Tufas are freshwater carbonate rocks that form in continental environments through a combination of physical, chemical, and biological processes. This study investigates the role of microorganisms in the precipitation of Quaternary tufa deposits in the Serra da Bodoquena Formation, in the Bonito region. Two sites along the Mimoso River, named Taíka and Mimosa, characterized by the pool–barrage–cascade depositional subenvironment, were selected for this study. Four distinct facies were identified: stromatolitic boundstones, phytoherm boundstones of algae, phytoherm boundstones of bryophytes, and phytoclastic rudstones. These facies were observed in diverse hydrological settings, including fast-flowing waters, such as waterfalls and cascades, as well as slow-flowing areas, such as pools and dams. The δ18O depletion indicated a meteoric origin for the fluid involved in carbonate precipitation. The low δ13C values were attributed to photosynthetic processes and the contribution of light carbon-enriched groundwater. The presence of Oocardium stratum and calcified organic mucilage from extracellular polymeric substance (EPS) corroborates the significant role of microorganisms in tufa formation, particularly in stromatolitic boundstones and phytoherm boundstones of algae. Rapid CO2 degassing significantly contributes to mineralization in fast-flowing waters. Micro-CT results offer detailed insights into the relationship between mechanical processes and biological influences in shaping porosity characteristics. The findings of this study significantly enhance our understanding of the role of microorganisms in tufa formation, highlighting the complex interplay between biotic and abiotic processes in the development of different tufa facies. Moreover, the insights gained from this study provide valuable implications for interpreting tufa deposits worldwide.

Hemoglycin, a space polymer of glycine and iron, has been identified in the carbonaceous chondritic meteorites Allende, Acfer 086, Kaba, Sutter's Mill and Orgueil. Its core form has a mass of 1494 Da and is basically an antiparallel pair of polyglycine strands linked at each end by an iron atom. The polymer forms two- and three- dimensional lattices with an inter-vertex distance of 4.9 nm. Here the extraction technique for meteorites is applied to a 2.1 Gya fossil stromatolite to reveal the presence of hemoglycin by mass spectrometry. Intact ooids from a recent (3000 Ya) stromatolite exhibited the same visible hemoglycin fluorescence in response to x-rays as an intact crystal from the Orgueil meteorite. X-ray analysis confirmed the existence in ooids of an internal three-dimensional lattice of 4.9 nm inter-vertex spacing, matching the spacing of lattices in meteoritic crystals. FTIR measurements of acid-treated ooid and a Sutter's Mill meteoritic crystal both show the presence, via the splitting of the Amide I band, of an extended anti-parallel beta sheet structure. It seems probable that the copious in-fall of carbonaceous meteoritic material, from Archaean times onward, has left traces of hemoglycin in sedimentary carbonates and potentially has influenced ooid formation.

The Lagoa Salgada is located in the Paraíba do Sul river delta plain on the coast of Rio de Janeiro state, Brazil, and is one of the few lagoons in the world that have well-developed recent stromatolites. Lagoa Salgada is a hypersaline lagoon formed in a very complex environmental system subjected to terrestrial and oceanic influences under different sea level regimes and climate variations. In addition, sediment and stromatolites are characterized by unusually positive inorganic δ13C VPDB values. For this reason, it has been the target of several geological and paleoenvironmental studies, which, in their great majority, require a geochronological technique in order to determine the changes in the environment over time. When radiocarbon (14C) dating is used, it is necessary to consider some details as the source of 14C in the environment and perform 14C ages calibration accordingly. In the present paper, a bibliographic survey was carried out in order to review the data treatment and improve the environmental evolution discussion based on accurate calibration. Using the Marine20 curve and an undetermined ΔR, we generated growth and depositional models to establish an overview of the formation of this lagoon.

The Drewer quarry located in the Rhenish Massif is a well-studied outcrop that comprises Upper Devonian (Famennian) to Lower Carboniferous (Viséan) strata. Within the Drewer deposits two black shale intervals have been described that are linked to two global oceanic anoxic events, the Hangenberg Event and the Lower Alum Shale Event. The black shales associated with the Middle Tournaisian Lower Alum Shale Event contain abundant phosphatic concretions, which were investigated using thin section petrography, powder X-ray diffraction, Fourier-transform infrared spectrometry and scanning electron microscopy. The concretions formed during several growth phases under anoxic and at least episodically sulphidic conditions within the sediment and served as a substrate for subsurface microbial mats that formed phosphatic microstromatolites. The microstromatolites occur either as partially branched columns of up to 600 µm in length attached to the phosphatic concretions or as smaller, bulbous aggregates surrounding the concretions. Element mapping identified the presence of pyrite and other metal sulphides within the phosphatic microstromatolites. The carbon and oxygen stable isotopic composition of phosphate-associated carbonate within the phosphatic microstromatolites suggests that the mat-forming microorganisms were probably anaerobic, chemotrophic microbial communities dwelling in the anoxic environment during the Lower Alum Shale Event. Such interpretation agrees with the deeper-water depositional setting of the Lower Alum Black Shale and its high content of organic matter, suggesting that chemotrophic microbial mats are potent agents of phosphogenesis in general, and of the formation of phosphatic stromatolites in particular.

