) and geochemistry (δ¹³C_carb, δ¹⁸O_carb), we show that all the studied microbialites formed in slightly evaporitic environments. Olenekian deposits in the Jena area and Anisian strata at Werbach contain stromatolites. Anisian successions at Hardheim, in contrast, host microbe-metazoan build-ups. Thus, the key difference is the absence or presence of nonspicular demosponges in microbialites. It is plausible that microbes and nonspicular demosponges had a mutualistic relationship, and it is tempting to speculate that the investigated microbial-metazoan build-ups reflect an ancient evolutionary and ecological association. The widespread occurrence of microbialites (e.g., stromatolites/microbe-metazoan build-ups) after the catastrophe may have resulted from suppressed ecological competition and the presence of vacant ecological niches. The distribution of stromatolites and/or microbe-metazoan build-ups might have been controlled by subtle differences in salinity and water depth, the latter influencing hydrodynamic processes and nutrient supply down to the microscale. To obtain a more complete picture of the distribution of such build-ups in the earth’s history, more fossil records need to be (re)investigated. For the time being, environmental and taphonomic studies of modern nonspicular demosponges are urgently required. Full article

Microbial mats represented the earliest complex ecosystems on Earth, since fossil mineralized examples (i.e., microbialites) date back to the Archean Eon. Some microbialites contain putative remains of organic matter (OM), however the processes and pathways that lead to the preservation of OM within microbialite minerals are still poorly understood. Here, a multidisciplinary study is presented (including petrographic, mineralogical and organic geochemical analyses), focusing on a modern calcifying mat from a hypersaline lake in the Kiritimati atoll (Central Pacific). The results show that this mat has a complex history, with two main growth phases under hypersaline conditions, separated by an interruption caused by desiccation and/or freshening of the lake. The mineral precipitates of the mat are predominantly aragonitic and two contrasting precipitation modes are observed: the main growth phases of the mat were characterized by the slow formation of irregular micritic particles with micropeloidal textures and subspherical particles, linked to the degradation of the exopolymer (EPS) matrix of the mat; whereas the interruption period was characterized by the rapid development of a thin but laterally continuous crust composed of superposed fibrous aragonite botryoids that entombed their contemporaneous benthic microbial community. These two precipitation modes triggered different preservation pathways for the OM of the mat as the thin crust shows a particular lipid biomarker signature, different from that of other layers and the relatively rapid precipitation of the crust protecting the underlying lipids from degradation, causing them to show a preservation equivalent to that of a modern active microbial community, despite them being >1100 years old. Equivalent thin mineral crusts occur in other microbialite examples and, thus, this study highlights them as excellent targets for the search of well-preserved biomarker signatures in fossil microbialites. Nevertheless, the results of this work warn for extreme caution when interpreting complex microbialite biomarker signatures, advising combined petrographic, mineralogical and geochemical investigations for the different microbialite layers and mineral microfabrics. Full article

Interpreting the paleoecosystems of ancient microbialites relies on our understanding of how modern microbialites form in relation with the bio-physico-chemical conditions of their environment. In this study, we investigated the formation of modern carbonate microbialites in the hydrothermal system of La Salsa in Laguna Pastos Grandes (Bolivia), which spans a wide range of physicochemical conditions and associated microbial communities. By combining dissolved inorganic carbon (DIC) isotope mass balance modeling, analysis of carbonates solubility diagram, and imaging of the microorganisms–mineral assemblages within microbial mats, we found that several modes of carbonate precipitation dominate in distinct portions of the hydrothermal system. (1) In high-[DIC] waters, undersaturated to slightly saturated with respect to calcite, cyanobacterial calcification is promoted by CO₂ degassing and photosynthetic activity within the microbial mats. (2) In alkaline waters undergoing sustained evaporation, the precipitation of an amorphous calcium carbonate phase seems to control the water a(Ca²⁺)/a(CO₃²⁻) ratio and to serve as a precursor to micritic calcite formation in microbial mats. (3) In saline ephemeral ponds, where the carbonate precipitation is the highest, calcite precipitation probably occurs through a different pathway, leading to a different calcite texture, i.e., aggregates of rhombohedral crystals. Full article

