Environmental extremes Temperature

Why should temperature make a difference to organisms? Organisms rely on biomolecules, all of which will break down at some temperature. For example, chlorophyll degrades near 75 °C (Rothschild and Mancinelli, 2001). The melting temperature of nucleic acids depends on the base composition (A-T bonds are less thermostable than are G-C bonds), the length of the polymer, and the solvent including the concentrations of ions in solution, but even DNA normally will degrade well below 100 °C.

Table 6.2. Examples of natural habitats with environmental extremes

Environmental

Type

Definition

Example

Example organisms

parameter

ecosystems

temperature

hyper-

growth > 80 °C

geyser

Pyrolobus fumarii,

thermophile

113 °C

thermophile

growth 60-80 °C

hotspring

mesophile

15-60 °C

Homo sapiens

psychrophile

<15 °C

ice, snow

Psychrobacter, some

insects

pH

alkaliphile

pH > 9

soda lakes

Natronobacterium,

Bacillus firmus OF4,

Spirulina spp.

(all pH 10.5)

acidophile

low pH loving

acid mine

Cyanidium caldarium,

drainoff,

Ferroplasma sp. (both

hotsprings

pH 0)

salinity

halophile

salt loving

salt lakes,

Halobacteriacea,

(2-5 M NaCl)

salt mines

Dunaliella salina

desiccation

xerophiles

anhydrobiotic

desert

Artemia salina;

nematodes, microbes,

fungi, lichens

pressure

barophile

weight loving,

deep ocean

unknown, for microbe,

piezophile

pressure loving

130 MPa

radiation

tolerates high

reactors,

Deinococcus

levels of

high solar

radiodurans

radiation

exposure,

e.g., at

altitude

oxygen

aerobe

tolerates oxygen

most of

Earth today

gravity

hypergravity

> 1 g

none known

hypogravity

< 1 g

none known

vacuum

tolerates vacuum

tardigrades, insects,

(space devoid of

microbes, seeds

matter)

chemical

gases, metals

can tolerate high

mine

Cyanidium caldarium

extremes

concentrations of

drainage

(pure CO2),

metal

Ferroplasma

(metalotolerant)

acidarmanus (Cu, As,

Cd, Zn); Ralstonia sp.

CH34 (Zn, Co, Cd,

Hg, Pb)

As temperature increases, proteins denature, which results in loss of quaternary, tertiary, and even secondary structures. This rules out their use as structural compounds.

Denatured enzymes lose their catalytic activity. But even when active, enzymes have a minimum, a maximum and an optimum temperature for function. By definition, at temperatures above or below the maximum, enzymatic function slows.

The solubility of gases in a liquid, including water, varies depending on temperature. As anyone who has opened a cold bottle of soda knows, the solubility of gases in liquids decreases with increasing temperature. As aquatic organisms often utilize soluble gases, such as CO2 or O2, high temperature decreases the availability of gases.

At low temperatures, water freezes. During freezing, solutes concentrate creating high, perhaps toxic, concentrations of solutes. Once the water is frozen, the ice crystals may act like tiny swords, piercing the cell membranes, and thus destroying cellular integrity.

These effects of temperature on biological compounds strongly suggest that life should have a fairly narrow temperature range, from above the freezing point of water to well below its boiling point. Thus, it was surprising to Tom Brock when, in July 1964, he detected living organisms in the run-off channels of hot springs in Yellowstone National Park. Later in the summer he noticed pink, gelatinous mats in Pool A of Octopus Spring, about six miles south of Old Faithful (Figure 6.1). At the time 'high temperature' was considered to be 60 °C. In 1966, Brock's graduate student, Hudson Freeze, cultured YT-1 (later known as Thermus aquaticus) from Mushroom Spring. T. aquaticus is the source for Taq polymerase which, when used as part of the polymerase chain reaction, revolutionized biology in the late twentieth century. Inspired by reports of this work (Brock, 1967), as well as the work of Woese (e.g., Woese and Fox (1977)) revealing the presence of the Archaea, Karl Stetter became the leading figure in thermophile research (see Chapter 7).

