All cells contain much more potassium, phosphate, and transition metals than modern (or reconstructed primeval) oceans, lakes, or rivers. Cells maintain ion gradients by using sophisticated, energydependent membrane enzymes (membrane pumps) that are embedded in elaborate ion-tight membranes. The first cells could possess neither ion-tight membranes nor membrane pumps, so the concentrations of small inorganic molecules and ions within protocells and in their environment would equilibrate. Hence, the ion composition of modern cells might reflect the inorganic ion composition of the habitats of protocells. We attempted to reconstruct the "hatcheries" of the first cells by combining geochemical analysis with phylogenomic scrutiny of the inorganic ion requirements of universal components of modern cells. These ubiquitous, and by inference primordial, proteins and functional systems show affinity to and functional requirement for K + , Zn 2+ , Mn 2+, and phosphate. Thus, protocells must have evolved in habitats with a high K + /Na + ratio and relatively high concentrations of Zn, Mn, and phosphorous compounds. Geochemical reconstruction shows that the ionic composition conducive to the origin of cells could not have existed in marine settings but is compatible with emissions of vapor-dominated zones of inland geothermal systems. Under the anoxic, CO 2 -dominated primordial atmosphere, the chemistry of basins at geothermal fields would resemble the internal milieu of modern cells. The precellular stages of evolution might have transpired in shallow ponds of condensed and cooled geothermal vapor that were lined with porous silicate minerals mixed with metal sulfides and enriched in K + , Zn 2+, and phosphorous compounds.

The stoichiometry and stability of arsenic gaseous complexes were determined in the system AsH 2 O Ϯ NaCl Ϯ HCl Ϯ H 2 S at temperatures up to 500°C and pressures up to 600 bar, from both measurements of As (III) and As (V) vapor-liquid and vapor-solid partitioning, and X-ray absorption fine structure (XAFS) spectroscopic study of As (III)-bearing aqueous fluids. Vapor-aqueous solution partitioning for As (III) was measured from 250 to 450°C at the saturated vapor pressure of the system (P sat) with a special titanium reactor that allows in situ sampling of the vapor phase. The values of partition coefficients for arsenious acid (H 3 AsO 3) between an aqueous solution (pure H 2 O) and its saturated vapor (K ϭ mAs vapor /mAs liquid) were found to be independent of As (III) solution concentrations (up to ϳ1 to 2 mol As/kg) and equal to 0.012 Ϯ 0.003, 0.063 Ϯ 0.023, and 0.145 Ϯ 0.020 at 250, 300, and 350°C, respectively. These results are interpreted by the formation, in the vapor phase, of As(OH) 3 (gas), similar to the aqueous As hydroxide complex dominant in the liquid phase. Arsenic chloride or sulfide gaseous complexes were found to be negligible in the presence of HCl or H 2 S (up to ϳ0.5 mol/kg of vapor). XAFS spectroscopic measurements carried out on As (III)-H 2 O(ϮNaCl) solutions up to 500°C demonstrate that the As(OH) 3 complex dominates As speciation both in dense H 2 O-NaCl fluids and low-density supercritical vapor. Vapor-liquid partition coefficients for As (III) measured in the H 2 O-NaCl system up to 450°C are consistent with the As speciation derived from these spectroscopic measurements and can be described by a simple relationship as a function of the vapor-to-liquid density ratio and temperature. Arsenic (III) partitioning between vapor and As-concentrated solutions (Ͼ2 mol As/kg) or As 2 O 3 solid is consistent with the formation, in the vapor phase, of both As 4 O 6 and As(OH) 3. Arsenic (V) (arsenic acid, H 3 AsO 4) vapor-liquid partitioning at 350°C for dilute aqueous solution was interpreted by the formation of AsO(OH) 3 in the vapor phase. The results obtained were combined with the corresponding properties for the aqueous As(III) hydroxide species to generate As(OH) 3 (gas) thermodynamic parameters. Equilibrium calculations carried out by using these data indicate that As(OH) 3 (gas) is by far the most dominant As complex in both volcanic gases and boiling hydrothermal systems. This species is likely to be responsible for the preferential partition of arsenic into the vapor phase as observed in fluid inclusions from high-temperature (400 to 700°C) Au-Cu (-Sn,-W) magmatic-hydrothermal ore deposits. The results of this study imply that hydrolysis and hydration could be also important for other metals and metalloids in the H 2 O-vapor phase. These processes should be taken into account to accurately model element fractionation and chemical equilibria during magma degassing and fluid boiling.

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Macrobioerosion is a common process in marine ecosystems. Many types of rock-boring organisms break down hard substrates, particularly carbonate rocks and calcareous structures such as dead corals and shells. In paleontology, the presence of rocks with boreholes and fossil macroboring assemblage members is one of the primary diagnostic features of shallow marine paleo-environments. Here we describe a silicate rock-boring organism and an associated community in submerged siltstone rock outcrops in Kaladan River, Myanmar. The rock-boring mussel Lignopholas fluminalis is a close relative of the marine piddocks, and its borings belong to the ichnospecies Gastrochaenolites anauchen. The neotectonic uplift of the area leading to gradual decrease of the sea level with subsequent shift from estuarine to freshwater environment was the most likely driver for the origin of this community. Our findings highlight that rocks with macroborings are not an exclusive indicator of marine paleo-ecosystems, but may also reflect freshwater habitats.