This is an electron micrograph taken using the UCMP Environmental Scanning Electron Microscope. It shows an extreme close-up of a bryozoan from Bahia de Animas, Baja California, Mexico. The calcareous shell of the bryozoan is perforated with a network of channels and tunnels, made by bacteria that bore into such shells. Such trace fossils have been found all through the last 600 million years of the fossil record, and add a new dimension to the study of the history of bacterial life. Courtesy UCMP
As for this question...it's impossible to really tell. We have no idea just what the limits are for living systems, so it is easy to speculate. As a hint to what the possibilities might be, here are the kinds of extreme life forms we already know about on Earth.
Thermophiles: The best studies high-temperature eukaryote is the acidophilic phototroph Cyanidium caldarium. Its optimal growth temperature was 45ºC and the maximum temperature at which growth occurred was 57ºC. The polychaete Alvinella pompejana, the Pompeii worm, lives in burrows on hydrothermal vent chimneys with a strong temperature gradient, which averages 68ºC but has frequent peaks exceeding 81ºC. It seems clear that we do not yet understand all the protective mechanisms operating which allow cells like the archaean Pyrococcus to grow above 100ºC, and indeed what the actual upper limit for life might be (Erauso et al., 1996; Stetter et al., 1990).
Psychrophiles: Adaptation to growth at low temperature has been most extensively studied in the Antarctic sea-ice, where a wide taxonomic range of flagellates have been observed. Here, in contrast to Heteromita, efforts to cultivate these strains, which normally grow at around -2ºC, failed completely if the temperature was raised above +2ºC,.
Acidophiles: Until recently only 4 organisms, all eukaryotes, were known to grow near pH 0; Cyanidium caldarium, and three fungi, Acontium cylatium, Cephalosporium sp., and Trichosporon cerebriae (Schleper et al., 1995). It has been observed that Cyanidium maintains its internal milieu at close to neutral pH (Beardall & Entwisle, 1984; Brock, 1978), and the means to do that is presumably a primary adaptation to this niche.
Alkalophiles: Two African soda lakes, with a pH of about 10, have been studied for their microbial populations, Lake Nakuru and Lake Simbi (Curds et al., 1986; Finlay et al., 1987). The former lake supports a very high population of flamingos, counted in millions, feeding largely on cyanobacteria (Brown, 1975) dominated by Spirulina, growing in the lake.
Barophiles: There are abundant eukaryotic communities on the continental shelf (up to 20 MPa). Indeed, there are metazoa present in the deepest oceans (Bruun, 1977), so there seems to be no fundamental reason why heterotrophic eukaryotic microbes should not be present at great depths given sufficient food. Holothurians (Sea Cucumbers; members of the phylum Echinodermata) are reported to be abundant in the world's deepest location, the Challenger Deep (in the West Pacific, 400 Km SW of Guam; approximately 110 MPa)
There are also bacteria that live inside solid rock...and others that can survive intense radiation exposure.
Visit the Extremophiles page at the Astrobiology Web for more details.
Copyright 1997 Dr. Sten Odenwald
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