Environmental consequences of geochemical change in hot spring ecosystems

Hydrothermal systems provide a natural laboratory for studying the effects of geochemical change over time, and for testing predictions of how geochemical change will affect microbial ecology. Hot springs in hydrothermal areas that express the results of subsurface boiling, phase separation, and differential movement of liquid phase and vapor phase constituents can fluctuate in temperature and composition. Since 1999 we have sampled several fluctuating hot springs at Yellowstone National Park, and those hat experience large geochemical changes provide opportunities to quantify the effects of fluctuations on chemical energy supplies. Annual samples from Obsidian Pool (Mud Volcano Area) showed that pH increased from 6.5 (in 1999) to 6.8 (’00), steadily decreased to 4.2 (’06), and then increased to 5.2 (’09), with temperature ranging from 76.4 to 85.3°C. Simultaneously the chloride concentration increased by 65% (from 18.5 ppm in 1999 to 30.7 ppm in 2009), indicative of increased hydrothermal input, and the sulfate concentration increased by over 300% (from 50.0 ppm in 2000 to 203.8 ppm in 2009), suggesting an increased gas-phase sulfide input and subsequent oxidation. Several energy yielding reactions at a pH of 6.5 no longer yield energy at pH of 4.2. This suggests that microorganisms that use those pathways had a negative selection pressure with the drop in pH. As an example, the chemical affinity for sulfur reduction to pyrite coupled to iron oxidation to goethite changed from 7.1 (pH = 6.5) to -1.3 kcal/mol e- (pH = 4.2), and once again had a positive value at pH = 5.2. This means that microorganisms using this pathway may once again inhabit the hot spring while many others from when the pH was 6.5 still have a negative selection pressure. The pH of another hot spring in the Sylvan Springs Area steadily increased from 3.7 (’04) to 7.6 (’08) while the temp. decreased from 52.9 to 41.9°C, chloride concentration increased by 32% (from 464 to 614 ppm), and the sulfate concentration decreased by 36 % (from 255 to 166 ppm). The changes suggest an increased liquid-phase hydrothermal input (increasing Cl) coupled with a decreased gas-phase input (sulfide, oxidized to sulfate). Many reactions that do not yield energy at pH = 3.7 become energy yielding at pH = 7.6, including methanogenesis from CO or CO2 coupled with H2S oxidation to pyrite. These examples from the geochemistry of fluctuating hot spring systems illustrate how predictions can be made about dynamic changes in microbial ecosystems that can be tested by molecular methods.