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Habitability at local scales

Habitability at local scales. . . . is determined by the distribution of resources and challenges within an environment. Any Questions?. Tori Hoehler NASA-Ames Research Center. Roadmap Habitability…. As observed in the world around us As currently applied in space sciences

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Habitability at local scales

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  1. Habitability at local scales . . . is determined by the distribution of resources and challenges within an environment Any Questions? Tori Hoehler NASA-Ames Research Center

  2. Roadmap Habitability… As observed in the world around us As currently applied in space sciences Areas for improvement - Defined* = And Energy And Evolution?

  3. What is Life? Attempts to define life are frequently challenged with exceptions . . .

  4. We presently lack the ability to define life, because we lack an adequate understanding of biology Cleland & Chyba “Never ask what something is; just ask ‘what does it do?’” Hilaire Belloc Example: Life processes (decodes, replicates, stores) information

  5. Defining Habitability A theoretical construct that informs our sense of possibility for life in the universe (“is it habitable?”) A discriminating tool with which to direct the allocation of limited observing resources (“how habitable is it?”)

  6. Defining Habitability Potential (probability?) for life to arise & evolve Diversity of life supported Resilience to environmental change Extant life supported?

  7. Habitability as Currently Applied ( and what we can do better)

  8. Raw Materials Energy Clement Conditions Solvent H? Requirements for Complexity (sponch, e-) (light, chemical) (water) (T, pH, S, etc.)

  9. State of the Art: The Habitability Check-List Water T, pH, P, S, Rad. (incl. ranges) Nutrients Energy All boxes checked = “habitable” (Right??)

  10. Follow the Water Water . . . Phase transitions well understood (predictable) Current or past occurrence can be discerned by multiple lines of evidence Assumption of water as critical helps to narrow the range of possible habitable conditions (THEMIS)

  11. Follow the Water Water . . . Is necessary but not sufficient for life (as we know it) Is a binary indicator of habitability (‘no’ or ‘maybe’) Absence of water makes a more definitive statement about habitability (THEMIS)

  12. +90º 180º 270º 0º 90º (MOLA Team) -90º Water, Water, Everywhere? Maybe? Maybe? Maybe? Maybe? Maybe? Maybe? Maybe? Maybe? Maybe? Maybe? Maybe? Maybe?

  13. The Reality of Habitability As Observed in the World Around Us

  14. Biomass Density: African Land Cover

  15. Photosynthetic Biomass Density: Surface Oceans ◘ ◘ ◘ ◘ ◘ ◘ ◘ ◘ ◘ ◘ ◘ ◘ (NASA / SeaWIFS/MODIS)

  16. Biomass Density: Pacific Ocean Sediments Log (cells·mL-1) 2 4 6 8 10 12 ●Eq. Pacific ●N.Pac.Gyre ●S.Pac.Gyre ●ODP ●Non-ODP Depth (mbsf) 107.5x (Kallmeyer et al., AGU 2009)

  17. Quantifying Habitability . . . Is it habitable? (what are life’s requirements & limitations?) + Resource availability: Quantity/Time + Quantitative impact of challenges + Inter-relation of resources and challenges How habitable is it?

  18. Energy and Habitability

  19. Energy… Is Needed For: Growth Maintenance Evolution

  20. Energetic Constraints on Biological Potential = (well, sort of . . . ) Like an electrical device, life has requirements for both VOLTAGE (ΔG) and POWER (energy/time)

  21. Energy Utilization: Growth and Maintenance (1/Y) Substrate (Energy) Utilization Rate (Q) (M) Growth Rate (μ) Sauer, Appl and Environ Microbiol, 1996

  22. Biomass Loss (Slope = f(environment)) Biomass (Lower Limit on Biomass) Biomass Gain (Cell Growth) (ME Threshold) Uninhabitable Energy/Time* *Power The Importance of Power: Constrains growth rate or steady-state biomass in continuum Energy-limited steady-state

  23. Temperature and Maintenance Energy -Ea/RT ME = A∙e (Tijhuis et al, 1993, Biotechnol. Bioeng.) (Price & Sowers, 2004, PNAS)

  24. pH and Maintenance Energy Interior pH maintained by pumping protons: pHin pHout ME = proton pumping rate x energy per proton Rate ≈ pH leakage rate Trans-membrane pH “leakage” rate Energy per proton = f(ΔpH)

  25. Breaking Points? Our concept of the physical and chemical limits to life is informed by culture work that pushes an organism to its limit with respect to one variable, while optimizing all others. These are absolute biochemical breaking points, which may not be the most relevant to consider for all environments “Strain 121” (credit: geobacter.org)

  26. Extremes & Energy Across a broad spectrum of physicochemical variables, the biological response to “extreme” typically involves energy expenditure Organisms have bioenergetic breaking points, which may be reached before biochemical ones – especially in energy-poor settings This provides a means of assessing the impact of disparate “extremes” on a common basis, energy “Strain 121” (credit: geobacter.org)

  27. ME (energy/biomass/time) Temperature limits on life: biochemical vs. bioenergetic Energy availability biochemical limit bioenergetic limit (Tijhuis et al, 1993, Biotechnol. Bioeng.)

