Life under pressure: from thermal vents to water worlds

By Dionysis Foustoukos

One of the most exciting discoveries related to the search of life on other planets is that Enceladus, one of Saturn’s moons, releases plumes of hydrogen from its surface, suggesting ongoing hydrothermal activity and thermodynamic disequilibrium in its subsurface, ice-covered ocean [Waite et al., 2017]. The implication for extraterrestrial life is that chemolithotrophic microorganisms—microbes that can use inorganic chemicals and minerals like hydrogen, sulfur, and nitrogen for energy, might inhabit this deep alien ocean.
 

Many astrobiologists believe these primitive H2-fueled microbes are strong candidates for Earth's earliest life-forms and for extraterrestrial organisms [Lane et al., 2010]. These findings also point to deep-sea hydrothermal systems on Earth as proxies to investigate the physiological and metabolic adaptations of microorganisms to conditions that may resemble those found on Enceladus and other ocean worlds. 

Most of Earth’s biosphere lives at the great depths of the ocean and within the subsurface of oceanic crust [Jannasch and Taylor, 1984; Oger and Jebbar, 2010]. This deep biosphere thrives under conditions of high hydrostatic pressure (>10 MPa, i.e., 100 bars) by utilizing metabolic strategies that permit adaptation to pressure, temperature and nutrient availability [Bartlett et al., 2007; Campbell et al., 2009]. The high-pressure adapted organisms (“piezophiles”) exhibit a phylogenetic diversity that extends to all three domains of life—Archaea, Bacteria, and Eukarya [Jebbar et al., 2015]. 

Such highly diverse microbiomes exist near deep-sea hydrothermal vents, where organisms have developed adaptations not only to pressure (> 20 MPa, i.e., 200 bars) but also to sharp gradients in temperature (4 – >100 °C) and chemical nutrient availability [Fang et al., 2010; Foustoukos et al., 2011; Hoehler and Jorgensen, 2013]. 

Considering the contribution of these microbial communities to the Earth’s microbiome and their association with the evolution of life on the early Earth [Martin et al., 2008], it is striking how little we know about the function and physiological responses of piezophiles to physical and chemical conditions resembling the deep biosphere [Jebbar et al., 2015].

At the Earth and Planets Laboratory, we are uniquely suited to study high-pressure microbes. 

How does life function in extreme environments?

In the search for life on other planetary bodies, such as Enceladus and Jupiter’s moon Europa, hydrothermal activity that could support environments suitable for extraterrestrial life should be considered. 

Understanding the evolution and environmental interaction of deep-sea hydrothermal microbiome on Earth can serve as a proxy to constrain the physiological and metabolic adaptations of microorganisms to life in extreme conditions.

Our innovative high-pressure microbiology techniques provide new avenues for studying metabolism in both pure cultures and natural deep-sea microbial communities under temperatures and pressures relevant to seafloor and subseafloor environments. For example, in a recent study, we constrained the cycle of elemental sulfur through microbial metabolism, while microorganisms are growing at seafloor pressures. 

Recently, in close collaboration with other institutions (i.e., Rutgers State University), we isolated and characterized a novel piezophilic Epsilonproteobacterium (Nautilia strain PV-1). This organism is an anaerobic NO3-reducing/H2-oxidizing autotrophic bacterium that was collected during incubation studies performed in the course of an oceanographic expedition.

Some of the key questions are: How do piezophiles adapt to pressure and chemical gradients present in deep-sea and subsurface environments? Which are the core cellular functions for pressure adaptation? Are those preserved as central and common functions among piezophiles? 

In these studies, we investigate the pressure/metabolic adaptation mechanisms of extremophiles associated with energy and carbon metabolism, membrane structure and competence by using a combination of gene and protein expression together with biochemical analyzes (e.g., biomass chemical/isotopic composition). Along with pure-culture studies, future work will include high-pressure incubations of diffuse-flow vent fluids delivered onboard oceanographic ships to isolate new microorganisms and to describe the physiology and function of natural mixed microbial communities. 

At the core of this effort is the integration of microbiological/molecular methods with (i) biochemistry; (ii) innovative experimental systems; (iii) thermodynamic modeling, and (iv) stable C-O-H-N isotope analysis. 

How do microbial ecosystems shape planet evolution?

Understanding the evolution and environmental interaction of the deep-sea hydrothermal microbiome on Earth can serve as a proxy to constrain the physiological and metabolic adaptations of microorganisms to the extreme for life conditions at primitive stages of planetary evolution. At EPL, we integrate microbial physiology, metabolism, and functions with ecosystem engineering to understand the functions and evolution of habitable worlds under the general umbrella of H2-based extreme autotrophic biospheres. 

