Evidence of oxygenic phototrophy in ancient phosphatic stromatolites from the Paleoproterozoic Vindhyan and Aravalli Supergroups, India

Avoiding and allowing apatite precipitation in oxygenic photolithotrophs

The essential elements Ca and P, taken up and used metabolically as Ca²⁺ and H₂ PO₄ ^- /HPO₄ ^2- respectively, could precipitate as one or more of the insoluble forms calcium phosphate (mainly apatite) if the free ion concentrations and pH are high enough. In the cytosol, chloroplast stroma, and mitochondrial matrix, the very low free Ca²⁺ concentration avoids calcium phosphate precipitation, apart from occasionally in the mitochondrial matrix. The low free Ca²⁺ concentration in these compartments is commonly thought of in terms of the role of Ca²⁺ in signalling. However, it also helps avoids calcium phosphate precipitation, and this could be its earliest function in evolution. In vacuoles, cell walls, and xylem conduits, there can be relatively high concentrations of Ca²⁺ and inorganic orthophosphate, but pH and/or other ligands for Ca²⁺ , suggests that calcium phosphate precipitates are rare. However, apatite is precipitated under metabolic control in shoot trichomes, and by evaporative water loss in hydathodes, in some terrestrial flowering plants. In aquatic macrophytes that deposit CaCO₃ on their cell walls or in their environment as a result of pH increase or removal of inhibitors of nucleation or crystal growth, phosphate is sometimes incorporated in the CaCO₃ . Calcium phosphate precipitation also occurs in some stromatolites.

2-Methylhopanoids in geographically distinct, arid biological soil crusts are primarily cyanobacterial in origin

Microbial palaeontology is largely reliant on the interpretation of geologically stable biomarkers or molecular fossils. Biomolecules that are both specific to particular groups of organisms and stable on a geological scale are invaluable for tracing the emergence and diversification of lifeforms, particularly in cases where mineral fossils are lacking. 2-Methylhopanoids and their diagenic product, 2-methylhopanes, are highly abundant bacterial membrane lipids, recoverable from samples in excess of a billion years old. In this work we used degenerate PCR, targeting 2-methylhopanoid biosynthesis genes, and sequencing to show that the ability to produce these molecules in arid biological soil crusts from deserts in diverse geographical locations (Utah, USA, and the Pilbara, Australia) is largely confined to cyanobacteria. These data suggest that 2-methylhopanes can be used as a proxy for cyanobacterial presence within these environments, contributing to our understanding of the emergence of terrestrial life on Earth.

Acid Mine Drainage as Energizing Microbial Niches for the Formation of Iron Stromatolites: The Tintillo River in Southwest Spain

The Iberian Pyrite Belt in southwest Spain hosts some of the largest and diverse extreme acidic environments with textural variation across rapidly changing biogeochemical gradients at multiple scales. After almost three decades of studies, mostly focused on molecular evolution and metagenomics, there is an increasing awareness of the multidisciplinary potential of these types of settings, especially for astrobiology. Since modern automatized exploration on extraterrestrial surfaces is essentially based on the morphological recognition of biosignatures, a macroscopic characterization of such sedimentary extreme environments and how they look is crucial to identify life properties, but it is a perspective that most molecular approaches frequently miss. Although acid mine drainage (AMD) systems are toxic and contaminated, they offer at the same time the bioengineering tools for natural remediation strategies. This work presents a biosedimentological characterization of the clastic iron stromatolites in the Tintillo river. They occur as laminated terraced iron formations that are the most distinctive sedimentary facies at the Tintillo river, which is polluted by AMD. Iron stromatolites originate from fluvial abiotic factors that interact with biological zonation. The authigenic precipitation of schwertmannite and jarosite results from microbial-mineral interactions between mineral and organic matrices. The Tintillo iron stromatolites are composed of bacterial filaments and diatoms as Nitzschia aurariae, Pinnularia aljustrelica, Stauroneis kriegeri, and Fragilaria sp. Furthermore, the active biosorption and bioleaching of sulfur are suggested by the black and white coloration of microbial filaments inside stromatolites. AMD systems are hazardous due to physical, chemical, and biological agents, but they also provide biogeochemical sources with which to infer past geochemical conditions on Earth and inform exploration efforts on extraterrestrial surfaces in the future.

