Hydration solids

Nature (2023)Cite this

Nature (2023)Cite this article

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Hygroscopic biological matter in plants, fungi and bacteria make up a large fraction of Earth's biomass1. Although metabolically inert, these water-responsive materials exchange water with the environment and actuate movement2,3,4,5 and have inspired technological uses6,7. Despite the variety in chemical composition, hygroscopic biological materials across multiple kingdoms of life exhibit similar mechanical behaviours including changes in size and stiffness with relative humidity8,9,10,11,12,13. Here we report atomic force microscopy measurements on the hygroscopic spores14,15 of a common soil bacterium and develop a theory that captures the observed equilibrium, non-equilibrium and water-responsive mechanical behaviours, finding that these are controlled by the hydration force16,17,18. Our theory based on the hydration force explains an extreme slowdown of water transport and successfully predicts a strong nonlinear elasticity and a transition in mechanical properties that differs from glassy and poroelastic behaviours. These results indicate that water not only endows biological matter with fluidity but also can—through the hydration force—control macroscopic properties and give rise to a ‘hydration solid’ with unusual properties. A large fraction of biological matter could belong to this distinct class of solid matter.

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Source data for Figs. 1a,d–f, 3a,b, 4a–i and 5b,c, and Extend Data Figs. 2, 4 and 5 are included with the paper. The raw data for cantilever deflections (Fig. 1c–f), spore height (Figs. 1a and 3a,b), force–distance curves (Fig. 4b–f) and dynamic stiffness measurements (Fig. 5c) are available in figshare (https://doi.org/10.6084/m9.figshare.22189823)58.

The MATLAB codes used for data processing, curve fitting, and plotting are available in figshare (https://doi.org/10.6084/m9.figshare.22189823)58.

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We acknowledge A. Driks (Department of Microbiology and Immunology, Loyola University Chicago, Maywood, IL, USA) who passed away before the completion of the work for contributing spores, and for discussions that informed the hygroelastic theory and for suggesting the use of known sieving properties of spores as an estimate of pore size. Funding was provided by US Department of Energy (DOE) Early Career Research Program, Office of Science, Basic Energy Sciences (BES), under award no. DE-SC0007999 (Fig. 1 and experimental data in Figs. 3 and 4); by the Office of Naval Research, under award nos. N00014-19-1-2200 (Fig. 5 and theoretical analyses in Figs. 3 and 4) and N00014-21-1-4004 (theoretical analyses in Figs. 3 and 4); by the National Institute of General Medical Sciences of the National Institutes of Health, under award nos. R35GM141953 (to J.D.) and R35GM145382 (to O.S.); and by the David and Lucile Packard Fellows Program. We acknowledge the use of facilities and instrumentation supported by NSF through the Columbia University, Columbia Nano Initiative, and the Materials Research Science and Engineering Center DMR-2011738.

Xi Chen

Present address: Advanced Science Research Center (ASRC) at the Graduate Center of the City University of New York, New York, NY, USA

Ahmet-Hamdi Cavusoglu

Present address: Merck Digital Sciences Studio (MDSS), Newark, NJ, USA

These authors contributed equally: Steven G. Harrellson, Michael DeLay

Department of Physics, Columbia University, New York, NY, USA

Steven G. Harrellson & Ozgur Sahin

Department of Biological Sciences, Columbia University, New York, NY, USA

Michael S. DeLay, Xi Chen & Ozgur Sahin

Department of Chemical Engineering, Columbia University, New York, NY, USA

Ahmet-Hamdi Cavusoglu

Department of Microbiology and Immunology, Columbia University, New York, NY, USA

