Some like it hot: understanding the limits of life using hyperthermophilic microbes
Abstract
Hyperthermophiles are those microorganisms that grow optimally above 80° C and have the highest known growth temperatures of any life (up to 121° C in pure culture). The study of hyperthermophiles has helped to constrain our understanding of the limits in two ways. First and most obvious, it provides upper temperature boundaries for where life may exist. In general, upper temperature limits vary with life forms: eukaryotes (55° C), photosynthetic bacteria (72° C), non-photosynthetic bacteria (100° C), methanogens (110° C), sulfur reduction (113° C), and iron reduction (121° C). The incubation of natural microbial assemblages suggests that life may exist at temperatures of at least 125° C, and studies of extracellular hydrolases show that the impact of life processes extend up to 130-140° C. Several factors have been implicated that permit hyperthermophilic life. Proteins are stabilized at high temperatures through increased rigidity brought on by several separate factors (e.g., increased hydrogen bonding, oligomerization, shortened loop structures). DNA is stabilized by a hyperthermophile-specific topoisomerase that introduces positive supercoils into the double helix and by DNA binding proteins that help to prevent strand melting. Hyperthermophiles also possess a remarkably efficient DNA repair mechanism. Lipids are stabilized by using ether linkages of isoprenoid alcohol hydrocarbons to the glycerol head and a high degree of covalent cross-linking between opposing lipids in the membrane bilayer. However, in order for life to exist above 140° C, organisms would require mechanisms to prevent denaturation within individual amino acids, nucleic acids, and other low molecular weight cofactors within the cytoplasm. The second way that the study of hyperthermophiles has helped to constrain the limits of life is by understanding how life exists in geothermal environments and deep within the Earth's crust without oxygen or sunlight. Hyperthermophiles can serve as the primary producers in these environments using volcanically-derived H2 and CO2 as energy and carbon sources and CO2 assimilation pathways other than the Calvin cycle. Insoluble iron, CO2 , and various sulfur compounds can serve as terminal electron acceptors. Unlike commonly-studied mesophilic bacteria, iron respiration in some hyperthermophiles does not require polyheme c-type cytochromes in the membrane, and direct contact with iron is not necessary for electron transfer. Environmental factors such as pH and reduction potential have a significant impact on the kinds of metabolisms that occur, and thermodynamic modeling suggests that minimum H2 activity increases with temperature in order to meet the minimum energy requirement for growth. These factors lead to a non-stochastic distribution of hyperthermophile metabolic types in any given environment. Taken together, these parameters will greatly aid in modeling growth in a variety of environments that will serve the exploration for life beyond our planet.
- Publication:
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37th COSPAR Scientific Assembly
- Pub Date:
- 2008
- Bibcode:
- 2008cosp...37.1259H