Microbialites are rocks formed by microbial communities that influence, promote, and/or control the precipitation of mineral phases. The oldest microbialites date back to the Archean and formed under globally anoxic conditions. Accordingly, modern microbialites developing under oxic conditions are limited analogs of their earliest predecessors. We discovered microbialites in the deep, seasonally anoxic water column of crater lakes Alchichica and Atexcac (Mexico). We studied the prokaryotic communities of plankton and microbialites from oxic and anoxic layers, alongside the temporal dynamics of microbialite communities incubated under strict continuous anoxia. 16S rRNA gene amplicon sequencing showed shifts in the relative abundance of taxa under anoxia, rather than changes in overall diversity. Regardless of their original depth, microbialite-associated communities from oxic and anoxic zones converged after ~25 weeks of laboratory exposure to a N2-H2-CO2 (85:10:5) atmosphere. Sequences assigned to likely anoxygenic photosynthetic taxa, notably within the Alpha- and Gammaproteobacteria, together with phyla associated with sulfate-reduction and other sulfur-cycling bacteria, increased in relative abundance. We isolated seven bacterial strains under anoxia, some of them biomineralizing. Several of these strains were cyanobacteria affiliated with the Leptolyngbyales, suggesting that they either continue to perform oxygenic photosynthesis or, hypothetically, switch to H2S-dependent anoxygenic photosynthesis. Our results indicate that these microbialite communities adapt to long-term anoxia (more than 70 weeks) under laboratory conditions, likely sustaining photosynthetic alkalinization and carbonate precipitation under such conditions.
Biogenic aragonite is a composite of mineral and biomolecules that exhibits superior material properties compared to its inorganic analog. This superiority underpins the success of the bioaragonite structures which support coral reefs and shell fisheries, yet the mechanisms improving the physical resilience of bioaragonite are not well understood. In this study, aragonite is synthesised under chemical conditions which simulate those of calcification sites in marine organisms. Aspartic acid, glutamic acid, and glycine in solution are incorporated into the aragonite at concentrations typically observed in marine biominerals and are observed to significantly improve aragonite Vickers hardness. Aspartic acid is incorporated more efficiently than glutamic acid or glycine, but for comparable amounts of amino acid incorporation, glycine improves aragonite hardness significantly more than either aspartic or glutamic acid. Glycine and glutamic acid incorporation reduce the aragonite grain size, thereby improving resilience to indentation via energy dissipation along grain boundaries. In contrast, aspartic acid also improves hardness on a smaller scale, possibly due to crystal structure distortions. These results suggest that amino acid incorporation influences biogenic aragonite hardness via multiple mechanisms and provide a prototype for exploring the influence of more complicated biomineral proteins.
The Carri Laufquen Lakes System is a highly alkaline, basalt-hosted lacustrine environment in northern Patagonia, Argentina. Diverse morphologies of Late Pleistocene sub-fossil stromatolites occur along the lakeshores. Using detailed optical and electron microscopy, spectroscopy and organic geochemical analyses, this study shows that stromatolite growth and early diagenetic processes were strongly influenced by microbially controlled organomineralisation. Mineralisation of extracellular polymeric substances and cyanobacterial sheaths promoted early lithification and exceptional preservation of stromatolite fabrics and biosignatures. Organic matter is closely associated with Al-bearing silicate minerals at the micro-nanoscale, enhancing fossilisation potential and preserving biogeochemical signatures. The resulting mineral assemblage in the Carri Laufquen stromatolites, dominated by Mg-carbonate with minor Al-silicates and carbonaceous material, resembles the geochemistry of ancient lacustrine deposits around the margins of Jezero crater, highlighting the propensity of such deposits for biosignature preservation, and providing a framework for detailed characterisation of such materials following Mars Sample Return.
Following the largest known Phanerozoic extinction, the end-Permian mass extinction, the Early Triassic is often considered an interval of slow biotic recovery. During this time, the minor Smithian-Spathian extinction occurred, further delaying the biotic recovery of this epoch. One mechanism linked to the slow recovery is widespread marine anoxic conditions, but the spatiotemporal changes to anoxia following the mass extinction remain poorly constrained. Here, we utilized local and global redox-sensitive metal proxies to track variations in (de-)oxygenation at Wallenbergfjellet, a section in Spitsbergen, Arctic Norway. Local proxies (e.g., Fe speciation and trace metal concentrations) indicate basinal anoxia and possibly euxinia prevalent in the Boreal Ocean during this interval. Importantly, thallium isotopes, a global paleoredox proxy sensitive to Mn oxide burial, have not previously been applied to the Early Triassic recovery interval that followed the end-Permian mass extinction, though basinal restriction may limit geographic extent of interpretations. Our results show a shift in ε205Tl values from ~ -4.0 in the Smithian to ~ -2.0 at the Smithian-Spathian boundary (SSB), consistent with a rapid expansion of global anoxia coincident with the boundary extinction. A rapid perturbation in thallium isotopes at the SSB (from -4.0 to -2.0 ε-units) indicates this interval experienced an expansion of anoxic conditions that can be linked to the minor extinction event during the recovery interval following the end-Permian mass extinction. This suggests that global anoxia persisted throughout the Early Triassic and contributed to delayed biotic recovery, with the SSB representing a major intensification of reducing conditions.
Microbial dolostones that now form the Arkaroola reef, northern Adelaide Rift Complex, and were deposited during the Cryogenian interglacial period, display a wide range of meso-scale stromatolite morphotypes in outcrop. Microcharacterisation involving optical microscopy, combined scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) and nanoscale secondary ion mass spectroscopy (NanoSIMS), was undertaken to identify microbial textures and relics in stromatolite samples. Microbial textures identified are micritic streaky and micropeloidal (grumous) microstructures as well as microcolumns, micritic threads and micritised tubular outlines resembling the Girvanella calcimicrobe. Within these microbial textures, multiple biotic components are discerned, including intact and flattened tubular bacterial sheaths, smooth and nanogranular non-crystalline extracellular polymeric substances (EPS), filamentous and spherical bacteriomorphs, as well as coccobacilli-like moulds. The biotic-mineralogical interface is characterised by microcrystalline rhombic dolomite, nano-sized pyrite spheroids and nano-sized flaky silicates in direct contact with biotic components. Interpreted as a mixed community of filamentous and coccoidal (cyano)bacteria, these microbes were important contributors to reef building. Preservation of microfossils was likely facilitated by synsedimentary silicification of EPS and bacterial sheaths. Microbial metabolic processes lowered kinetic barriers for authigenic precipitation of clay minerals on bacterial surfaces serving as nucleation sites. These synsedimentary clays strengthened initially soft microfabrics and potentially helped establish chemical conditions that promoted early diagenetic precipitation of protodolomite. The findings of this study show that carbonates may intricately preserve microfossils where associated with synsedimentary authigenic clays and microcrystalline dolomite. These results contribute to a deeper understanding of the formation and growth of microbial reefs during the Cryogenian interglacial period.
