Tectonically-driven oxidant production in the hot biosphere

Reconstructed genomes of the Last Universal Common Ancestor (LUCA) indicate thermophily, autotrophy, H2 dependence, and, paradoxically, the presence of genes for O2 and H2O2 cycling despite an anoxic early Earth with only trace H2O2 from atmospheric photochemistry. The prevailing explanation is later acquisition via lateral gene transfer after oxygenic photosynthesis evolved. The authors hypothesize instead that geological processes could have supplied significant oxidants (H2O2 and O2) prior to oxygenic photosynthesis. Two mechanistic sources are proposed: (1) mechanochemical generation of reactive surface sites (Si· and SiO·) during cataclasis of silicate rocks that react with water to form H2 and potentially H2O2 via hydroxyl radicals; and (2) activation and cleavage of pre-existing intragranular oxidized defects (peroxy bridges, Si–O–O–Si) formed during magmatic crystallization and by alpha-recoil, which under stress and heat migrate to surfaces to yield SiO· that can generate H2O2. The central research question is whether temperature, under oxygen-limited conditions, controls oxidant yields from silicate rock–water reactions in a way relevant to hot subsurface environments and early Earth niches for life.

Prior models estimate only nanomolar H2O2 from Archean atmospheric UV photochemistry, insufficient to explain LUCA's oxidant-cycling genes. Experimental studies show H2 generation from crushed silicates via Si· reactions with water and inhibition at high temperatures when SiO· consumes H·. Several room-temperature studies reporting H2O2 often involved crushing or water addition in air; O2 enhances H2O2 formation via superoxide (SiOO·) on silicate surfaces. H2O2 generation via O2-mediated pathways ceases after heating minerals in water at 60 °C for 24 h, implying a distinct, temperature-dependent, O2-independent pathway. Peroxy bridges in igneous/metamorphic rocks are ubiquitous (~100 ppm), can be activated by stress, and may supply H2O2 to the subsurface. Prior work suggests mineral structure influences H2O2 yields (inverse with tetrahedra corner-sharing). Reports also indicate trace O2 formation with peroxy cleavage and that abrasion energy and milling conditions affect ROS generation.

Rock selection and preparation: Commercial granite (continental crust analog), basalt and peridotite (oceanic crust analogs) were sourced. Samples were shattered, jaw-crushed, sieved (1–3 mm), washed, dried, and milled in a gas-tight, stainless steel-encased agate planetary ball mill (Fritsch P6) at 500 rpm (34 g) for 30 min under N2 (5.0 grade, <10 ppm O2). The mill was evacuated and N2-flushed (7 cycles). Operations were conducted in an N2-flushed glove bag (<0.1% O2). Approximately 2 g aliquots were dispensed into pre-cleaned, furnaced borosilicate serum vials and crimped with treated butyl stoppers.

Water and atmosphere: Deoxygenated 18.2 MΩ·cm water was autoclaved (121 °C, 1 h), cooled, and N2-bubbled (~4 h) to ≤8.4 µmol L−1 O2. For some pilot tests, vials were N2-flushed post-sealing; for subsequent experiments, flushing was omitted based on glove bag O2 monitoring.

Experimental designs: (1) Flash-heating pilot with granite: 4 mL deoxygenated water was added to rock-filled vials at 0 °C, then vials were flash-heated for 1 min to 30, 60, or 121 °C (or held at 0 °C), rapidly cooled, and destructively sampled at 1, 24, and 168 h. (2) Continuous heating with granite, basalt, peridotite: water added at room temperature; vials incubated at 60, 80, 104, or 121 °C for 1, 24, or 168 h (121 °C only at ~1 h). Grain sizes after milling were 20.6–22.8 µm. Blanks (water only) underwent identical treatments.

Analytical methods: H2 was quantified by GC (ThermoFisher GC with He pulsed discharge detector for pilot; SRI GC with RGA/TCD for continuous runs), calibrated against certified standards (10–20,000 ppm). Detection limit: 0.2 nmol g−1. H2O2 was quantified spectrophotometrically via Cu(I)-DMP complex at 454 nm with fresh standards (DL: 9.2 nmol g−1). Hydroxyl radicals (•OH) were probed using PCBA trapping and HPLC (no significant detection). Fe2+ was measured (ferrozine method referenced). Thermogravimetric/differential scanning calorimetry (TG-DSC) with QMS monitored evolved gases. Controls and blanks assessed contributions from borosilicate glass defects. Statistical analyses included Mann–Whitney U tests for significance vs blanks, ANOVA with post-hoc LSD for temperature comparisons, and correlations (Fe2+ vs H2). Mass balance constrained possible O2-derived H2O2 contributions. Mineralogical compositions were characterized (XRD).

