Physiological Limits

Each species has a unique set of limitations on the conditions under which individuals, and therefore populations, can grow and reproduce. Shelford's Law of Tolerance states that there is a maximum and minimum value for each environmental factor, beyond which a given species cannot survive. This is usually discussed with respect to environmental characteristics known as "modulators," such as temperature, pH, or salinity. Modulators impact the physiology of organisms by altering the conformation of proteins and cell membranes and the thermodynamic and kinetic favorability of biochemical reactions (see Chap. 2). For each environmental modulator, species also have an optimal range, within which maximum population growth occurs. Tolerance to modulators can be interactive; for example, tolerance to temperature extremes may be broader at one pH than another. Normally the geographical range of a species coincides with areas where environmental conditions are within the optimal ranges for the species, with the most optimal conditions at the center of the geographical range. The effects of species being in habitats with modulators outside their tolerance levels are listed in Table 8.1, along with biochemical strategies used by microorganisms that exist under these "extreme" conditions.

Resources are physical components of the environment that are captured by organisms for their use, such as N, energy, territories, or nesting sites. Shelford's law can be applied to most resources, but the responses to different resources are highly interactive. This is partially captured in Liebig's Law of the Minimum, which states that the resource in lowest supply relative to organismal needs will limit growth. At very low levels of a resource, the organism is unable to accumulate the resource in adequate quantities for metabolism. At very high levels, resources can also be toxic or inhibit growth.

The response of a species to environmental conditions or resources depends on the genetic makeup of the species. The limits and optima are determined through natural selection and other mechanisms that affect the genome. All organisms have some degree of "phenotypic plasticity," or the ability to adapt to the environment. In microorganisms, a change in an environmental condition can induce expression of alternative phenotypes (e.g., proteins, phospholipids) that are adapted to the new conditions, broadening the range of conditions acceptable for the species. The cost associated with this ability is extra genetic material that must be duplicated with each cell division, resulting in lower efficiency of resource use. This strategy can be efficient in fluctuating environments such as the soil surface. At the other extreme, endosymbionts that live continuously within the host and depend on the homeostasis of their host can have reduced genomes (Silva et al., 2001; Gil et al., 2003).

TABLE 8.1 The Effects of Physical Stresses (Modulators) on Microorganisms and the Biochemical Adaptations They Induce (Modified from Paul and Clark, 1996)

Organisms that have

Modulator Effects on cell Biochemical adaptation required adaptation

Temperature

Water deficit or salt stress pH

Denaturation of enzyme; change in membrane fluidity

Dehydration and inhibition of enzyme activity

Protein denaturation; enzyme inhibition

Aeration stress

Oxygen radicals damage membrane lipids, proteins, and DNA

Production of proteases and ATP-dependent chaperones (Derré et al., 1999); production of cold-tolerant enzymes by amino acid substitution (Lonn et al., 2002); increases in intracellular trehalose and polyol concentrations and unsaturated membrane lipids, secretion of antifreeze proteins and enzymes active at low temperatures (Robinson, 2001)

Changes in composition of polysaccharides produced (Coutinho et al., 1999); maintaining salt in cytoplasm and uptake or synthesis of compatible solutes (RoeBler and Muller, 2001)

Increased intrasubunit stability in proteins afforded by increased hydrogen bonds and stronger salt bridges (Settembre et al., 2004); organisms that can secrete a surplus of protons or block extracellular protons from the cytoplasm by blocking membrane composition (de Jonge et al., 2003); stress regulator genes (de Vries et al., 2001)

Detoxification of oxygen radicals by catalase and superoxide dismutase (Wu and Conrad, 2001)

Thermophiles, psychrophiles

Osmophiles, xerophiles, halophiles

Acidophiles

Obligate anaerobes, methanogens, sulfur and N users

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