Microbial ecosystems associated with “active” to “inactive” hydrothermal systems: massive sulfide deposits a CO2 sink in the deep-sea

Sea-floor massive sulfide deposits (SMS) harbor over 10⁸ tons of mineral resources along the mid-ocean ridges representing a significant polymetallic reservoir [1]. These SMS deposits are mainly composed of iron and sulfur, but also contain copper, zinc, gold, silver and rare-earth elements, making them attractive for the mining industry. Little is known about the geo-biodiversity and ecosystem services associated with these SMS deposits especially the inactive or extinct ones. Deep dark chemosynthetic microbial ecosystems associated with these deep-sea habitats interact with the hydrothermal deposits during their aging process through biogeochemical metal and carbon cycles. Hence, characterization and quantification of carbon incorporation are required to estimate the carbon fluxes associated with active and inactive hydrothermal systems [2-4]. During the HERMINE 2 [5] and BICOSE 3 [6] oceanographic expeditions on Mid-Atlantic Ridge (MAR), we collected 36 mineralized samples on SMS deposits characterized by different relative ages and various degrees of oxidation. Carbon fixation autotrophic and heterotrophic rates were measured using radio-labelled substrates at in situ conditions (pressure and temperature) to evaluate distribution and environmental controls on energy and carbon fluxes. Phylogenetic and metabolic microbial diversity were also investigated by a metagenomics approach to define microbial functional pathways driving alteration and biogeochemical metal and carbon cycles in the hydrothermal deep biosphere. Here we show, for the first time at a slow spreading ridge (i.e MAR), that high rates of CO₂ fixation support large microbial biomasses that inhabit “active” to “inactive” SMS. Given the large volumes of SMS habitats, these deposits could represent a significant CO₂ sink in the deep-sea and should therefore be considered for environmental management.

References:

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[2].      Dover, V. & Lee, C. Inactive Sulfide Ecosystems in the Deep Sea: A Review. Front. Mar. Sci. 6, (2019).

[3].      Cathalot, C. et al. Hydrothermal plumes as hotspots for deep-ocean heterotrophic microbial biomass production. Nat Commun 12, 6861 (2021).

[4].      Achberger, A. M. et al. Inactive hydrothermal vent microbial communities are important contributors to deep ocean primary productivity. Nat Microbiol 9, 657–668 (2024).

[5].      PELLETER Ewan & CATHALOT Cécile. HERMINE2 cruise,Pourquoi pas ? R/V. Preprint at https://doi.org/10.17600/18001851 (2022).

[6].      CAMBON Marie-Anne. BICOSE 3 cruise,Pourquoi pas ? R/V. Preprint at https://doi.org/10.17600/18002399 (2023).