• Temperature dependence: Substantial H2O2 generation occurred only at elevated temperatures near the boiling point of water (notably 104 °C) under oxygen-limited conditions; little H2O2 was released at <80 °C.
• H2 vs H2O2 tradeoff: At 104 °C, H2 production was significantly inhibited while H2O2 production increased, consistent with SiO· reacting with H· (Eq. 3) and forming H2O2 via •OH intermediates (Eqs. 4–5). ANOVA for H2 decrease at 104 °C: F2,24=6.408, P=0.006; LSD comparisons vs 60 and 80 °C both P=0.005. ANOVA for H2O2 increase at 104 °C: F2,24=6.475, P=0.006; LSD vs 60 °C P=0.004; vs 80 °C P=0.005.
• Quantitative yields: After 1 week at 104 °C, mean H2O2 yields were approximately 0.70 µmol g−1 (granite), 3.44 µmol g−1 (peridotite), and 1.13 µmol g−1 (basalt), equivalent to 171, 836, and 299 µM at experimental loadings. H2 detection limits were 0.2 nmol g−1; H2O2 detection limits 9.2 nmol g−1. •OH was not detected above blanks (t32,233=1.46, P=0.154), consistent with rapid consumption.
• Rock type and structure: Mean H2O2 generation followed peridotite > basalt > granite, broadly consistent with mineral structural control (nesosilicate/inosilicate > tectosilicate) under O2-limited, high-T conditions.
• Blanks and O2 contributions: Blanks (water-only in borosilicate vials) produced detectable H2 and H2O2 due to glass Si/SiO/SiOO defects. Mass balance showed trace O2 could account for at most ~0.073 µmol g−1 H2O2 in rock experiments, far less than observed at 104 °C, implicating SiO· from cataclasis and peroxy bridges as primary sources under these conditions.
• Peroxy bridges as oxidant reservoirs: Ubiquitous peroxy bridges (~100 ppm; ~2940 nmol g−1 potential H2O2) in igneous/metamorphic rocks are of the correct magnitude to explain net H2O2 accumulated at 104 °C (means 667–3273 nmol g−1). Both cataclasis-generated SiO· and stress-activated peroxy bridges likely contributed.
• Kinetics and upper thermal limit: Over >1 h timescales, SiO· was reactive at 121 °C, reducing H2 generation, and oxidant generation was evident within the upper thermal limit of microbial growth (≤122 °C).
• Global flux estimates: Assuming 100 ppm peroxy bridges and modern oceanic crust production/subduction (~19 km3 yr−1; density 3 g cm−3), potential H2O2 generation is ~1.67×10^11 mol yr−1 (~6.4×10^5 molecules cm−2 s−1 Earth-surface-normalized). Cataclasis maximum flux estimate ~1.1×10^9 molecules cm−2 s−1. These exceed pre-photosynthetic Archean atmospheric photochemical H2O2 estimates (~10^6 molecules cm−2 s−1).
• Biological relevance: The temperature window for H2O2 production overlaps growth ranges of hyperthermophilic, deeply branching microaerophiles (e.g., Aquifex aeolicus 85–95 °C; Pyrobaculum aerophilum 75–104 °C). Energetics at 100 °C: H2 oxidation by H2O2 (−353.03 kJ mol−1 H2) or O2 (−294.68 kJ mol−1 H2) yields far more energy than methanogenesis using CO2 (~−45.3 kJ mol−1 H2).

Findings demonstrate temperature as a critical control on abiotic oxidant generation from crushed silicate rocks under oxygen-limited conditions. At ~104 °C, SiO· sites from cataclasis and stress-activated peroxy bridges drive hydroxyl radical formation and H2O2 accumulation while suppressing H2. This resolves the paradox of LUCA's oxidant-cycling genes by identifying a geological, anoxia-compatible oxidant source in hot subsurface fractures. The observed yields and kinetics are compatible with the physiology of hyperthermophilic, hydrogen-oxidizing microaerophiles and with enzymatic systems handling both H2O2 (catalase) and O2 respiration. Geophysically, tectonic stressing, faulting, and magmatic intrusion focus these reactions into fracture networks, potentially delivering 100s µM to mM local H2O2 consistent with experimental concentrations. Modern global flux calculations suggest that peroxy bridges and cataclasis could influence present-day subsurface redox ecology near active faults. On the early Earth, even without modern-style plate tectonics, vertical tectonics, intrusion, and impacts would have provided stress and heat, enabling similar oxidant production in ultramafic to felsic crustal lithologies. The energetic benefits of using H2O2/O2 as electron acceptors, exothermic H2O2 disproportionation, and potential roles in protometabolism and prebiotic chemistry underscore the significance of this mechanism for both the hot biosphere and origins-of-life scenarios.

Under oxygen-limited conditions, heating water–crushed silicate rock systems to near-boiling (~104 °C) produces substantial H2O2 (hundreds to thousands of nmol g−1) while inhibiting H2, with minimal oxidant production below ~80 °C. The dominant H2O2 source is most consistent with activation of pre-existing oxidized peroxy bridge defects and SiO· sites generated by cataclasis, which become reactive at elevated temperatures. This temperature window overlaps the growth ranges of ancient hyperthermophiles, suggesting tectonically driven oxidant production could shape subsurface microbial ecology today and have supplied oxidants to early Earth niches, helping reconcile LUCA's oxidant-cycling genes. Future work should quantify in situ fluxes in natural faulted systems, evaluate spatial focusing and transport in fractures, extend rock type coverage (including Archean lithologies), refine the roles of stress vs temperature, and integrate biotic–abiotic feedbacks under realistic geochemical conditions.

• Laboratory conditions: Experiments used crushed rocks and controlled N2 atmospheres; natural systems may differ in stress histories, fluid compositions, and transport.
• Potential artefacts: Milling energy/temperature may activate surface sites and generate small pre-incubation pulses of H2O2; blanks showed ROS from borosilicate glass defects.
• Oxygen traces: Although minimized, trace O2 could form SiOO· in blanks; mass balance suggests minor contributions in rock experiments but cannot be entirely excluded.
• Temporal/thermal coverage: 121 °C treatments were measured only at ~1 h; longer high-T incubations were not assessed. Room-temperature controls from literature differ in crushing energies.
• Geochemical complexity: Possible minor H2 contributions from Fe2+–water reactions (serpentinization-type), though Fe2+ was orders of magnitude lower than H2 and uncorrelated. Mineralogical variability and fluid chemistry (pH, ions) effects were not fully explored.
• Scaling and heterogeneity: Extrapolation to crustal scales assumes peroxy bridge concentrations and full activation; natural stress/heat distributions and fracture focusing introduce heterogeneity.