Jonathan Dworkin

Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, USA

Howard A. Stone

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M.D., X.C. and O.S. designed the experiments probing relaxation kinetics with nanomechanical cantilever sensors. M.D. and X.C. developed the experimental apparatus for relaxation kinetics measurements. M.D. conducted the relaxation kinetics measurements. M.D., S.G.H, A.-H.C. and O.S. contributed to the analysis of relaxation kinetics measurements. M.D. and S.G.H. conducted spore height measurements. X.C. conducted force–distance curve measurements. S.G.H. and O.S. analysed measurements of water-responsive size change and nonlinear elasticity of spores. S.G.H. and O.S. designed experiments probing the hygroelastic transition. S.G.H. conducted the experiments probing the hygroelastic transition and analysed the data. H.A.S. contributed to the theoretical analysis of the hygroelastic transition. J.D. contributed materials. O.S. conceived the hygroelastic model. S.G.H. aided in the development and refinement of the hygroelastic model. M.D., S.G.H., J.D., H.A.S. and O.S. contributed to the discussions. O.S. designed the research. M.D., S.G.H. and O.S. prepared the paper with input from all authors.

Correspondence to Ozgur Sahin.

The authors declare no competing interests.

Nature thanks Peter Hoffmann and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Scanning electron microscopy image of a spore-coated cantilever. Scale bar ~20 µm. The cantilever is T-shaped, and it is approximately 300 µm long and 30 µm wide, except near the free end where the width is approximately 60 µm.

Representative cantilever deflection signals following photothermal pulses shown for a range of relative humidity levels. Deflection signals are normalized so that peak-to-peak deflection corresponds to 1. All curves are obtained with the same spore-coated cantilever. They are representative of curves from all five cantilevers used in Fig. 1d.

Source data

a, Scanning electron microscopy image of the spores of B. subtilis. Scale bar: 500 nm. Wild type spores of B. subtilis are approximately 650 nm in diameter and 1 um to 1.5 um in length. b, Illustration of the cross section of spores of B. subtilis showing the cortex and coat layers that surround the core, which contains the genetic material. The coat is a proteinaceous, water-permeable layer49. AFM images of the outer surface of the coat reveals an assembly of parallel rodlets formed by coat proteins with ~ 8 nm periodicity59. The cortex, also a water permeable layer, is a loosely crosslinked network of peptidoglycan that is similar in structure to that of vegetative cells. The vegetative peptidoglycan in B. subtilis has an average diameter of 4 nm, as observed in AFM images60. The thicknesses of the coat and the cortex layers are approximately 70 nm each (see Methods). The spore core contains proteins and DNA. The core is dehydrated61 and the DNA is packed in a crystalline state62. Elastic modulus measurements of DNA films in crystalline state show the Young's modulus of these films to be approximately 1.1 GPa63, suggesting that the core is a stiff solid rather than a fluid. This assumption is also supported by the observation that soluble biomolecules are immobile in the dehydrated cores of dormant spores but gain mobility upon germination when the core gets hydrated64. The spore water, however, exhibits rotational mobility, as indicated by the observations of short rotational correlation times of D2O in spores65. This observation also indicates that water in spores is not in an ice-like (solid) state.

Shifts in the resonance frequency of spore-coated cantilevers are sensitive to the amount of water exchange. We used this effect to compare relative effects of the photothermal pulse and RH. Here we plot the relative changes in fundamental resonance frequency of a cantilever coated with spores in two cases: (left, red bar) as a result of photothermal pulse and (right, purple bar) in response to a change in relative humidity from 80% to 10%. The results are given as percentages. They indicate that the perturbation due to the photothermal pulse is small, and therefore, the transient deflections of the cantilever in response to photothermal pulses reflect approximately the state of the spore at the set-point level of the relative humidity.

Source data

The spore quantities are represented by spore mass estimated from the shifts in cantilever resonance frequencies. Time constants are plotted for wild type B. subtilis (square) and B. subtilis cotE gerE (circle) spores. According to the data, there is a lack of clear association between the time constant and the total spore mass, however the effect of spore type results in a statistically significant change in time constants: Mean time constant at 50% RH for wild type B. subtilis [square] is ~118 ms and ~47.1 ms for B. subtilis cotE gerE [circle] (one-tailed T, p < .01).

Source data

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Harrellson, S.G., DeLay, M.S., Chen, X. et al. Hydration solids. Nature (2023). https://doi.org/10.1038/s41586-023-06144-y

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Received: 05 October 2017

Accepted: 27 April 2023

Published: 07 June 2023

DOI: https://doi.org/10.1038/s41586-023-06144-y

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