Our hypothesis-driven investigations follow the life-cycle of bacteria that thrive in hydrogen-rich conditions from their interactions at their immediate habitats all the way into their co-evolution with atmospheres and oceans, by utilizing reservoirs of abiotic materials that are available on planets. We use laboratory observations, field studies at deep-sea hydrothermal vents, and geochemical modeling to link microbial ecosystems with planetary evolution.

By focusing on H2-based metabolism, we expand the framework of microbial and planetary co-evolution as a more general mechanism for compositional change at the planetary level, both at other points in Earth’s history (e.g., Great Oxidation Event) and on other bodies. Our studies aim to shed new light on what makes up a planet’s biosignature by better explaining the relationship between an initial abiotic composition of a planet and how the planet's atmosphere and composition is ultimately shaped by its microbial communities.

In a series of ongoing experimental studies, we are exploring how H2-fueled microorganisms function and respond to their surrounding physical (pressure/temperature) and chemical conditions by observing the activity of key metabolic enzymes and gene expressions. This work requires a wide variety of methods including isotopic, elemental, and protein/gene expression analysis of vent fluids, sulfide structures, and biofilms collected from deep-sea environments, and a novel approach to high-pressure co-culturing of H2-fueled autotrophs—organisms that can create food from their environment— with fermenting heterotrophs—organisms that have to consume organic matter to produce their own food.

We also want to learn how microbially-derived volatiles—like carbon dioxide and water—are introduced, transported, and stored in the atmosphere, hydrosphere, and the interior of terrestrial-like planets. These features are profoundly linked to habitability, the origin of life, and the observable evidence of conditions favorable for life. To study this,  we are combining experimental studies of biomass and abiotic organic matter hydrothermal degradation with the culturing of anaerobic autotrophic and fermentative heterotrophic bacteria under extreme deep-seafloor pressures.

References:

Bartlett, D. H., F. M. Lauro, and E. A. Eloe (2007), Microbial adaptation to high pressure, in Physiology and Biochemistry of Extremophiles, edited by C. Gerday and N. Glansdorff, pp. 333-348, American Society for Microbiology Press, Washington, DC.

Campbell, B. J., J. L. Smith, T. E. Hanson, M. G. Klotz, L. Y. Stein, C. K. Lee, D. Y. Wu, J. M. Robinson, H. M. Khouri, J. A. Eisen, and S. C. Cary (2009), Adaptations to submarine hydrothermal environments exemplified by the genome of Nautilia profundicola, Plos Genet, 5(2), Art# e1000362, doi:doi: 10.1371/journal.pgen.1000362.

Fang, J. S., L. Zhang, and D. A. Bazylinski (2010), Deep-sea piezosphere and piezophiles: geomicrobiology and biogeochemistry, Trends in Microbiology, 18(9), 413-422, doi:doi: 10.1016/j.tim.2010.06.006.

Foustoukos, D., J. L. Houghton, W. E. J. Seyfried, S. Sievert, and G. D. Cody (2011), Kinetics of H2-O2 redox equilibria and formation of metastable H2O2 under low temperature hydrothermal conditions, Geochimica et Cosmochimica Acta, 75, 1594-1607.

Hoehler, T. M., and B. B. Jorgensen (2013), Microbial life under extreme energy limitation, Nat Rev Microbiol, 11(2), 83-94, doi:doi: 10.1038/Nrmicro2939.

Jannasch, H. W., and C. D. Taylor (1984), Deep-Sea Microbiology, Annual Review of Microbiology, 38, 487-514, doi:doi: 10.1146/annurev.mi.38.100184.002415.

Jebbar, M., B. Franzetti, E. Girard, and P. Oger (2015), Microbial diversity and adaptation to high hydrostatic pressure in deep-sea hydrothermal vents prokaryotes, Extremophiles, 19(4), 721-740, doi:doi: 10.1007/s00792-015-0760-3.

Lane, N., J. F. Allen, and W. Martin (2010), How did LUCA make a living? Chemiosmosis in the origin of life, Bioessays, 32(4), 271-280, doi:10.1002/bies.200900131.

Martin, W., J. Baross, D. Kelley, and M. J. Russell (2008), Hydrothermal vents and the origin of life, Nat Rev Microbiol, 6(11), 805-814, doi:10.1038/nrmicro1991.

Oger, P. M., and M. Jebbar (2010), The many ways of coping with pressure, Res Microbiol, 161(10), 799-809, doi:doi: 10.1016/j.resmic.2010.09.017.

Waite, J. H., C. R. Glein, R. S. Perryman, B. D. Teolis, B. A. Magee, G. Miller, J. Grimes, M. E. Perry, K. E. Miller, A. Bouquet, J. I. Lunine, T. Brockwell, and S. J. Bolton (2017), Cassini finds molecular hydrogen in the Enceladus plume: Evidence for hydrothermal processes, Science, 356(6334), 155-159, doi:doi: 10.1126/science.aai8703.