Energetics of sulfate-reduction in terrestrial hydrothermal systems
Introduction
When the Earth was formed about 4.5 billion years ago, the atmosphere was devoid of molecular oxygen. The first prokaryotic life forms appeared approximately 3.5 billion years ago, according to evidence of microfossils in sedimentary deposits and the presence of stromatolites. These early microorganisms were most likely obligate anaerobes that were able to derive their energy from the reduction of sulfate (Levett, 1990). Similarly, phylogenetic evidence based on comparisons of 16S ribosomal RNA sequences supports the hypothesis that these ancestral bacteria may have also been thermophilic anaerobes (Wiegel & Adams, 1998). If this is accurate, then elucidating the metabolic properties of present-day anaerobic thermophiles can provide insight into the origins of life.
 

 

A Desire for Energy
Like all living organisms, microorganisms require a continuous supply of energy for growth and maintenance. This need can be met through the utilization of chemical energy. This energy is secured through dissimilatory sulfate reduction, which is coupled to the oxidation of dissolved organic compounds through the following reactions:
 
2 CH3CHOHCOO- (lactate)  +  3SO42-  +  2H+  ↔  6HCO3-  +  3H2S(aq)  (1)
 
2 CH3CHOHCOOH (lactic acid)  +  3SO42-  +  6H+  ↔  6CO2(g)  +  3H2S(aq)  +  6H2O (2)
A substantial amount of chemical energy from sulfate reduction lies within the hot springs at Yellowstone National Park. Geochemical analyses were combined with thermodynamic data in order to compare the Gibbs energy available to sulfate-reducing bacteria in two extremely different hot springs.
Obsidian Pool
"Little Hottie"

 

Equation used to calculate values of ΔrG:
 
ΔrG = ΔrGº  + RT ln Q
 
R = 1.987 cal/mol-K
T = ºC + 273.15
Q = Π aiνi,r

 
   

   Using the data provided in the table, we can explore the effects of changes in lactate and lactic acid activities on the total energy. As Figure 1 demonstrates, more energy is available to the organism with an increased activity of substrate. In addition, there appears to be more energy from reaction 2 in the acidic spring “Little Hottie” than reaction 1 in the neutral spring Obsidian Pool. In these calculations , only the activities of the organic compounds were allowed to change, all other activities were held at the constant values shown in the table. The amount of energy would also change if the activities of other products and reactants were to change. In order to explore the effects of these changes on the growth rates of microorganisms, various media compositions can be designed using a thermodynamic analysis.

Figure 1: Thermodynamic calculations of Gibbs energy

 

Modified Baar’s medium for sulfate-reducers

 

MgSO4·7H2O              2.0g

Na3C6H5O7·2H2O     5.0g

CaSO4·2H2O               1.0g

NH4Cl                         1.0g

K2HPO4                      0.5g

C3H5NaO3                  3.5g

yeast                            1.0g

distilled H2O                 1L

Media manipulation
The goal of my research is to determine the microbial response to the Gibbs energy within a geochemical system. In laboratory experiments, this system is the growth medium. Sensitivity analysis was conducted in order to aid in the formulation of media with differing Gibbs energy. Through this analysis, it has been determined that the Gibbs energy is more sensitive to the products than it is to the reactants. Note that the lines on the graphs shown below for the products are steeper than those for the reactants. This is contrary to the popular view that an increase in substrates will provide more energy.
2 CH3CHOHCOO- (lactate)  +  3SO42- +  2H+  ↔  6HCO3-  +  3H2S(aq)

 

Future work: Laboratory experiments involving differing Gibbs energy to microorganisms.

     Formulate media of different Gibbs energy.

Develop protocols for analysis of products and reactants.

Conduct experiments involving Desulfovibrio vulgaris (Hildenborough).

Conduct experiments involving a thermophilic sulfate-reducer (to be determined).

 

References
Amend, J. P., and E.L. Shock, Energetics of overall metabolic reactions of thermophilic and hyperthermophylic Archaea and Bacteria, FEMS Microbiol. Rev., 25, 175-243, 2001. 
Dawes, Edwin A., Microbial Energetics, Blackie & Son, Glasgow and London, 1986.
Johnson, J.W., Oelkers, E.H. and H.C. Helgeson, SUPCRT92: A software package for calculating the standard molal properties of minerals, gases, aqueous species, and reactions from 1 to 5000 bar and 0 to 1000C. Comput. Geosci., 18, 899-947, 1992.
Levett, P.N., Anaerobic bacteria, Open University Press, Philadelphia, 1990.
Wiegel, J and M. Adams,Thermophiles: The keys to molecular evolution and the origin of life?, Taylor and Francis, Philadelphia, 1998.