Radiolytic hydrogen and microbial respiration in subsurface sediments

Subsurface microbial ecosystems rely on electron donors for respiration. Most of these are derived from organic matter decay. However, organic matter becomes scarce at greater depths. This gap motivated researchers to explore alternative sources of electron donors. Radiolysis of water is one such process that may generate molecular hydrogen. Prior research has shown that radiolysis can occur in subsurface environments. But the extent to which it supports microbial respiration remains unclear. This study aimed to assess the significance of radiolytic hydrogen in anoxic marine sediments. It focused on comparing radiolytic H₂ production with organic-fueled respiration rates.

The goal was to determine how much radiolytic hydrogen contributes to microbial respiration in marine sediments. Researchers wanted to quantify this process at multiple Ocean Drilling Program (ODP) sites. They compared calculated radiolytic H₂ production rates with net respiration estimates. The study aimed to identify under what conditions radiolysis becomes a major electron donor source. It also sought to understand how this process might function in low-organic environments. The researchers considered the role of radiolysis in sustaining microbial life in deep sediments. They examined whether this process could be a key energy source in low-organic settings. The findings might inform models of non-photosynthetic ecosystems on other planets.

The team used a radiolysis model to estimate H₂ production rates. Inputs included concentrations of uranium, thorium, and potassium in sediments. They also considered porosity, grain density, and water content. Net respiration rates were calculated from dissolved electron acceptor fluxes. These included sulfate, nitrate, and other electron acceptors. The researchers compared radiolytic H₂ yields with respiration rates at multiple ODP sites. They focused on sites with varying levels of organic carbon content. The study aimed to identify trends in H₂ availability across different sediment depths.

Radiolytic H₂ production rates increased in sediments with lower organic carbon content. At ODP Site 1231, radiolysis may fuel up to 10% of respiration. In even lower-organic sediments, radiolysis could be the main electron donor source. The highest H₂ yields were observed in sediments with low organic matter. Uranium and thorium concentrations strongly influenced radiolytic H₂ production. The model predicted that radiolysis becomes more significant as organic respiration declines. These findings suggest a shift in electron donor sources in deep sediments. The study highlights the potential role of radiolysis in sustaining microbial life in low-organic environments.

The study suggests that radiolysis may become a key electron donor source in low-organic sediments. Radiolytic H₂ production increases as organic-fueled respiration decreases. At ODP Site 1231, radiolysis may support 10% of microbial respiration. In even lower-organic environments, this process could be the main energy source. The findings align with the authors’ hypothesis about radiolysis in deep sediments. They propose that radiolysis could sustain life in subsurface ecosystems. The study also suggests that marine sediments may model non-photosynthetic ecosystems elsewhere. The authors suggest that these findings could inform astrobiological models of Mars and Europa.