1.2: Limits of Life
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- 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 an immense biomass and an incredible biodiversity. Therefore, it is a challenge to find where the limits to this adaptability are.
Try to come up with the best answer for the following question:
Which of the following may be the most limiting physical factor for animals living in aquatic environments? Why?
- being wet all the time
- access to nutrients
- access to oxygen
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 a large biomass and an 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 temperatures. Particularly, at temperatures of 100 °C and beyond, some low molecular weight compounds, such as ATP and NAD, hydrolyze quite rapidly, and thermolabile amino acids, like cysteine and glutamic acid, are decomposing [Stetter, 1999]. The pressure tolerance of life, though, is high and extends to at least 1100 bar  [Stan-Lotter, 2007].
Organisms, particularly microbes and other microscopic organisms, are quite tolerant to extreme pH values, from just below 0 to about 13. Ferroplasma sp. and Cephalosporium are examples of organisms that live at low pH-values, Natronobacterium, and several species of protists and rotifers are examples of organisms that live at very high pH-values [Schulze-Makuch & Irwin, 2008, Baross et al., 2007].
However, life on Earth is relatively sensitive to a lack of water: bacteria, archaea, and fungi can only metabolize at water activities down to about 0.6 [Stevenson, et al., 2014]. Adaptation to water with high salt content, however, is quite common, as some Halobacteria and archaea can grow in 35% NaCl solution [Schulze-Makuch & Irwin, 2008].
Another physical limit to life is radiation, both UV, and ionizing radiation. Tolerances to radiation vary widely. Tardigrades, microscopic animals that usually live in mosses and lichen, can withstand ionizing radiation doses up to 5000 Gy when in the dormant state [Schulze-Makuchh & Seckbach, 2013] and display additional special adaptation traits, such as anhydrobiosis and cryptobiosis [Watanabe, 2006] (elaborated on below). Deinococcus radiodurans can still tolerate higher radiation doses and grow at doses upward of 10,000 Gy.
One non-chemical limitation of life on earth is related to scaling of body size.
Some examples of physical and chemical limitations of life were listed above. One aspect that was not discussed is size. How do you think size is a physical limitation for sustaining life for animals? Find one specific example with one of your peers. Once you have found your example, you should think about why the size is the limit in your example.
Add exercises text here. For the automatic numbDescribe five different limitations to life on Earth.
- Stan-Lotter, H. Extremophiles, the physicochemical limits of life (growth and survival). In Complete Course in Astrobiology; Horneck, G., Rettberg, P., Eds.; Wiley-VCH: Weinheim, Germany, 2007; pp. 121–150. ↵