Prokaryotes and Their Habitats
Prokaryotes are well recognized as essential members of the biosphere. They inhabit all possible locations in which life exists from those offering ideal conditions for growth and reproduction to those representing extreme environments at the borderline o
- PDF / 1,203,733 Bytes
- 48 Pages / 539 x 751 pts Page_size
- 64 Downloads / 260 Views
CHAPTER 1.6 se toyrakorP
dna
r i ehT
Prokaryotes and Their Habitats HANS G. SCHLEGEL AND HOLGER W. JANNASCH
Prokaryotes are well recognized as essential members of the biosphere. They inhabit all possible locations in which life exists from those offering ideal conditions for growth and reproduction to those representing extreme environments at the borderline of abiotic conditions. The ubiquity of microorganisms is based on three major properties: their small size for easy dispersal by air and water, their metabolic versatility and flexibility, and their ability to tolerate unfavorable conditions. A predominant population is commonly composed of species able to grow under the particular conditions of a habitat. Many other species may also be present but in low numbers of individuals. As a rule, ecosystems of indistinct physicochemical and nutritional characteristics, such as many soils or sea water, which neither suppress nor specifically support microbial growth, usually carry low numbers of microorganisms but a high diversity of species. In contrast, ecosystems of strong environmental characteristics, such as acid mine waters, salt brines, and hot springs, commonly contain high cell numbers of very few species. Experimental enrichment procedures bring about the predominance of certain species by controlling the supply of specific nutrients or the use of certain physicochemical conditions. If the growth conditions of a particular microorganism are known and reproducible, enrichment and isolation usually pose no problem. But if the particular requirements for growth of an organism are unknown, isolation procedures may be difficult to discover (Pfennig, 1961; Schlegel and Pfennig, 1961). For that reason, a number of organisms long known from microscopical observations, such a Thiovulum or Achromatium, have not yet been isolated in pure culture. Furthermore, organisms that have hitherto unknown growth characteristics and that are too small and inconspicuous for easy microscopical recognition have often escaped detection. An excellent example is Desulfuromonas acetoxidans (Pfennig and Biebl, 1976). In characterizing an ecosystem microbiologically, it is important to distinguish between 1) organisms introduced incidentally by air, soil runoff, etc., physiologically just making the best of it, and 2) organisms typically adapted to the particular habitat and not occurring in any other except in the form of survival stages. An example of the former is Escherichia coli, as frequently found in polluted waters. An example of the latter is the above-mentioned Thiovulum sp., whose need for dissolved oxygen and hydrogen sulfide at the same time requires a high motility combined with chemotactic orientation in an aquatic oxic/anoxic interface.
This chapter was taken unchanged from the second edition.
Although their morphological differentiation is limited, prokaryotes have evolved a number of structural and chemical mechanisms that enable them to inhabit various extreme environments. The presence of a specified pigment, for
Data Loading...
