Bacterial endospores — dormant, highly resistant resting cells — dominate the microbial biomass of the uppermost kilometer of marine sediment. The founding global survey (Wörmer et al., 2019) estimated 2.5×10282.5\times 10^{28} to 1.9×10291.9\times 10^{29} endospores (4.6–35 Pg C, exceeding the vegetative-cell biomass), but the estimate spans nearly an order of magnitude because of an uncertain dipicolinic-acid-to-endospore conversion factor, biased extrapolation models, and uneven geographic coverage. The in situ rates of endospore production, germination, and inactivation that maintain this pool, and the temperature, energy, and time controls, or other factors, that ultimately limit its survival during burial, remain undetermined. The problem is to deliver a quantitative, uncertainty-bounded account of the global marine subseafloor endospore pool and its response to burial: a reconciled global abundance and biomass estimate, in situ rates of endospore production and germination, and the survival controls that quantitatively explain the observed size, depth and age distribution of the pool.

Background

Marine sediments host a deep biosphere in which microorganisms survive burial for thousands to millions of years under extreme energy limitation. A key survival strategy is dormancy: bacterial endospores are metabolically quiescent, resistant resting cells formed mainly by bacteria of the phylum Firmicutes. Endospores are impermeable to the DNA stains used to count normal (vegetative) cells, so they are invisible to standard cell counts; they are instead quantified through dipicolinic acid (DPA), a biomarker molecule unique to endospores that is extracted from sediment by autoclaving and measured by the fluorescence of its complex with terbium. Converting a DPA concentration to an endospore count requires a DPA-per-endospore quota; the global study used 2.24×10−162.24\times 10^{-16} mol per endospore, a value based on only six laboratory isolates. Wörmer et al. (2019) analyzed 331 sediment samples from 47 sites spanning diverse depositional types and geographical origins and found that endospores dominate below about 25 meters below seafloor, with three independent modeling approaches (a global concentration-versus-depth regression, parameterization from single sites, and conversion of a published global vegetative-cell model via measured endospore-to-cell ratios) giving global estimates of 8.2×10288.2\times 10^{28}, 2.5×10282.5\times 10^{28}, and 1.9×10291.9\times 10^{29} endospores — a spread caused by regression bias toward a few organic-rich sites, limited single-site coverage, and analytical offsets between quantification methods. Thermophilic endospores ('thermospores'), produced by bacteria living in warm deep subsurface habitats, are nonetheless ubiquitous in cold sediments and appear more resistant to thermal inactivation than mesophilic ones; laboratory thermal-decay curves (decimal reduction values, the time needed to reduce a spore population tenfold at a given temperature) and burial-related geothermal heating were used to argue that burial acts as a selection filter on endospore subgroups. The authors proposed that the pool acts as a seed bank whose low-frequency germination could help sustain the very slow growth observed in the deep biosphere, but this remained a hypothesis: no in situ rate measurements existed, the global estimate carried order-of-magnitude uncertainty, and the ultimate factor limiting long-term survival of these cells was left unresolved.

Problem Statement

Deliver a quantitative, uncertainty-bounded account of the global bacterial endospore pool in the uppermost kilometer of marine sediment and of its response to burial, comprising three jointly linked components that together quantitatively explain the observed size, depth and age distribution of the pool: (1) Abundance — determine the global endospore abundance and biomass with quantified uncertainty, resolving the dipicolinic acid (DPA) per-endospore quota variability (its dependence on taxonomy, endospore size, and environmental conditions), the analytical offsets among quantification methods, and the extrapolation biases that currently produce the 2.5×10282.5\times 10^{28}–1.9×10291.9\times 10^{29} spread. (2) Dynamics — determine the in situ rates at which endospores are produced and germinate, the viable fraction of the pool, and the extent to which low-frequency germination contributes to maintaining deep-biosphere biomass and to measured subseafloor metabolic fluxes. (3) Controls — determine which temperature, energy, and time thresholds and mechanisms, or other factors, ultimately limit endospore survival and inactivation during burial, and test whether they quantitatively explain the observed depth- and age-dependent decline of the pool, accepting a negative or alternative result. Quantifying other specialized dormant cell types (exospores, akinetes, cysts, myxospores) is a distinguishable adjacent problem flagged by the same source and is not required for completion.

References

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