Thermodynamic investigations of microbial metabolism and abiotic

117 Pages·2012·5.11 MB·English

Thermodynamic investigations of microbial
metabolism and abiotic organic synthesis in seafloor
hydrothermal systems
Michael Hentscher
Dissertation
zur Erlangung des Doktorgrades Dr. rer. nat.
Bremen, 2012
Fachbereich Geowissenschaften
Universität Bremen

Thermodynamics
- fit for fun ...
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Referent:
Prof. Dr. Wolfgang Bach
Korreferent:
Prof. Dr. Kai Uwe Hinrichs
Tag des öffentlichen Promotionskolloquiums:
27.06.2012 18:00 Uhr
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Table of Contents
Abstract VII
Zusammenfassung IX
1. Introduction 1
1.1 Hydrothermal systems in the deep sea . . . . . . . . . . . . . . . . .1
1.2 Habitats created by hydrothermal circulation of seawater . . . . . . .2
1.3 Bioenergetics of hydrothermal vents . . . . . . . . . . . . . . . . . .3
1.4 Abiotic organosynthesis . . . . . . . . . . . . . . . . . . . . . . . .4
1.5 Motivation and outline of this thesis . . . . . . . . . . . . . . . . . .5
1.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2. Geochemically induced shifts in catabolic energy yields explain
past ecological changes of diffuse vents in the East Pacific Rise
9°50’N area 13
2.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3.1 Calculation of affinity . . . . . . . . . . . . . . . . . . . . . 15
2.4 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . 22
2.4.1 Case studies . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.6 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3. Geochemical modeling of CO behavior in water-peridotite inter-
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actions: organic synthesis versus carbonate precipitation 33
3.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.3 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.4.1 Predicted mineral assemblages . . . . . . . . . . . . . . . . 37
3.4.2 Predictions of organic carbon stability . . . . . . . . . . . . 41
3.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.5.1 Organic synthesis . . . . . . . . . . . . . . . . . . . . . . . 42
3.5.2 Methanogenesis versus carbonate precipitation . . . . . . . . 44
3.5.3 Implications for Lost City . . . . . . . . . . . . . . . . . . . 47
3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
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3.7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4. Thermodynamic assessment of catabolic and anabolic energetics in
submarine hydrothermal systems 55
4.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.2.1 Hydrothermal systems . . . . . . . . . . . . . . . . . . . . . 59
4.3 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.4 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . 63
4.4.1 Bioenergetics in hydrothermal vents . . . . . . . . . . . . . . 63
4.4.2 Potential for ridge flank microbial processes . . . . . . . . . 66
4.4.3 Vent fluid chemistry as a guide to microbial metabolism . . . 68
4.5 Summary and conclusion . . . . . . . . . . . . . . . . . . . . . . . 70
4.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5. Synthesis and Outlook 79
5.1 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.3 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
6. Appendix-A: Geochemically induced shifts in catabolic energy
yields explain past ecological changes of diffuse vents in the East
Pacific Rise 9°50’N area 87
6.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
7. Appendix-B: Catabolic and anabolic energy for chemolithoauto-
trophs in deep-sea hydrothermal systems hosted in different rock
types 89
7.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
8. Appendix-C: Colonization of subsurface microbial observatories
deployed in young ocean crust 91
8.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
9. Appendix-D: Driving forces behind the biotope structures in two
low-temperature hydrothermal venting sites on the southern Mid-
Atlantic Ridge 93
9.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
10. Appendix-E: Short-term microbial and physico-chemical variabil-
ity in low-temperature hydrothermal fluids near 5°S on the Mid-
Atlantic Ridge 95
10.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
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11. Appendix-F: Geochemistry of vent fluid particles formed during
initial hydrothermal fluid–seawater mixing along the Mid-Atlantic
Ridge 97
11.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Danksagung 99
Lebenslauf 100
Publikationsliste 101
Erklärung 103
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Abstract
Hydrothermal circulation of seawater within the oceanic crust creates conditions suit-
able for chemosynthesis-based microbial life. Synthesis of abiotic organic compounds takes
place during seawater-basement rock interaction at elevated temperatures. Low temperature
circulation in the recharge zone allows chemolithoautotrophs to gain energy by oxidizing or
reduction of minerals, while deeper and hotter regions (reaction zone) are dominated by rock
alteration and produce the reduced conditions which are required in abiotic organic synthe-
sis. The discharge zone, where cold seawater mixes with upwelling hydrothermal fluid is
another habitat for diverse microbial life and locus of abiotic organic synthesis.
Thermodynamic calculations are helpful in assessing these processes in hydrothermal
systems. In this thesis thermodynamic reaction path models have been used to simulate
water-rock interactions in the oceanic lithosphere to evaluate the potential of abiotic organo-
synthesis in sub-seafloor environments. Batch mixing models were used to determine the
Gibbs energy available for metabolic reaction along given mixing paths for hydrothermal
fluid and seawater. The results highlight the important role variable water-to-rock ratios
(W/R), fluid mixing ratios and temperature play in influencing abiotic organosynthesis pro-
cesses. By merging model results with geochemical field data and microbiological obser-
vations it is possible to elucidate the bioenergetics of microbial metabolism in a range of
hydrothermal environments from diffuse vent fields to ridge flank seawater aquifers.
Furthermore, thermodynamic computations were used to test the hypothesis that
changing vent fluid compositions directly impact vent biology. In an example from the East
Pacific Rise (EPR) 9°50’N the energy yield of a wide range of catabolic reactions in diffuse
vents was calculated. From these calculations it is apparent that the energy for sulfide oxidiz-
ing dropped notably before the demise of the colonies of Riftia (a giant tubeworm living in
symbiosis with sulfur oxidizing bacteria). In a second vent field of the EPR 9°50’N area the
catabolic activity of microbial communities in the subsurface was assessed by geochemi-
cal modeling. Elements behaving conservatively during the fluid mixing process were used
to infer the mixing ratio of seawater and hydrothermal vent fluid. Concentrations of non-
conservative elements were then compared with the actually measured composition of dif-
fuse fluids. The difference between measured and calculated fluid composition is indicative
of the consumption or the release of a chemical species. The growth of hydrogenotrophic
communities effective lowers the hydrogen concentration in a hydrothermal fluid. Indeed,
the calculations show that the fluid composition measured at the vent site allows subseafloor
methanogenesis. The results are in line with these findings and indicate that a period of
active growth of hydrogenotrophic communities was followed by a period of steady state.
In a second study, the potential for abiotic organosynthesis in serpentinization systems
was examined by reaction path modeling of harzburgite – seawater interactions at a range of
temperatures. Serpentinization reactions release hydrogen, which drives the reduction of dis-
solved inorganic carbon to organic compounds (e.g., formic acid, methanol, and methane).
While methane is the thermodynamically stable species, there is evidence from hydrother-
mal experiments indicative of sluggish reduction from methanol to methane. In calculations
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