A 571 million-year-old alkaline volcanic lake photosynthesizing microbial community, the Anti-atlas, Morocco

The Ediacaran period coincides with the emergence of ancestral animal lineages and cyanobacteria capable of thriving in nutrient deficient oceans which together with photosynthetic eukaryotic dominance, culminated in the rapid oxygenation of the Ediacaran atmosphere. However, ecological evidence for the colonization of the Ediacaran terrestrial biosphere by photosynthetic communities and their contribution to the oxygenation of the biosphere at this time is very sparse. Here, we expand the repertoire of Ediacaran habitable environments to a specific microbial community that thrived in an extreme alkaline volcanic lake 571 Myr ago in the Anti-atlas of Morocco. The microbial fabrics preserve evidence of primary growth structures, comprised of two main microbialitic units, with the lower section consisting of irregular and patchy thrombolytic mesoclots associated with composite microbialitic domes. Calcirudite interbeds, dominated by wave-rippled sandy calcarenites and stromatoclasts, fill the interdome troughs and seal the dome tops. A meter-thick epiclastic stromatolite bed grading upwards from a dominantly flat to wavy laminated base, transitions into low convex laminae consisting of decimeter to meter-thick dome-shaped stromatolitic columns, overlies the thrombolitic and composite microbialitic facies. Microbialitic beds constructed during periods of limited clastic input, and underlain by coarse-grained microbialite-derived clasts and by the wave-rippled calcarenites, suggest high-energy events associated with lake expansion. High-resolution microscopy revealed spherulitic aggregates and structures reminiscent of coccoidal microbial cell casts and mineralized extra-polymeric substances (EPS). The primary fabrics and multistage diagenetic features, represented by active carbonate production, photosynthesizing microbial communities, photosynthetic gas bubbles, gas escape structures, and tufted mats, suggest specialized oxygenic photosynthesizers thriving in alkaline volcanic lakes, contributed toward oxygen variability in the Ediacaran terrestrial biosphere.

Living phosphatic stromatolites in a low-phosphorus environment: Implications for the use of phosphorus as a proxy for phosphate levels in paleo-systems

In the geological record, fossil phosphatic stromatolites date back to the Great Oxidation Event in the Paleoproterozoic, but living phosphatic stromatolites have not been described previously. Here, we report on cyanobacterial stromatolites in a supratidal freshwater environment at Cape Recife, South African southern coast, precipitating Ca carbonate alternating with episodes of Ca phosphate deposition. In their structure and composition, the living stromatolites from Cape Recife closely resemble their fossilized analogues, showing phosphatic zonation, microbial casts, tunnel structures and phosphatic crusts of biogenic origin. The microbial communities appear to be also similar to those proposed to have formed fossil phosphatic stromatolites. Phosphatic domains in the material from Cape Recife are spatially and texturally associated with carbonate precipitates, but form distinct entities separated by sharp boundaries. Electron Probe Micro-Analysis shows that Ca/P ratios and the overall chemical compositions of phosphatic precipitates are in the range of octacalcium phosphate, amorphous tricalcium phosphate and apatite. The coincidence in time of the emergence of phosphatic stromatolites in the fossil record with a major episode of atmospheric oxidation led to the assumption that at times of increased oxygen release the underlying increased biological production may have been linked to elevated phosphorus availability. The stromatolites at Cape Recife, however, form in an environment where ambient phosphorus concentrations do not exceed 0.28 μM, one to two orders of magnitude below the previously predicted minimum threshold of >5 μM for biogenic phosphate precipitation in paleo-systems. Accordingly, we contest the previously proposed suitability of phosphatic stromatolites as a proxy for high ambient phosphate concentrations in supratidal to shallow ocean settings in earth history.

Microbially induced potassium enrichment in Paleoproterozoic shales and implications for reverse weathering on early Earth

Illitisation requires potassium incorporation into a smectite precursor, a process akin to reverse weathering. However, it remains unclear whether microbes facilitate K+ uptake to the sediments and whether illitisation was important in the geological past. The 2.1 billion-year-old Francevillian Series of Gabon has been shown to host mat-related structures (MRS) and, in this regard, these rocks offer a unique opportunity to test whether ancient microbes induced illitisation. Here, we show high K content confined to illite particles that are abundant in the facies bearing MRS, but not in the host sandstone and black shale. This observation suggests that microbial biofilms trapped K+ from the seawater and released it into the pore-waters during respiration, resulting in illitisation. The K-rich illite developed exclusively in the fossilized MRS thus provides a new biosignature for metasediments derived from K-feldspar-depleted rocks that were abundant crustal components on ancient Earth.

