1.2 – Limits of Life
Introduction - Limits of Life
This section was adapted from Shulze-Makuch, Shulze-Makuch, and Houtkooper, 2015. The physical, chemical, and physiological limits of life. Life 5(3): 1472-1486. doi:10.3390/life5031472.
1.2. Describe physical, chemical and evolutionary limitations to sustaining life. |
Life on Earth exists in nearly every niche on our planet. However, life forms are not equally successful in all conditions on Earth, and one challenge is to find under which conditions life can be sustained. Certainly, life on Earth is very adaptable, which has led to immense biomass and incredible biodiversity. Therefore, it is a challenge to find where the limits to this adaptability are.
| Question 1.2
Try to come up with the best answer to the following question: |
On Earth, life is based on carbon as a major building block, water as a solvent and chemical bonds and light as life-sustaining energy sources, which seems to be an ideal combination on a terrestrial planet like Earth, which has an average surface temperature of about 15 °C under a pressure of 1 bar. In principle, other building blocks than carbon are feasible, such as silicon, but these would require very different planetary conditions to be used in a comparable fashion to carbon on Earth. Water as a solvent has great benefits and also some challenges (reviewed by Schulze-Makuch and Irwin [Schulze-Makuch & Irwin., 2008]), but being the most abundant molecule on our planet that exists in liquid form, life had simply to adapt to some of its drawbacks. Light is plentiful on our planet, and organic compounds with covalent bonds are versatile at average Earth temperatures, thus providing a powerful combination that resulted on Earth in a biosphere with large biomass and incredible biodiversity, including complex life.
There are limits to the conditions under which life can exist on our planet. Most pronounced is the temperature envelope under which active life can exist. Organismic growth can usually occur at temperatures from at least −15 °C to about 113 °C. There are also reports in the literature that the temperature range may even be broader. For example, metabolic activity was inferred down to temperatures of −40 °C due to anomalous concentrations of gases [Campen et al., 2003], and the upper-temperature limit may be as high as about 122 °C. At that temperature, methanogenic archaea could be cultured under a pressure of 20 MPa [Takai et al., 2008], only limited by the solubility of lipids in water and protein stability [McKay, 2014]. In principle, if the biochemistry of organisms could be adapted to these extremes, perhaps even higher temperatures may be tolerated, but the practical limit due to energetic and biochemical constraints under which life can still metabolize and reproduce is surely much lower [Schulze-Makuch, 2015]. Hyperthermophilic microorganisms require specialized cell components, like proteins and membranes, to be stable and function at high temper