Radionuclides are ubiquitous in sediment and rock, where their decay leads to the production of hydrogen and oxidized chemicals via radiolysis of water; these radiolytic products provide the dominant fuel for microbial activity in marine sediment older than a few million years; they may also be significant for sustaining life in subseafloor sediment and subsurface environments of other planets, according to a new study.
Global distribution of production rates of radiolytic hydrogen and radiolytic oxidants in marine sediment; rates are expressed in mol electron equivalents/cm2/year; in electron equivalents, water radiolysis produces hydrogen and oxidized chemicals at equal rates; although abyssal clay has a volumetric radiolytic production rate an order of magnitude higher than the volumetric rate for continental-margin sediment types, the sediment layer that blankets open-ocean regions is much thinner and consequently has much lower vertically integrated radiolytic production than the sediment of continental margins. Image credit: Sauvage et al., doi: 10.1038/s41467-021-21218-z.
“This work provides an important new perspective on the availability of resources that subsurface microbial communities can use to sustain themselves,” said Dr. Justine Sauvage, a postdoctoral researcher in the Department of Marine Sciences at the University of Gothenburg.
“This is fundamental to understand life on Earth and to constrain the habitability of other planetary bodies, such as Mars.”
In contrast to the conventional view that life in sediment is fueled by products of photosynthesis, an ecosystem fueled by radiolysis of water begins just meters below the seafloor in much of the open ocean.
“The resulting molecules become the primary source of food and energy for the microbes living in the sediment,” sai Professor Steven D’Hondt, a researcher in the Graduate School of Oceanography at the University of Rhode Island.
“The marine sediment actually amplifies the production of these usable chemicals. If you have the same amount of irradiation in pure water and in wet sediment, you get a lot more hydrogen from wet sediment. The sediment makes the production of hydrogen much more effective.”
“Why the process is amplified in wet sediment is unclear, but minerals in the sediment may behave like a semiconductor, making the process more efficient.”
“This study is a unique combination of sophisticated laboratory experiments integrated into a global biological context,” said Professor Arthur Spivack, also from the Graduate School of Oceanography at the University of Rhode Island.
In a series of laboratory experiments, the researchers irradiated vials of wet sediment from various locations in the Pacific and Atlantic Oceans, collected by the Integrated Ocean Drilling Program and by U.S. research vessels.
They compared the production of hydrogen to similarly irradiated vials of seawater and distilled water. The sediment amplified the results by as much as a factor of 30.
“We experimentally quantified hydrogen yields for alpha- and gamma-irradiation of pure water, seawater, and seawater-saturated marine sediment with a typical abyssal clay porosity (80–85%) for all abundant marine sediment types (abyssal clay, nannofossil-bearing clay (calcareous marl), clay-bearing siliceous ooze, calcareous ooze, and lithogenous sediment), which collectively cover ~70% of Earth’s surface,” they explained.
“Our results show that for pure water, seawater, and marine sediment slurries, hydrogen production increases linearly with absorbed alpha- and gamma-ray dose.”
“The implications of the findings are significant. If you can support life in subsurface marine sediment and other subsurface environments from natural radioactive splitting of water, then maybe you can support life the same way in other worlds,” Professor D’Hondt said.
The results also have implications for the nuclear industry, including for how nuclear waste is stored and how nuclear accidents are managed.
“If you store nuclear waste in sediment or rock, it may generate hydrogen and oxidants faster than in pure water,” Professor D’Hondt said.
“That natural catalysis may make those storage systems more corrosive than is generally realized.”
The findings were published in the journal Nature Communications.
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J.F. Sauvage et al. 2021. The contribution of water radiolysis to marine sedimentary life. Nat Commun 12, 1297; doi: 10.1038/s41467-021-21218-z
