Chapter 4.3.4 : Life in High-Temperature Environments

Generalized schematic of terrestrial geothermal systems such as those in Yellowstone National Park. Local or regional aquifers sourced by meteoric water follow fault lines or permeable layers and become heated by proximity to shallow magma bodies or heated rocks. Heated water may boil in the subsurface due to adiabatic decompression as fluids rise. Dissolved gases such as H2, H2S, and NH3 become enriched in vapor phases, which source either fumaroles (a) or acidic pools or mudpots (b). These so-called vapor-dominated systems become acidified to pH 1.5–3.5 by biotic and abiotic oxidation of H2S to sulfuric acid at or near the surface; acid weathers the host rock and solubilizes clays. The liquid phase, typically with lower concentrations of volatiles, but enriched in soluble ions such as Na+ and Cl−, is buffered to pH 7–9 by the carbonate buffering system and may source large, clear pools or streams with substantial discharge (c). doi:10.1128/9781555818821.ch4.3.4.f1

Terrestrial geothermal systems at a variety of scales. (a–c) Abundant microbial growth in Octopus Spring, a high-temperature, alkaline spring in Yellowstone National Park's Lower Geyser Basin. (a) View of the source pool (∼92°C; pH ∼ 8.0) from above the “head” of the Octopus, with outflow streams (“tentacles”) flowing away and to the right (arrow, site of pink streamer community). (b) Pink streamer community in the outflow of Octopus Spring at ∼84 to 88°C, with abundant growth of Thermocrinis ruber and yet-uncultivated lineages ( 33 ) (width of view, ∼0.3 m). (c) Scanning electron micrograph (SEM) of a pure culture of Thermocrinis ruber isolated from Octopus Spring showing flow-dependent filamentous morphology (bar, 2 µm; used with permission from [ 34 ]). (d–f) Sharp transition of phototrophic biofilm growth in Mud Hot Spring (Sandy's Spring West) in the U.S. Great Basin. (d) View of the source pool (∼86°C; pH ∼ 7.2) with the upper temperature limit of photosynthesis clearly visible (arrow, ∼73°C). (e) Close up of the outflow channel showing the upper temperature limit of photosynthesis (arrow; width of view, ∼0.5 m). (f) SEM of a pure culture of Thermoflexus hugenholtzii, an abundant resident of sediments in Mud Hot Spring and nearby Great Boiling Spring ( 35 , 36 ) (bar, arrow, ∼2 µm; arrows, septa between individual cells in filaments). (g–i) Diretiyanqu (“Experimental Site”), an acidic system in Tengchong, China, and abundant microorganisms. (g) Large, geothermally altered erosional feature typical of many acidic geothermal systems (arrow, area in focus in H). (h) One of several hot, acidic pools (∼86°C; pH ∼ 2.6) actively gassing and with thermoacidophilic algae (Cyanidiales) visible in vapor condensate (arrow; width of view, ∼0.5 m). (i) Transmission electron micrograph of Sulfolobus tengchongensis, the dominant microorganism at this site ( 2 ) with Sulfolobus tengchongensis spindle-shaped virus (STSV1) (arrow; bar, 1 µm; used with permission from [ 37 ]). doi:10.1128/9781555818821.ch4.3.4.f2

Example core tetraether lipid structures. (a) Example isoprenoid dialkyl glycerol tetraethers (iGDGTs) found in thermophilic Crenarchaeota and Euryarchaeota; increasing cyclization is associated with increased temperature and/or decreased pH (top to bottom). (b) Thaumarchaeol (formerly crenarchaeol), a distinctive lipid of both thermophilic and mesophilic Thaumarchaeota. (c) Example branched GDGTs (bGDGTs) produced by mesophilic Acidobacteria and likely produced by unknown moderate thermophiles; methylation is inversely proportional to temperature in terrestrial geothermal systems. Figure modified from ( 122 ). doi:10.1128/9781555818821.ch4.3.4.f3