Mars has held large amounts of running and standing water throughout its history, as evidenced by numerous morphologies attributed to rivers, outflow channels, lakes, and possibly an ocean. This work examines the crater Antoniadi located in the Syrtis Major quadrangle. Some parts of the central area of the crater exhibit giant polygonal mud cracks, typical of endured lake bottom, on top of which a dark, tens of kilometers-long network of dendritic (i.e., arborescent) morphologies emerges, at first resembling the remnant of river networks. The network, which is composed of tabular sub-units, is in relief overlying hardened mud, a puzzling feature that, in principle, could be explained as landscape inversion resulting from stronger erosion of the lake bottom compared to the endured crust of the riverine sediments. However, the polygonal mud cracks have pristine boundaries, which indicate limited erosion. Furthermore, the orientation of part of the network is the opposite of what the flow of water would entail. Further analyses indicate the similarity of the dendrites with controlled diffusion processes rather than with the river network, and the presence of morphologies incompatible with river, alluvial, or underground sapping processes, such as overlapping of branches belonging to different dendrites or growth along fault lines. An alternative explanation worth exploring due to its potential astrobiological importance is that the network is the product of ancient reef-building microbialites on the shallow Antoniadi lake, which enjoyed the fortunate presence of a heat source supplied by the Syrtis Major volcano. The comparison with the terrestrial examples and the dating of the bottom of the crater (formed at 3.8 Ga and subjected to a resurfacing event at 3.6 Ga attributed to the lacustrine drape) contribute to reinforcing (but cannot definitely prove) the scenario of microbialitic origin for dendrites. Thus, the present analysis based on the images available from the orbiters cannot be considered proof of the presence of microbialites in ancient Mars. It is concluded that the Antoniadi crater could be an interesting target for the research of past Martian life in future landing missions. Full article

As we expand the search for life beyond Earth, a water-dominated planet, we turn our eyes to other aquatic worlds. Microbial life found in Earth’s many extreme habitats are considered useful analogs to life forms we are likely to find in extraterrestrial bodies of water. Modern-day benthic microbial mats inhabiting the low-oxygen, high-sulfur submerged sinkholes of temperate Lake Huron (Michigan, USA) and microbialites inhabiting the shallow, high-carbonate waters of subtropical Laguna Bacalar (Yucatan Peninsula, Mexico) serve as potential working models for exploration of extraterrestrial life. In Lake Huron, delicate mats comprising motile filaments of purple-pigmented cyanobacteria capable of oxygenic and anoxygenic photosynthesis and pigment-free chemosynthetic sulfur-oxidizing bacteria lie atop soft, organic-rich sediments. In Laguna Bacalar, lithification by cyanobacteria forms massive carbonate reef structures along the shoreline. Herein, we document studies of these two distinct earthly microbial mat ecosystems and ponder how similar or modified methods of study (e.g., robotics) would be applicable to prospective mat worlds in other planets and their moons (e.g., subsurface Mars and under-ice oceans of Europa). Further studies of modern-day microbial mat and microbialite ecosystems can add to the knowledge of Earth’s biodiversity and guide the search for life in extraterrestrial hydrospheres. Full article

Monohydrocalcite (CaCO₃·H₂O) is a mineral rarely found in natural environments. Here, we report finding of this mineral in the composition of the microbialites in Laguna de los Cisnes (Isla Grande, Chile), a saline alkaline lake with high Mg/Ca ratio. We have made a detailed structural and mineralogical description of these microbialites with the use of light and scanning electron microscopy, infrared spectroscopy and X-ray analysis. The predominantly carbonate composition of microbialites was revealed. Carbonates were represented mainly by high-magnesium calcites and monohydrocalcite. Calcite and aragonite were found in minor quantities. In addition, a small amount of silicates and amorphous hydromagnesite were found. The yellowish-brown surface layer of microbialites consists of numerous crystals within a mineralized exopolysaccharide (EPS) matrix. A large number of unicellular and filamentous algae, as well as areas of released EPS, are also seen here. Below is a slimy green layer. This layer is not mineralized; it represents an "algal-bacterial mat" consisting of algae, cyanobacteria, and diatoms developed in EPS. Chisel-shaped crystals of monohydrocalcite and its amorphous spherical precursors are numerous in these upper layers. The deeper layers are mineralized; they predominantly consist of Mg-carbonates with varying degrees of Mg. Algae and cyanobacteria are decomposed or fossilized there. Thus, monohydrocalcite occurs in the composition of the microbialites, being one of the main mineral components. As in other lacustrine localities, it is formed in the presence of algae and cyanobacteria. To our knowledge, this is the first report on the discovery of monohydrocalcite in South America. This research was funded by the Ministry of Science and Higher Education of the Russian Federation. Full article