The deep biosphere is Earth's largest microbial habitat, yet how its biomass is maintained and reshuffled remains poorly understood. Gittins et al. (2022) documented on the Scotian Slope (Northwest Atlantic) a subsurface microbial dispersal loop in which geological processes circulate living biomass out of and back into the deep subsurface: viable thermophilic bacterial endospores are exported upward from deep petroleum-bearing sediments by hydrocarbon seepage along faults, dispersed laterally in the permanently cold deep sea, and returned to the seabed by sediment burial, after which they can eventually germinate and grow where new petroleum systems establish. Whether this loop operates globally, on what time scales its upward and downward legs proceed, how quantitatively important it is for maintaining deep biosphere biomass, and whether buried endospores actually reactivate and grow where new petroleum systems establish all remain unresolved. The problem is to establish, with co-registered geological and microbiological evidence from settings beyond the Scotian Slope, the generality, rates, and quantitative significance of this geologically mediated microbial dispersal loop.

Background

The marine deep biosphere — the microbial populations inhabiting sediments and rock beneath the seafloor — is the largest microbial habitat on Earth, holding on the order of 102910^{29} cells, and its inhabitants survive at extraordinarily low energy fluxes. Bacterial endospores are dormant, highly resistant resting stages produced by members of the phylum Firmicutes; they accumulate in marine sediments and can remain viable for thousands to millions of years. Their signature molecule, dipicolinic acid (DPA), serves as a biomarker for quantifying endospores in sediments. Thermophilic endospores ("thermospores"), whose parent bacteria grow optimally at roughly 40–70°C, are routinely found in permanently cold seabed sediments — first highlighted by Hubert et al. (2009) in Svalbard fjords — and their presence where growth is impossible is a classic biogeographic puzzle pointing to dispersal from hot source habitats.

Cold seeps are seafloor vents of methane- and hydrocarbon-rich fluids. Where the hydrocarbons are thermogenic — generated by heat-driven maturation of deeply buried organic matter — they must migrate upward along geological conduits such as faults, providing a physical route connecting deep, hot, anoxic petroleum systems (source rocks and oil reservoirs, which host abundant endospore-forming thermophilic communities) with the cold ocean floor.

Gittins et al. (2022) combined seismic and acoustic imaging, hydrocarbon geochemistry, high-temperature anoxic incubations, and genomics on Scotian Slope sediments to show that thermospore populations in the cold seabed correlate with underlying seep conduits and migrated thermogenic hydrocarbons; that their genomes are adapted to anoxic petroleum systems and closely resemble oil-reservoir microbiomes worldwide; that spores remain viable down to at least 9 meters below seafloor (mbsf), germinating when incubated hot; and that DPA depth profiles imply continuous endospore burial at roughly 10810^{8} spores m−2^{-2} year−1^{-1}. They estimated tens to hundreds of years for upward seepage transport out of ~2000-m-deep reservoirs, and on the order of 10 million years for burial to carry spores back down to depths warm enough (>40°C) for germination. Together these observations define a subsurface microbial dispersal loop: geofluid seepage exports dormant cells from deep petroleum systems to the ocean, and sedimentation returns them, enabling environmental selection — germination, growth, and population replenishment — where new petroleum systems establish.

The loop was demonstrated at a single passive margin using order-of-magnitude rate estimates and indirect evidence for the return leg. Whether it operates wherever hydrocarbon seepage and sedimentation are coupled, at what measured rates, with what quantitative significance for deep biosphere biomass, and whether buried endospores truly reactivate in newly forming petroleum systems, remains unestablished.

Problem Statement

Establish whether the geologically mediated subsurface microbial dispersal loop operates as a globally significant mechanism maintaining deep biosphere biomass. This comprises four linked aspects: (i) determine the prevalence of the loop in geological settings beyond the Scotian Slope where hydrocarbon seepage and sedimentation are coupled; (ii) constrain the time scales of both legs of the loop — upward transport of viable cells by seepage and downward return by sediment burial — beyond the current order-of-magnitude estimates; (iii) quantify the loop's significance for maintaining deep biosphere biomass; and (iv) directly establish whether buried endospores reactivate and grow in newly forming petroleum systems. These aspects together address one overarching question: is geological-process-mediated circulation of dormant cells a general mechanism by which the deep biosphere is seeded and maintained, or a local peculiarity of the Scotian Slope? Whether the circulation is a global mechanism or a local peculiarity turns jointly on the loop's prevalence, the time scales of both legs, and its quantitative significance for deep biosphere biomass, not on prevalence alone.

References

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