Oxygen penetrates surface waters and enters the soil to some extent. These areas have also evolved oxygen-dependent life forms, but also important are the accumulated organiccompounds that have dissolved in water and that continually enrich the soil. Such compounds transform a barren dust or sand to the rich topsoil that supports plant life and forms the basis for all food chains on land. Similarly, organic compounds are important to oceanic plankton and zooplankton. Plankton form the basis for life in the surface waters of the ocean. That life supplies not only organic compounds at the surface, but also contributes to the quiet rain of detritus that reaches the ocean floor and sustains life even at the greatest ocean depth. Such fife lives in permanent darkness, with minimal oxygen and almost always low temperatures. But the fact that it exists has changed the ocean from an originally lifeless dilute soup of
1. The methanogens have at least two enzymes, related to methane metabolism, and not known to occur in other organisms.
2. The methanogens contain no cytochromes, the proteins widely used by other organisms for electron transport.
3. Most prokaryotes have peptidoglycan in their cell walls; Wthanogens have none.
4. The transfer RNA (tRNA) of other organisms carries a distinctive squence TψCG in one part of each tRNA molecule. In its place, the methanogens have either ψψCG or UψCG. (ψ is a modified uridine.)
5. Changes occur in rRNA after it has been transcribed. (The change of uridine to "pseudo-uridine" or ψ, in 4 above, is an example.) Such changes in the methanogen 16S rRNA are very different from those found in other prokaryotes.
Altogether, then, significant differences are seen between the methanogens and typical prokaryotes. Whether this will justify a new kingdom of organisms, as hinted at by Carl Woese and his colleagues, remains to be seen. For the present it is convenient to refer to the methanogens as a very special group of prokaryotes. spontaneously formed compounds to a collection of complex ecosystems with interdependent inhabitants. This awareness of co-evolution, the concept that life evolves along with the changes it produces in its environment, is emerging as a key concept in our search for life on other planets. This search is called exobiology--life beyond the limits of this earth, or outer life--and one of its leading practitioners in the United States, Cyril Ponnamperuma, of the University of Maryland, uses it as a criterion for predicting the probabilities of extraterrestrial life. In his laboratory Ponnamperuma mimics extraterrestrial conditions on the moon or on Mars, for example, so that he can study the spontaneous formation of molecules needed for life. When the first satellite descriptions were received on earth from Mars, Ponnamperuma set up his Mars atmosphere on earth and correctly showed that life on Mars was highly unlikely, a result confirmed by subsequent tests performed by the spacecraft that landed on the surface of Mars.