The deep terrestrial subsurface was long treated as an exclusively prokaryotic habitat, yet Borgonie et al. (2015) found nematodes, flatworms, rotifers, annelids, arthropods, protozoa and fungi thriving in kilometre-deep palaeometeoric fracture waters of South African gold mines, with nematode population densities of roughly one million individuals per square metre of biofilm — high enough to suggest a substantial eukaryal biomass. The authors cautioned that these figures could not be reliably extrapolated globally: detailed eukaryal data existed from only four fissure-water sampling points, all in mined, hydrologically perturbed settings. A decade of follow-up work has extended the geographic and taxonomic record (Fennoscandian Shield groundwater fungi, fungal consortia in Swedish granite fractures, abundant novel fungi in deep shale, genomic adaptations of a subsurface nematode) and largely explained how surface organisms enter (seismically driven recharge), but no study has quantified the eukaryotic share of deep-subsurface biomass or diversity across geological settings, and no quantitative eukaryal undisturbed-system baseline exists against which to judge mining-induced enhancement. The problem posed here is to determine, with quantitative and uncertainty-aware evidence, the magnitude and cross-setting generality of the eukaryotic contribution to deep terrestrial subsurface biomass and diversity beyond the few mined sites studied to date.

Background

The deep biosphere — life in rocks, sediments and groundwater hundreds of metres to kilometres below Earth's surface — is estimated to hold a large fraction of all microbial cells on the planet, dominated in census terms by bacteria and archaea. In the continental crust, life concentrates in fractures (fissures) that conduct groundwater; where that water originated as rain or rivers that percolated downward long ago, it is called palaeometeoric water, and in South African gold mines it can be thousands of years old. Access to this environment comes almost exclusively through mines and scientific boreholes, because advancing tunnels intersect water-bearing fractures. An earlier study in South African gold mines (Borgonie et al., Nature, 2011) discovered a new nematode worm living 1.3 km underground in fracture water, nicknamed the 'worm from hell', proving that multicellular animals could inhabit the deep subsurface. Borgonie et al. (Nature Communications, 2015) then performed the first systematic eukaryotic survey of these fracture waters: by filtering borehole flow over long periods (up to two years) and combining video documentation of biofilms (surface-attached microbial communities on the rock faces) with microscopy, culturing and 18S rRNA and internal transcribed spacer (ITS) gene sequencing — standard barcodes for identifying eukaryotes and fungi respectively — they recovered 17 eukaryotic species at 1.0–1.4 km depth in the Kopanang and Driefontein mines, including Nematoda (roundworms), Platyhelminthes (flatworms), Rotifera, Annelida, Arthropoda, Protozoa and Fungi. All were known surface species, mostly freshwater opportunists (organisms that exploit transient favourable conditions), and ecological calculations indicated that food (bacteria), not dissolved oxygen, limits their population growth. Extrapolated nematode densities were about one million individuals per square metre, higher than typical surface freshwater biofilms, prompting the title claim that eukaryotic opportunists dominate this deep-subsurface ecosystem. The authors, however, explicitly framed the limits of this conclusion: (i) with detailed eukaryal data from only four fissure-water sampling points worldwide, and none from outside mining districts, the figures 'cannot be reliably globally extrapolated'; (ii) mining artificially augments fracture flow (accelerated fracture opening and mixing of waters), so observed densities may sit at the high end of the natural range — natural baseline dynamics are unknown; (iii) the circumstances and timescales by which surface organisms enter, transit and survive in fracture water were unknown. Subsequent work transformed part (iii): Borgonie et al. (Scientific Reports, 2019) showed that surface river animals and 1.4-km-deep mine populations of the same metazoan species are genetically identical for 18S rDNA and even interbreed, and that seismic activity measurably increases hydraulic conductivity in faults, producing a steady downward flow of surface organisms, of which some survive and others perish. Genomic work on the deep nematode Halicephalobus mephisto identified expanded heat-shock (Hsp70) and AIG1 protein families consistent with adaptation to subsurface stress (Weinstein et al., 2019). Fungi have since been found active in deep Fennoscandian crystalline groundwater and rock surfaces (Sohlberg et al., 2015; Drake et al., 2017; Nuppunen-Puputti et al., 2020), and a 2026 survey of organic-rich shale documented abundant, diverse and novel fungal communities at 247–556 m depth with a median fungal-to-bacterial biomass ratio near 1:4.7 in that ecosystem (Moon et al., 2026) — overturning the assumption of eukaryotic scarcity locally, yet still a single setting. What remains missing is the original paper's unanswered global question: the eukaryotic share of deep terrestrial subsurface biomass and diversity, and whether the mined-site densities reflect nature or the mining perturbation itself.

Problem Statement

Quantify, with uncertainty, the contribution of eukaryotes to the biomass and diversity of the terrestrial deep subsurface — on the biomass axis, the eukaryal share of the prokaryote-dominated deep-subsurface total as a pooled estimate with its uncertainty, reported so that the source's own claim that eukaryotic opportunists 'dominate' can be assessed against it; on the diversity axis, eukaryal richness at a stated taxonomic rank documented across contrasting geological or hydrogeological settings, at least one of them non-mined, rather than richness confined to a single setting — in the generality that Borgonie et al. (2015) could not: quantify eukaryal abundance, biomass and diversity in deep (hundreds of metres to kilometres) fracture waters and rock-hosted communities across contrasting geological and hydrogeological settings, and establish how the resulting densities compare with the mined, flow-augmented South African sites. The single scientific objective — an uncertainty-aware, multi-setting determination of the eukaryotic share of deep-subsurface communities — jointly requires: (a) quantitative eukaryal abundance and biomass data (metazoans, protozoa, fungi) from deep fracture water and rock surfaces across the assembled settings, from methods such as microscopy, cultivation, and 18S rRNA/ITS amplicon or metagenomic sequencing, with contamination controls adequate for low-biomass samples — satisfied by published datasets, with new sampling not required for (a) to be complete, as for (b); (b) at least one natural-baseline set of quantitative eukaryal measurements from hydraulically undisturbed, non-mined settings — satisfied by already-published datasets from non-mined scientific boreholes where they exist, together with a specified sampling design (newly drilled, sterilized-pathway boreholes) for the settings those datasets do not cover; executing that design is not required for (b) to be complete — so that natural baseline population densities can be compared against mine-augmented-flow densities, resolving whether the high South African nematode densities are natural or industrially enhanced; and (c) an explicit extrapolation, with uncertainty, of the eukaryotic contribution to deep terrestrial subsurface biomass and diversity; where the data assembled under (a)–(b) bound that contribution too weakly to extrapolate, reporting the bounds they do support, together with the gaps that limit them, is a permitted partial result for (c) but does not by itself resolve (a) or (b). These components are linked: (b) supplies the baseline that (a) needs to interpret mined-site data, and (c) is the synthesis of both. Entry and survival mechanisms, which the source also asked about, may inform the interpretation but are not required deliverables, since the qualitative entry question has since been answered by seismic-recharge evidence (Borgonie et al., 2019).

References

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