The contribution of water radiolysis to marine sedimentary life

Abstract

Water radiolysis continuously produces H2 and oxidized chemicals in wet sediment and rock. Radiolytic H2 has been identified as the primary electron donor (food) for microorganisms in continental aquifers kilometers below Earth’s surface. Radiolytic products may also be significant for sustaining life in subseafloor sediment and subsurface environments of other planets. However, the extent to which most subsurface ecosystems rely on radiolytic products has been poorly constrained, due to incomplete understanding of radiolytic chemical yields in natural environments. Here we show that all common marine sediment types catalyse radiolytic H2 production, amplifying yields by up to 27X relative to pure water. In electron equivalents, the global rate of radiolytic H2 production in marine sediment appears to be 1-2% of the global organic flux to the seafloor. However, most organic matter is consumed at or near the seafloor, whereas radiolytic H2 is produced at all sediment depths. Comparison of radiolytic H2 consumption rates to organic oxidation rates suggests that water radiolysis is the principal source of biologically accessible energy for microbial communities in marine sediment older than a few million years. Where water permeates similarly catalytic material on other worlds, life may also be sustained by water radiolysis.

Earth-like Habitable Environments in the Subsurface of Mars

“…Because sulfate is likely the limiting nutrient for hydrogenotrophic sulfate-reducing metabolisms in a martian subsurface biosphere rather than H 2 , the highest concentrations of sulfate-reducing microorganisms would exist in a water-bearing crustal section with high sulfide abundance, low sulfide grain sizes, and relatively high radionuclide concentrations. Regions rich in zeolites, which enhance radiolytic gas production (Kumagai et al, 2013;Sauvage et al, 2021), would also sustain higher sulfatereducing bacteria concentrations. Future missions should focus on characterizing where these criteria are met, as these would be prime landing site targets for extant life investigation from a redox energy perspective.…”

“…These different production efficiencies are summarized in the work of Tarnas et al (2018). The presence of certain minerals, such as zeolites, can significantly increase H 2 and oxidant production efficiency from radiolysis (Kumagai et al, 2013;Sauvage et al, 2021). Zeolites are found in both bedrock (Ehlmann et al, 2009) and dust (Ruff, 2004) on Mars; thus this mineral is expected to enhance radiolytic H 2 and oxidant production in some parts of the crust, but this effect is not modeled here in order to focus on the most conservative estimates possible.…”

Limits on the contribution of early endogenous radiolysis to oxidation in carbonaceous chondrites’ parent bodies

“…Variation of key parameters such as the W/R ratio or the time for starting aqueous alteration cannot raise this contribution to a significant level unless some unrealistic values are assumed. Recent experimental evidence using Earth sediments indicates radiolysis yields may be considerably larger than those used in the present work (Sauvage et al 2021). Considering the maximum enhanced yields, the oxidant production by endogenous radiolysis may become a larger contributor to sulfate formation (up to 11% of sulfur can be converted into sulfates in the CM case; see Appendix B) and generally to the evolution of the parent body redox state.…”

Evaluating the Microbial Habitability of Rogue Planets and Proposing Speculative Scenarios on How They Might Act as Vectors for Panspermia

“…Nevertheless, radioactivity is largely responsible for the heating of the interior of a planetary body, which then could be utilized by life as outlined above under Section 3.3 . In addition, recent experimental studies of marine sediment and sedimentary minerals [ 52 , 53 ] have found that radiolytic H 2 yields per unit radiation are magnified by up to 27 times relative to pure water, depending on sediment composition. And this radiolytic H 2 is produced at all sediment depths, suggesting that water radiolysis may be the key driver of microbial activity in a broad range of settings in marine sediments on Earth older than a few million years.…”