SummarySince a key requirement of known life forms is available water (water activity; a w), recent searches for signatures of past life in terrestrial and extraterrestrial environments have targeted places known to have contained significant quantities of biologically available water. However, early life on Earth inhabited high-salt environments, suggesting an ability to withstand low water-activity. The lower limit of water activity that enables cell division appears to be ∼ 0.605 which, until now, was only known to be exhibited by a single eukaryote, the sugar-tolerant, fungal xerophile Xeromyces bisporus. The first forms of life on Earth were, though, prokaryotic. Recent evidence now indicates that some halophilic Archaea and Bacteria have water-activity limits more or less equal to those of X. bisporus. We discuss water activity in relation to the limits of Earth's present-day biosphere; the possibility of microbial multiplication by utilizing water from thin, aqueous films or non-liquid sources; whether prokaryotes were the first organisms able to multiply close to the 0.605-a w limit; and whether extraterrestrial aqueous milieux of ≥ 0.605 aw can resemble fertile microbial habitats found on Earth.

Critical to the origin of life are the ingredients of life, of course, but also the physical and chemical conditions in which prebiotic chemical reactions can take place. These factors place constraints on the types of Hadean environment in which life could have emerged. Many locations, ranging from hydrothermal vents and pumice rafts, through volcanic-hosted splash pools to continental springs and rivers, have been proposed for the emergence of life on Earth, each with respective advantages and certain disadvantages. However, there is another, hitherto unrecognized environment that, on the Hadean Earth (4.5–4.0 Ga), would have been more important than any other in terms of spatial and temporal scale: the sedimentary layer between oceanic crust and seawater. Using as an example sediments from the 3.5–3.33 Ga Barberton Greenstone Belt, South Africa, analogous at least on a local scale to those of the Hadean eon, we document constant permeation of the porous, carbonaceous, and reactive sedimentary layer by hydrothermal fluids emanating from the crust. This partially UV-protected, subaqueous sedimentary environment, characterized by physical and chemical gradients, represented a widespread system of miniature chemical reactors in which the production and complexification of prebiotic molecules could have led to the origin of life. Key Words: Origin of life—Hadean environment—Mineral surface reactions—Hydrothermal fluids—Archean volcanic sediments. Astrobiology 18, 259–293.

Anoxic irradiation of a type IIICD iron meteorite known to contain the phosphide mineral schreibersite (Fe,Ni)3P in the presence of ethanol/water affords the reactive oxyacid H-phosphinic acid (H3PO2) as the dominant phosphorus product.

Experimental SectionFenton Chemistry: A series of solutions were prepared with Na 2 HPO 3 or NaH 2 PO 2 in concentrations between 0.001 M to 0.1 M, iron (as FeCl 2 × 4H 2 O or as FeSO 4 ) concentrations between 0.001 M to 0.1 M, and H 2 O 2 concentrations between 0.002 M to 0.5 M (Table S1), each with a starting pH of 7±1, unless otherwise noted. Peroxide was added last to promote the reactions. One experiment was performed with a formaldehyde concentration of 1.2 M to act as a reductant. The solutions were stirred for 1-20 days, a 2 mL aliquot of each solution was extracted and placed in 2 mL of a 1:1 mixture of 10 M NaOH solution and D 2 O to quench the reaction, precipitate Fe III and prepare the sample for analysis by NMR. Decanted samples were analyzed using 31 P NMR on a Varian 300 four-nucleus probe FT-NMR spectrometer at 121.43 MHz and 24.5 ºC for 256 to 6000 scans following prior work [6a,c]

The element phosphorus (P) is central to ecosystem growth and is proposed to be a limiting nutrient for life. The Archean ocean may have been strongly phosphorus-limited due to the selective binding of phosphate to iron oxyhydroxide. Here we report a new route to solubilizing phosphorus in the ancient oceans: reduction of phosphate to phosphite by iron(II) at low (<200 °C) diagenetic temperatures. Reduction of phosphate to phosphite was likely widespread in the Archean, as the reaction occurs rapidly and is demonstrated from thermochemical modeling, experimental analogs, and detection of phosphite in early Archean rocks. We further demonstrate that the higher solubility of phosphite compared to phosphate results in the liberation of phosphorus from ferruginous sediments. This phosphite is relatively stable after its formation, allowing its accumulation in the early oceans. As such, phosphorus, not as phosphate but as phosphite, could have been a major nutrient in early pre-oxygenated oceans.