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Research ArticleArticles

The thermodynamic ladder in geomicrobiology

Craig M. Bethke, Robert A. Sanford, Matthew F. Kirk, Qusheng Jin and Theodore M. Flynn
American Journal of Science March 2011, 311 (3) 183-210; DOI: https://doi.org/10.2475/03.2011.01
Craig M. Bethke
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  • For correspondence: bethke@illinois.edu
Robert A. Sanford
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Matthew F. Kirk
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Qusheng Jin
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Theodore M. Flynn
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Abstract

A tenet of geomicrobiology is that anaerobic life in the subsurface arranges itself into zones, according to a thermodynamic ladder. Iron reducers, given access to ferric minerals, use their energetic advantage to preclude sulfate reduction. Sulfate reducers exclude methanogens in the same way, by this tenet, wherever the environment provides sulfate. Examining usable energy—the energy in excess of a cell's internal stores—in subsurface environments, we find that in groundwater of near neutral pH the three functional groups see roughly equivalent amounts. Iron reducers hold a clear energetic advantage under acidic conditions, but may be unable to grow in alkaline environments. The calculations fail to identify a fixed thermodynamic hierarchy among the groups. In long-term bioreactor experiments, usable energy did not govern microbial activity. Iron reducers and sulfate reducers, instead of competing for energy, entered into a tightly balanced mutualistic relationship. Results of the study show thermodynamics does not invariably favor iron reducers relative to sulfate reducers, which in turn do not necessarily have an energetic advantage over methanogens. The distribution of microbial life in the subsurface is controlled by ecologic and physiologic factors, and cannot be understood in terms of thermodynamics alone.

  • Geomicrobiology
  • thermodynamics
  • iron reduction
  • sulfate reduction
  • methanogenesis
  • microbial redox processes
  • mutualism
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American Journal of Science
Vol. 311, Issue 3
1 Mar 2011
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The thermodynamic ladder in geomicrobiology
Craig M. Bethke, Robert A. Sanford, Matthew F. Kirk, Qusheng Jin, Theodore M. Flynn
American Journal of Science Mar 2011, 311 (3) 183-210; DOI: 10.2475/03.2011.01

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The thermodynamic ladder in geomicrobiology
Craig M. Bethke, Robert A. Sanford, Matthew F. Kirk, Qusheng Jin, Theodore M. Flynn
American Journal of Science Mar 2011, 311 (3) 183-210; DOI: 10.2475/03.2011.01
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  • Article
    • Abstract
    • Introduction
    • MICROBIAL CATALYSIS
    • AVAILABLE ENERGY
    • USABLE ENERGY
    • BIOREACTOR EXPERIMENTS
    • DISCUSSION
    • CONCLUDING REMARKS
    • ACKNOWLEDGMENTS
    • APPENDIX
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