Box 1 Classification Nomenclature

A nomenclature based on the classification of microorganisms by nutritional requirements is designed to reflect three properties: the principal process for generating metabolic energy, the source of electrons for energy-converting reactions, and the form of environmental carbon assimilated for growth. The processes for generating metabolic energy can be divided into two classes: chemical oxidation and light absorption, designated by the prefixes chemo- and photo-, respectively. Likewise, the sources of electrons for biological energy-converting reactions can be divided into two broad classes: inorganic and organic, designated by the prefixes litho- and organo-, respectively. Finally, the sources of carbon assimilated for growth can be classified as either inorganic or organic, designated by the prefixes auto- and hetero-, respectively. A photoorganoheterotroph, for example, uses light energy to excite electrons extracted from organic compounds to support the assimilation and transformation of carbon from organic compounds. If humans were microbes, they would be classified as chemoorganoheterotrophs.

interaction is usually irrelevant to the existence of the producing organism but may be damaging or even fatal to the infected organism.

Infectious agents, which may be actively or opportunistically invasive, must multiply in or on the host in order to cause damage. The capacity of a microbe to infect a host requires an intimate interaction between the pathogen and the host and often depends on highly specific interactions between cell surfaces of the host and pathogen. Above all, infectious agents must overcome the defenses that have evolved in most potential hosts as a consequence of persistent, unremitting challenges by potential pathogens on Earth. Living organisms defend themselves mechanically and chemically in diverse ways against agents that are themselves constantly changing.

The chances that invasive properties would have evolved in putative martian microbes in the absence of evolutionary selection pressure for such properties is vanishingly small. Subcellular disease agents, such as viruses and prions, are biologically part of their host organisms, and an extraterrestrial source of such agents is extremely unlikely.

THE POTENTIAL FOR ECOLOGICAL EFFECTS

Although hypothetical extraterrestrial biota could have properties different from those of Earth biota, it must be assumed that the chemical reactions governing their metabolism would be largely the same. Putative martian microorganisms would likely be functionally similar to some of Earth's soil bacteria, but because the range of habitats available on Mars is much narrower than that on



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