Time-distinctive features within carbonate rocks, specifically the presence, abundance and distribution of stromatolites, molar-tooth fabric and specific morphologies of marine cement, have been identified as potential indicators of global-scale changes in the chemistry of marine environments. Recently, Cantine et al. (2019) introduced a database approach seeking to quantify spatial and temporal patterns in these carbonate features through the Precambrian. Despite the coarse temporal scale, results support earlier inferences of temporal change in carbonate sedimentation. Here, we use original field notes to dissect late Mesoproterozoic (˜1.3 to 1.0 Ga) carbonate strata at a high resolution, analyse time-distinctive carbonate fabrics within a database context and compare sedimentological patterns within this narrow time range to observations of the Proterozoic as a whole. Late Mesoproterozoic strata contain a variety of features (e.g. stromatolites, seafloor precipitates, herringbone carbonate, molar-tooth carbonate), often in close spatial and temporal proximity, that are commonly considered to be temporally restricted during the Precambrian. The spatial distribution of such features within Mesoproterozoic basins demonstrates the importance of recognizing even rare occurrences of time-distinctive facies and permits inference of environmental drivers that may have interacted to affect carbonate precipitation. Such spatial variability reflects a subtle division of Mesoproterozoic carbonate platform environments driven by globally high sea level, elevated carbonate saturation and a low-oxygen water column. The heterogeneous, mosaic nature of environments appears to be a hallmark of Mesoproterozoic carbonate sedimentation and emphasizes the importance of these basins in understanding longer-term trends in carbonate deposition.

During the Noachian period, 4.1-3.7 Gys ago, the Martian environment was moderately similar to the one on present Earth. Liquid water was widespread in a neutral environment, volcanic activity and heat flow more vigorous, and atmospheric pressure and temperature were higher than today. These conditions may have favoured the spread of life on the surface of Mars. The recognition that different planets and moons share rocky material cast in space by meteoroid impact entails that life creation is not necessary for each single planetary body, but could travel through the Solar system on board of rock fragments. Studies conducted on the past forms of Martian life have already highlighted possible positive matches with microbialite-like structures, referable to the geo-environmental conditions in the Noachian and Hesperian. However, by necessity, these studies are on predominantly micro and meso-scopic scale structures and doubts arise as to their attribution to the biogenic world. We suggest that in the identification of Martian life, we are currently in a position similar to the one of Kalkowsky who in 1908, based solely on morphological and sedimentological arguments, hypothesized the (now accepted) view of the biotic origin of stromatolites. Our analysis of thousands of images from Spirit, Opportunity and Curiosity has provided a selection of images of ring-shaped, domal and coniform macrostructures that resemble terrestrial microbialites such as the ring-shaped stromatolites of Lake Thetis, and stacked cones reminiscent of the group of terrestrial Conophyton. Notably, the latter were detected by Curiosity in the mudstone known as ‘Sheepbed’, the same outcrop where past organic molecules have been detected and where the occurrence of microbial-induced sedimentary structures (MISS) and of many more microbialitic micro, meso and macrostructures has already been hypothesized. Some of the structures discussed in this work are so complex that alternative biological hypotheses can be formulated as possible algae. Alternate, non-abiotic explanations are examined but we find difficult to explain some of such structures in the context of normal sedimentary processes, both syngenetic or epigenetic.

This study, using the Microscopic Imager (MI) of NASA Rover Exploration Mission's (REM) ‘Opportunity’, aims to explain the origin of laminated sediments lying at Meridiani Planum of Mars, and of the strange spherules, known as blueberries, about which several hypotheses have been formulated. To this purpose, images of the sedimentary textures of layers and fragments captured by REM have been analysed; sediments that NASA has already established as ‘pertinent to water presence’. Our study shows that such laminated sediments and the spherules they contain could be organosedimentary structures, probably produced by microorganisms. The laminated structures are characterized by a sequence of a thin pair of layers, which have the features of skeletal/agglutinated laminae and whose basic constituents are made by a partition of septa and vacuoles radially arranged around a central one. The growth of these supposed organosedimentary masses is based on the ‘built flexibility’ of such a basal element; it may be a coalescing microfossil formed by progressive film accretion (calcimicrobe), in a variety of geometrical gross forms, such as a repeated couplet sequence of laminae or domal mass and large composite polycentric spherule, both in elevation. The acquired structural and textural data seem to be consistent with the existence of life on Mars and could explain an origin of sediments at Meridiani Planum similar to that of terrestrial stromatolites. The Martian deposits, probably produced by cyanobacterial activity, and the embedded blueberries could represent a recurrent and multiform product of colonies with sheath forms, resembling in shape those of the fossil genus Archaeosphaeroides (stromatolites of Fig Tree, South Africa).