Early Archean origin of Photosystem II

Photosystem II is a photochemical reaction center that catalyzes the light-driven oxidation of water to molecular oxygen. Water oxidation is the distinctive photochemical reaction that permitted the evolution of oxygenic photosynthesis and the eventual rise of eukaryotes. At what point during the history of life an ancestral photosystem evolved the capacity to oxidize water still remains unknown. Here, we study the evolution of the core reaction center proteins of Photosystem II using sequence and structural comparisons in combination with Bayesian relaxed molecular clocks. Our results indicate that a homodimeric photosystem with sufficient oxidizing power to split water had already appeared in the early Archean about a billion years before the most recent common ancestor of all described Cyanobacteria capable of oxygenic photosynthesis, and well before the diversification of some of the known groups of anoxygenic photosynthetic bacteria. Based on a structural and functional rationale, we hypothesize that this early Archean photosystem was capable of water oxidation to oxygen and had already evolved protection mechanisms against the formation of reactive oxygen species. This would place primordial forms of oxygenic photosynthesis at a very early stage in the evolutionary history of life.

Jurassic Non-Carbonate Microbialites from the Betic-Rifian Cordillera (Tethys Western End): Textures, Mineralogy, and Environmental Reconstruction

The term microbialite is commonly applied for describing carbonate organo-sedimentary deposits that have accreted as a result of the activity of benthic microbial communities (BMC). However, non-carbonate microbialites are progressively well-known and show a great diversity of organisms, processes, and mineralogical compositions. This article reviews three types of Jurassic microbialites from four different environmental contexts from the Betic-Rifian Cordillera (South Spain and North Morocco): marine hardgrounds, submarine caves, hydrothermal vents, and submarine volcanic deposits. The Middle-Late Jurassic transition in the External Subbetic (Betic Cordillera) and the Jbel Moussa Group (Rifian Calcareous Chain) was characterized by the fragmentation of the carbonate epicontinental platforms that favored these different settings: (A) Many stratigraphic breaks are recorded as hardgrounds with surficial hydrogenetic Fe crusts and macro-oncoids related to chemo-organotrophic behavior of BMC that served as a specific trap for Fe and Mn enrichment; (B) Cryptic hydrogenetic Fe-Mn crusts (or endostromatolites) grew in the walls of submarine cavities and fractures mainly constituted by Frutexites (chemosynthetic and cryptobiontic microorganism) locally associated to serpulids; (C) Hydrothermal Mn crusts are mainly constituted by different types of filaments and bacillus-shaped bacteria, whose mineralogy and geochemistry point to a submarine hydrothermal origin; (D) Finally, glauconite laminated crusts, constituted by branched cylindrical filaments, have grown in cryptic spaces among the pillow-lava bodies, probably related to the metabolism of chemo-organotrophic microbes under oxic conditions at temperatures between 30 and 90 °C. In most of the cases described in this work, microbial organisms forming microbialites were extremophiles.

Early Archean origin of heterodimeric Photosystem I

When and how oxygenic photosynthesis originated remains controversial. Wide uncertainties exist for the earliest detection of biogenic oxygen in the geochemical record or the origin of water oxidation in ancestral lineages of the phylum Cyanobacteria. A unique trait of oxygenic photosynthesis is that the process uses a Type I reaction centre with a heterodimeric core, also known as Photosystem I, made of two distinct but homologous subunits, PsaA and PsaB. In contrast, all other known Type I reaction centres in anoxygenic phototrophs have a homodimeric core. A compelling hypothesis for the evolution of a heterodimeric Type I reaction centre is that the gene duplication that allowed the divergence of PsaA and PsaB was an adaptation to incorporate photoprotective mechanisms against the formation of reactive oxygen species, therefore occurring after the origin of water oxidation to oxygen. Here I show, using sequence comparisons and Bayesian relaxed molecular clocks that this gene duplication event may have occurred in the early Archean more than 3.4 billion years ago, long before the most recent common ancestor of crown group Cyanobacteria and the Great Oxidation Event. If the origin of water oxidation predated this gene duplication event, then that would place primordial forms of oxygenic photosynthesis at a very early stage in the evolutionary history of life.