Ferric iron triggers greenalite formation in simulated Archean seawater

Sedimentary rock deposits provide the best records of (bio)geochemical cycles in the ancient ocean. Studies of these sedimentary archives show that greenalite, an Fe(II) silicate with low levels of Fe(III), was an early chemical precipitate from the Archean ocean. To better understand the formation of greenalite, we explored controls on iron silicate precipitation through experiments in simulated Archean seawater under exclusively ferrous conditions or supplemented with low Fe(III). Our results confirm a pH-driven process promoting the precipitation of iron-rich silicate phases, and they also reveal an important mechanism in which minor concentrations of Fe(III) promote the precipitation of well-ordered greenalite among other phases. This discovery of an Fe(III)-triggering iron silicate formation process suggests that Archean greenalite could represent signals of iron oxidation reactions, potentially mediated by life, in circumneutral ancient seawater.

Biominerals: Formation, Function, Properties

Triggered by geochemical cycles, large-scale terrestrial processes and evolution, a tremendous biodiversity evolved over the geological record and produced proto- and metazoa with biomineralized hard tissue, characterized by unique structural designs and exquisite performance [...]

The diversity of molecular mechanisms of carbonate biomineralization by bacteria

Although biomineralization of CaCO3 is widespread in Bacteria and Archaea, the molecular mechanisms involved in this process remain less known than those used by Eukaryotes. A better understanding of these mechanisms is crucial for a broad diversity of studies including those (i) aiming at assessing the role of bacteria in the geochemical cycles of Ca and C, (ii) investigating the process of fossilization, and (iii) engineering applications using bacterially mediated CaCO3 mineralization. Different types of bacterially-mediated mineralization modes have been distinguished depending on whether they are influenced (by extracellular organic molecules), induced (by metabolic activity) or controlled (by specific genes). In the first two types, mineralization is usually extracellular, while it is intracellular for the two ascertained cases of controlled bacterial mineralization. In this review, we list a large number of cases illustrating the three different modes of bacterially-mediated CaCO3 mineralization. Overall, this shows the broad diversity of metabolic pathways, organic molecules and thereby microorganisms that can biomineralize CaCO3. Providing an improved understanding of the mechanisms involved and a good knowledge of the molecular drivers of carbonatogenesis, the increasing number of (meta)-omics studies may help in the future to estimate the significance of bacterially mediated CaCO3 mineralization.

The Role of Diagenesis in Shaping the Geochemistry of the Marine Carbonate Record

Carbonate sediments and rocks are valuable archives of Earth's past whose geochemical compositions inform our understanding of Earth's surface evolution. Yet carbonates are also reactive minerals and often undergo compositional alteration between the time of deposition and sampling and analysis. These changes may be mineralogical, structural, and/or chemical, and they are broadly referred to as diagenesis. Building on work over the past 40 years, we present an overview of key carbonate diagenesis terminology and a process-based framework for evaluating the geochemical impacts of carbonate diagenesis; we also highlight recent experimental and field observations that suggest metal isotopes as valuable diagenetic indicators. Our primary objectives are to demonstrate the value of coupling quantitative and analytical approaches, specifically with regard to metal isotopes and Mg/Ca, and to focus attention on key avenues for future work, including the role of authigenesis in impacting global geochemical cycles and the isotopic composition of the rock record. ▪ Quantitative frameworks utilizing well-understood diagenetic indicators and basic geochemical parameters allow us to assess the extent of diagenetic alteration in carbonate sediments. ▪ The reactivity, duration of reaction, and degree of isotopic or elemental/chemical disequilibrium determine the extent to which carbonates may be altered. ▪ Metal isotopic ratios (δ44Ca, δ26Mg, 87Sr/86Sr) can be used to constrain the extent and rate of carbonate recrystallization. ▪ Diagenetic signals may be globally synchronous, while diagenetic fluxes may impact global geochemical cycles.

Early Cyanobacteria and the Innovation of Microbial Sunscreens

ABSTRACT Metabolism drives life; thus, understanding how and when various branches of metabolism evolved provides a critical piece to understanding how life has integrated itself into the geochemical cycles of our planet over billions of years. Although the most transformative metabolisms that have significantly altered the trajectory of Earth are inherently linked to primary metabolism, natural products that stem from specialized metabolic pathways are also key components to many auxiliary facets of life. Cyanobacteria are primarily known as the original inventors of oxygenic photosynthesis, using sunlight to split water to create our dioxygen-filled atmosphere; however, many of them also have evolved to produce small molecules that function as sunscreens to protect themselves from ultraviolet radiation. Determining when cyanobacteria first evolved the ability to biosynthesize such compounds is an important piece to understanding the rise of oxygen and the eventual success of the phylum.