South Pacific Gyre

Average cell abundances and net rates of respiration are a few orders of magnitude lower than in any other subseafloor biosphere previously studied.

In other subseafloor biospheres, microbial respiration will break down organic material and consume all the oxygen near the seafloor leaving the deeper portions of the sediment column anoxic.

[6][7] Benthic microbes in organic-poor sediments in oligotrophic oceanic regions, such as the South Pacific Gyre, are hypothesized to metabolize radiolytic hydrogen (H2) as a primary energy source.

[2] Typically, deep-ocean benthic microbial life utilizes the organic carbon exported from surface waters.

In oligotrophic regions where sediments are poor in organic material, subsurface benthic life exploits other primary energy sources, such as molecular hydrogen (H2).

[10][8][2][9] Radioactive decay of naturally occurring uranium (238U and 235U), thorium (232Th), and potassium (40K) in seafloor sediments collectively bombard the interstitial water with α, β, and γ radiation.

These criteria indicate that certain sediment types, such as abyssal clays and siliceous oozes, may have higher radiolytic H2 production relative to other seafloor strata.

Measured negative fluxes of O2 in the surface layer demonstrate that a relatively low abundance of aerobic microbes that are oxidizing the minimally deposited organic matter from the ocean above.

[9][2] So, despite the known occurrence of radiolytic H2 production, molecular hydrogen is below the detectable limit in the SPG cores, leading to the hypothesis that H2 is the primary energy source in low-organic seafloor sediments below the surface layer.