  28. Multi-Parameter Habitability Case Study: Methanogenesis in Serpentinizing Systems Mg1.8Fe0.2SiO4 + 1.37H2O → 0.5Mg3Si2O5(OH)4 + 0.3Mg(OH)2 + 0.067Fe3O4 + 0.067H2 CO2 + 4H2 → CH4 + 2H2O (Image: Kelley et al, 2001)

  29. The Poster Child: Lost City Hydrothermal Field Hydrogen Activity up to 15mM Temperature up to 90˚C pH to 11 Evidence for methane-associated biology (Kelley et al, 2001)

  30. Variability in Serpentine-Associated Fluids 1Proskurowski et al, 2008, Lang et al, 2010; 2McCollom et al unpublished; 3Cardace et al, AGU 2009

  31. (Brazelton et al., 2012, Frontiers Microbio. )

  32. The Good: Elevated H2 concentrations represent abundant substrate for H2-utilizing methanogens The Bad: Elevated temperatures and extremes of pH may significantly impact both biochemistry and bioenergetics The Ugly: CO2 may be extremely limiting, both as a methanogenic substrate and as a carbon source for autotrophy Methanogenesis in Serpentinizing Systems? CO2+ 4H2 → CH4 + 2H2O

  33. [S]bulk [S] substrate concentration [S]BEQ ΔGmin defines [S] at r=0 0 Energy Budget with Cell-Scale Reaction Transport Model radial distance from cell interior (r) Bulk fluid concentrations Diffusion-reaction model considers: Temperature, pH, salinity Membrane transport, enzyme kinetics*

  34. Influence of pH and {H2} on Methanogenic Energy Flow (Generation & Demand) (Seawater, 50°C, DIC = 100 mM, CH4 = 100 mM; diffusive-only trans-membrane C transport) Generation / Demand Generation - Demand fg C cell-1 d-1 (Biomass Density Potential) (Growth Rate Potential)

  35. Thermodynamics in Serpentine-Associated Fluids Methanogen Power Budget

  36. Exploring Parameter Space (courtesy Sanjoy Som) GWB numerically coupled to cell-scale RT model -- Caveat emptor: Equilibrium chemistry; does not include kinetics What Matters? Rock Composition (esp. Fe & Ca content) Fluid Composition (Fresh vs. Seawater) Water-Rock Ratio (“sweet spots” exist) Membrane transport processes

  37. CO2 + 4H2 → CH4 + 2H2O The Basics ΔG effect pH (speciation)  flux effect Power: Gen/Demand Fresh Water, 100C, Neutral-only X-membrane transport Som et al., in prep

  38. Fluid Composition Important(esp. at high W:R) Mg content of seawater buffers pH through formation of Brucite. Lower pH = Higher CO2 Flux. Most evident at high W:R. Same Rocks and Temperature, but with Seawater Som et al., in prep

  39. Rock Composition(Fe and Ca affect H2 and pH) Fresh Water, 100C Som et al., in prep

  40. Variability in Biological Potential W:R W:R Seawater, Low Ca Fresh Water, 18% Cpx Som et al., in prep

  41. Why does energy flux matter? Case Study: Europa + -

  42. Energy and Biomass Dynamics (plus a few numbers from Europa) McCollom ‘99 vent fluid 10-3x Earth flux into oxygenated ocean Maint. only (no growth) 10-5x Earth flux into oxygenated ocean Hand et al., 2010 upper limit O2 flux into reduced ocean* Global Biomass (cells) Biomass Turnover @ 1/yr Anaerobic metabolism Hand et al., 2010 lower limit O2 flux into reduced ocean* 50x 10x your gut flora Global Flux of Limiting Substrate (mol/yr) your diet

  43. Biomass Density: Pacific Ocean Sediments Log (cells·mL-1) 2 4 6 8 10 12 ●Eq. Pacific ●N.Pac.Gyre ●S.Pac.Gyre ●ODP ●Non-ODP Depth (mbsf) (Kallmeyer et al., AGU 2009)

  44. Defining Habitability Potential (probability?) for life to arise & evolve Diversity of life supported Nobody understands the origin of life. If they say they do, they are probably trying to fool you. -- Ken Nealson Resilience to environmental change Extant life supported?

  45. Evolution as an Element of Habitability? Biological innovation can, itself, influence habitability and detectability Rate of exploration of genetic space is a function of: • Standing genetic stock (= f(energy)) • Replication Rate (= f(energy)) Some Fun with Numbers: Your Gut Flora 1014 cells Mutation rate 10-2.5 to -4 Turnover approx 1/day >1010 mutants/day Deep Sediments 108–fold fewer cells/vol >105-fold slower turnover >1013–fold slower evolution?

  46. Questions?

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