[biorxiv.org] The subsurface of our planet hosts 15% of Earth’s biomass and plays a key role in mediating the exchange of volatiles and elements between deep, long-residence-time geological reservoirs and rapidly cycling surface environments, influencing planetary climate and habitability.
While a significant fraction of subsurface microorganisms rely on surface-derived organic carbon, an unknown portion is sustained through chemolithotrophic carbon fixation. Despite its importance, the global diversity and distribution of microbial carbon fixation pathways in the subsurface, and the environmental drivers shaping them, remain poorly constrained.
Here we systematically characterise carbon fixation pathways for 412 subsurface metagenomes, including 242 new metagenomes, and compare them to surface-derived datasets. We find that subsurface environments span a broader physicochemical space than surface systems and support a higher abundance and diversity of carbon fixation strategies.
Using colocated geochemical data spanning >50 variables, we show that the reductive tricarboxylic acid cycle and the reductive acetyl-CoA pathway are enriched in reducing, geochemically evolved fluids.
We use the metagenomic results together with previously published carbon fixation rates in the subsurface to derive a global continental subsurface carbon fixation rate of ∼2.65 Pg C yr−1 (range: 0.31–2.99). This represents ∼2% of terrestrial photosynthetic primary production, and is an order of magnitude higher than geological fluxes between the surface and the subsurface.
These results identify the subsurface as a reservoir of autotrophic strategies organized along geochemical gradients, contributing substantially to the global carbon cycle.
Global distribution and physicochemical space of subsurface and surface metagenomes. a, Global map of all metagenomic sampling sites included in this study, comprising geothermal and subsurface fluids, sediments, and biofilms from the CoEvolve dataset, as well as publicly available surface (soil, marine, freshwater) and subsurface samples from public repositories. b, Distribution of samples in pH–temperature space, highlighting the substantially broader physicochemical range covered by subsurface environments compared to surface ecosystems. c, Piper diagram summarising aqueous geochemistry for samples with available data. The lower triangles represent the relative proportions of major cations (Ca2+ , Mg2+ , Na++K+ ) and anions (Cl– , SO4 2- , HCO3 – ), while the central diamond integrates these to define hydrochemical facies. Seawater samples are represented by a narrow area (blue diamond circled). Soil and sediment samples are absent from this plot. — biorxiv.org
Global carbon fixation in Earth’s subsurface biosphere, biorxiv.org
Astrobiology