In 1977, the exploratory submarine Alvin travelled through the Panama Canal for the first time. Geological work in the Galapagos Rift was completed during February and March. The major discovery of an abundance of exotic animal life on and in the immediate proximity of warm water vents prompted theories about the generation of life. Because no light can penetrate through the deep waters, scientists concluded that the animal chemistry there is based on chemosynthesis, not photosynthesis. Since then, Alvin has located more than 24 hydrothermal sites in the Atlantic and Pacific Oceans. It also has allowed researchers to find and record about 300 new species of animals, including bacteria, foot-long clams and mussels, tiny shrimp, arthropods, and red-tipped tube worms that can grow up to 3 m long in some vents.

Fig. 6.1. Sources of thermophiles from Yellowstone National Park. Outflow channel of Octopus Spring, approximately 6 miles south of Old Faithful. Inset is close-up of the gelatinous (pink) mats, which contain the hyperthermophilic bacterium, Thermocrinis ruber (Reysenbach et al., 1994; Huber et al., 1998). The mats are growing at approximately 81 °C. Note that Octopus is 7352 ft above sea-level, and therefore the boiling point of water is depressed. In the front of the photo is a section of the source, which is boiling, yet it too contains living hyperthermophiles. Photo captured 3 August 2006.

Fig. 6.1. Sources of thermophiles from Yellowstone National Park. Outflow channel of Octopus Spring, approximately 6 miles south of Old Faithful. Inset is close-up of the gelatinous (pink) mats, which contain the hyperthermophilic bacterium, Thermocrinis ruber (Reysenbach et al., 1994; Huber et al., 1998). The mats are growing at approximately 81 °C. Note that Octopus is 7352 ft above sea-level, and therefore the boiling point of water is depressed. In the front of the photo is a section of the source, which is boiling, yet it too contains living hyperthermophiles. Photo captured 3 August 2006.

The ability to withstand fire is not normally considered an extreme-environment adaption, but it is worth mention here. Fire-triggered germination is widespread in the plant families Fabaceae, Rhamnaceae, Convolvulaceae, Malvaceae, Cistaceae, and Sterculiaceae. The trigger is either heat shock or the chemical products of combustion, rather than fire itself (Keeley and Fotheringham, 1998).

The ability of organisms to thrive in cold temperature environments has a much longer history. The great nineteenth-century microbiologist, Ferdinand Cohn, noticed algae that grow on snow, a phenomenon often referred to as 'watermelon snow' (Figure 6.2). Cold temperature organisms, or psychrophiles, were studied from the marine environment by Certes in the 1980s and Conn in the early twentieth century. ZoBell and his students - particularly Dick Morita - studied these organisms and were the first to get deep-sea samples from the 1957 Galathea expeditions to ocean trenches. They were the first to study these psychrophiles and barophiles. By the 1980s, multiple field campaigns were being staged to the Dry Valleys of the Antarctic to study the microbes in the permanently ice-covered lakes (Figure 6.3). These lakes were seen as a model system for

Fig. 6.2. Watermelon snow, Lassen National Park.

Fig. 6.3. Nymph Creek, an acidic creek several miles north of Norris Annex. At the time the photo was taken, the water was 41 °C. The water appears green due to Cyanidium caldarium, and an acidophilic Chlorella (Belly et al., 1973). Photo captured 3 August 2006. Color photographs are available on the book website http://www.cambridge.org/9780521875486.

Fig. 6.3. Nymph Creek, an acidic creek several miles north of Norris Annex. At the time the photo was taken, the water was 41 °C. The water appears green due to Cyanidium caldarium, and an acidophilic Chlorella (Belly et al., 1973). Photo captured 3 August 2006. Color photographs are available on the book website http://www.cambridge.org/9780521875486.

potential organisms living in rocks (e.g., Friedmann (1982), McKay and Friedmann (1988)), under ice-covered regions of ancient Mars, or even present-day Europa and now Enceladus (reviewed in Priscu et al. (1998), McKay et al. (2005); see Chapter 12).

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