The cyanobacterial fossil record is among the oldest for any group of organisms, possibly reaching back to 3500 Ma ago. The molecular phylogeny of cyanobacteria is complementary to the fossil findings, confirming the antiquity of the group, the role of cyanobacteria in the evolution of planetary primary production, and the symbiotic origins of plastids in algae and plants from cyanobacterial ancestors. The study of fossil cyanobacteria followed the discovery of Precambrian microbial fossils by S.A Tyler and E.S. Barghoorn in 1954, and is still developing. Most fossil cyanobacteria are preserved in permineralized conditions in cherts and phosphorites or as organic compressions in shales. The interpretation of fossil cyanobacteria is aided by the study of modern counterparts, preferably within their natural habitats. These comparisons include the post mortem degradation of cellular remains. The fortuitous preservation and fossilization of ancient cyanobacterial communities in growth position, i.e. in the synsedimentary context, allows one to draw conclusions about their palaeoenvironment, including interactions between cyanobacteria and ancient sediments. These relations are based on cyanobacterial ecological requirements, and they compare well with behavioural responses of modern cyanobacteria in microbial mats and modern stromatolites. The general trend in the evolution of cyanobacteria is one of gradually increasing complexity and diversity, but the group shows a conservative maintenance of morphological adaptations to successful ecological niches. Accordingly, a large proportion of ancient morphological types is still represented among modern cyanobacteria. Fossil to Recent counterparts are identified for several coccoid and filamentous cyanobacteria. Evidence for heterocystous cyanobacteria is indirect, through identification of fossil akinetes.

Geomicrobiological analysis of calcifying biofilms of three alkaline salt lakes characterized by moderate to high carbonate alkalinity indicates that microbial carbonate rock formation is not directly linked to cyanobacterial carbon fixation. The present review summarizes results from two published case studies that have been carried out at Pyramid Lake, USA, and Lake Nuoertu, PR China. New observations and data are presented for a current project on Satonda Crater Lake, Indonesia, that revise previous conclusions concerning the relationship between cyanobacteria and biofilm calcification. Extracellular polymeric substances (EPS) in the investigated lakes are mostly produced by cyanobacteria; their properties are discussed as key factors in biofilm calcification. In particular, EPS are capable of binding divalent cations (e.g. Ca2+) from the liquid phase by their carboxylate and sulphate groups. Therefore, despite a high supersaturation of the lake water with respect to calcium carbonate minerals, precipitation does not take place immediately. A delayed onset of precipitation can be achieved by a continuous Ca2+ supply that exceeds the Ca2+-binding capacity of the EPS, and/or an exoenzymatic degradation (decarboxylation, cleavage) of mucous substances that reduces the binding capacity and causes secondary Ca2+ release. The resulting microcrystalline precipitates are randomly distributed within the EPS, usually away from any of the living cyanobacteria. This suggests that the effect of photosynthetic CO2 fixation in increasing supersaturation is of secondary importance at high alkalinities. In contrast to biofilm-covered surfaces, calcium carbonate minerals nucleate and grow rapidly at surfaces poor in EPS when the critical supersaturation level for non-enzymatically controlled carbonate precipitation is reached. Examples of such surfaces poor in EPS are dead, lysed green algal cells and thin, discontinuous biofilms in voids of microbial reef rocks. Calcium carbonate crystals directly linked to cyanobacterial cells or filaments have been observed only exceptionally, e.g. on Calothrix.

A brief history of research into the unusual laminated and spherulitic East Kirkton Limestone (Brigantian, Viséan) is given. It documents the paucity of fossil finds until 1984, when S. P. Wood discovered a previously unsuspected terrestrial biota. Some early finds, such as stromatolites, have since been overlooked, while others were misinterpreted, e.g. a ‘fish-bone’ collected before 1870 proves to be amphibian. A formal lithostratigraphic description is given of the three beds that comprise the c. 15 m sequence in East Kirkton Quarry: the East Kirkton Limestone, overlain by Little Cliff Shale and Geikie Tuff, the latter two being here formally named. Lithological descriptions and a unit-by-unit distribution of major elements of the biota are provided for a measured section comprising 88 units. This is based on bed-by-bed excavations carried out 1985-92, supplemented by nine boreholes. Two 3 m-thick massive limestones lenses are recorded from the quarry and a probable third was proved in a borehole. Brief reports are given on the ‘stromatolites’ (by M. R. Walter), bivalve molluscs (R. M. C. Eagar and R. B. Wilson), ostracods (J. Pollard) and gypsum pseudomorphs.