First shotgun metagenomics study of Juan de Fuca deep-sea sediments reveals distinct microbial communities above, within, between, and below sulfate methane transition zones

The marine deep subsurface is home to a vast microbial ecosystem, affecting biogeochemical cycles on a global scale. One of the better-studied deep biospheres is the Juan de Fuca (JdF) Ridge, where hydrothermal fluid introduces oxidants into the sediment from below, resulting in two sulfate methane transition zones (SMTZs). In this study, we present the first shotgun metagenomics study of unamplified DNA from sediment samples from different depths in this stratified environment. Bioinformatic analyses showed a shift from a heterotrophic, Chloroflexota-dominated community above the upper SMTZ to a chemolithoautotrophic Proteobacteria-dominated community below the secondary SMTZ. The reintroduction of sulfate likely enables respiration and boosts active cells that oxidize acetate, iron, and complex carbohydrates to degrade dead biomass in this low-abundance, low-diversity environment. In addition, analyses showed many proteins of unknown function as well as novel metagenome-assembled genomes (MAGs). The study provides new insights into microbial communities in this habitat, enabled by an improved DNA extraction protocol that allows a less biased view of taxonomic composition and metabolic activities, as well as uncovering novel taxa. Our approach presents the first successful attempt at unamplified shotgun sequencing samples from beyond 50 meters below the seafloor and opens new ways for capturing the true diversity and functional potential of deep-sea sediments.

The trisulfur radical ion S3 •- controls platinum transport by hydrothermal fluids

Platinum group elements (PGE) are considered to be very poorly soluble in aqueous fluids in most natural hydrothermal-magmatic contexts and industrial processes. Here, we combined in situ X-ray absorption spectroscopy and solubility experiments with atomistic and thermodynamic simulations to demonstrate that the trisulfur radical ion S₃ ^•- forms very stable and soluble complexes with both Pt^II and Pt^IV in sulfur-bearing aqueous solution at elevated temperatures (∼300 °C). These Pt-bearing species enable (re)mobilization, transfer, and focused precipitation of platinum up to 10,000 times more efficiently than any other common inorganic ligand, such as hydroxide, chloride, sulfate, or sulfide. Our results imply a far more important contribution of sulfur-bearing hydrothermal fluids to PGE transfer and accumulation in the Earth's crust than believed previously. This discovery challenges traditional models of PGE economic concentration from silicate and sulfide melts and provides new possibilities for resource prospecting in hydrothermal shallow crust settings. The exceptionally high capacity of the S₃ ^•- ion to bind platinum may also offer new routes for PGE selective extraction from ore and hydrothermal synthesis of noble metal nanomaterials.

Colloidal transport and flocculation are the cause of the hyperenrichment of gold in nature

Hydrothermal veins supply much of the Earth’s gold. Their propensity to contain “bonanza” occurrences of gold that have concentrations millions of times greater than the concentration of gold in Earth’s crust makes them important targets for exploration and resource development. The mechanisms by which such hyperenrichment occurs are enigmatic. The accepted wisdom is that this enrichment reflects the saturation and precipitation of gold from hydrothermal fluids. Laboratory experiments and measurements of active hydrothermal systems, however, have shown that the solubility of this noble metal is exceptionally low. Here, we demonstrate that this issue is resolved by the physical transport of gold in the solid state as nanoparticles and their flocculated aggregates, thereby explaining the paradox of bonanza gold ore formation.

Aqueous complexation has long been considered the only viable means of transporting gold to depositional sites in hydrothermal ore-forming systems. A major weakness of this hypothesis is that it cannot readily explain the formation of ultrahigh-grade gold veins. This is a consequence of the relatively low gold concentrations typical of ore fluids (tens of parts per billion [ppb]) and the fact that these “bonanza” veins can contain weight-percent levels of gold in some epithermal and orogenic deposits. Here, we present direct evidence for a hypothesis that could explain these veins, namely, the transport of the gold as colloidal particles and their flocculation in nanoscale calcite veinlets. These gold-bearing nanoveinlets bear a remarkable resemblance to centimeter-scale ore veins in many hydrothermal gold deposits and give unique insight into the scale invariability of colloidal flocculation in forming hyperenriched gold deposits. Using this evidence, we propose a model for the development of bonanza gold veins in high-grade deposits. We argue that gold transport in these systems is largely mechanical and is the result of exceptionally high degrees of supersaturation that preclude precipitation of gold crystals and instead lead to the formation of colloidal particles, which flocculate and form much larger masses. These flocculated masses aggregate locally, where they are seismically pumped into fractures to locally form veins composed largely of gold. This model explains how bonanza veins may form from fluids containing ppb concentrations of gold and does not require prior encapsulation of colloidal gold particles in silica gel, as proposed by previous studies.

Sulfur radical species form gold deposits on Earth

Current models of the formation and distribution of gold deposits on Earth are based on the long-standing paradigm that hydrogen sulfide and chloride are the ligands responsible for gold mobilization and precipitation by fluids across the lithosphere. Here we challenge this view by demonstrating, using in situ X-ray absorption spectroscopy and solubility measurements, coupled with molecular dynamics and thermodynamic simulations, that sulfur radical species, such as the trisulfur ion S3(-), form very stable and soluble complexes with Au(+) in aqueous solution at elevated temperatures (>250 °C) and pressures (>100 bar). These species enable extraction, transport, and focused precipitation of gold by sulfur-rich fluids 10-100 times more efficiently than sulfide and chloride only. As a result, S3(-) exerts an important control on the source, concentration, and distribution of gold in its major economic deposits from magmatic, hydrothermal, and metamorphic settings. The growth and decay of S3(-) during the fluid generation and evolution is one of the key factors that determine the fate of gold in the lithosphere.