The other host rock: Sulfidic and methane-rich gabbro-hosted hyperalkaline fluids likely host novel chemotrophic communities

Abstract:

Gabbroic rocks comprise a significant component of the oceanic crust and its exhumed counterparts in continents (e.g., ophiolites) yet gabbro-hosted fluids are poorly studied and their potentials to support life are less understood than basalt- and peridotite-hosted ecosystems. In this work we investigated the compositions and bioenergetic potentials of low-temperature fluids (<40°C) sampled from gabbroic bodies in the Samail ophiolite in Oman. Like peridotite-hosted fluids in Oman, gabbro-hosted fluids can attain hyperalkaline pH (>11) and highly reduced compositions. Concentrations of dissolved components (Mg, Si, Al) in both fluid types overlap but gabbro-hosted fluids tend to be more enriched in Si and Al and depleted in Mg. While gabbro-hosted fluids are depleted in dissolved H₂, these fluids are enriched in CH₄ and contain 1 to 2 orders of magnitude more dissolved sulfide (~100 μM) than peridotite-hosted fluids in Oman. These differences in fluid compositions lead to varying potentials to support various chemotrophic strategies. On average, there is less energy available for hydrogenotrophs (aerobic H₂ oxidizers, and hydrogenotrophic methanogens and sulfate reducers) present in fluids associated with gabbros. In contrast, the highest energy (~5 cal/kg fluid) available for aerobic sulfide oxidizers is associated with a gabbro-hosted spring site. Moreover, anaerobic methane oxidation using sulfate is highly favored in gabbro-hosted fluids (~0.5 cal/kg) relative to peridotite-hosted fluids (~0.03 cal/kg). This work shows that fluid-rock reactions enabled by differences in the host rock compositions can ultimately lead to varying potentials to support different ecosystems. Our calculations suggest communities of microbes living in gabbro-hosted fluids are likely different from those present in similarly high pH peridotite-hosted ecosystems. Like on Earth, the crust of Mars and rocky interiors of ocean worlds in the solar system could be heterogenous and intrusive gabbroic rocks are likely present beneath surficial rocks. This is strong motivation to investigate gabbro-hosted systems, in parallel with our growing understanding on basalt- and ultramafic-hosted environments, to better capture the varying potentials of rocky bodies in the solar system to support life.