| Summary: | The physical environment in the cryptoendolithic habitat in Antarctica were investigated using two complementary approaches: field measurements and theoretical models. Light, air temperature, snow, wind and rock temperature and moisture on Linnaeus Terrace, Antarctica, were monitored for three years through a satellite-mediated automatic weather station. These data were coupled with existing information on light, temperature, and moisture requirements and used to estimate the period of biological activity. Metabolic activity in the community is restricted to the period from mid November to mid February, with the total number of hours of biological activity less than about 1000 hours per year. The measured temperature data were supplemented through the construction of a computer model of the temperature regime. This model enabled the parameters affecting the horizontal and altitudinal distribution of the community to be examined. Colonization is possible on surfaces with zenith angle less than $15\sp\circ.$ At greater zenith angles, colonization is restricted to surfaces with azimuth angles less than $135\sp\circ$ or greater than $225\sp\circ.$ The upper elevational limit of the community is less than 2500 m. The thermal regime probably does not influence the zonation of the community within a rock. The light regime within the community was investigated though the development of a general computer model of light transmission in close-packed particulate media. The apparent extinction coefficients in dry colonized rocks varies from 1.5 to 3.0 mm$\sp{-1},$ depending on the amount of iron-oxide in the crust and the degree of colonization. Saturating amounts of water decrease the extinction coefficient to about 65% of its dry value. In absolute numbers, the maximum irradiance at the top of the Hemichloris zone is about 1 $\mu{\rm M}$ photons ${\rm m}\sp{-2}$ ${\rm s}\sp{-1}$ when dry, 10 $\mu{\rm M}$ photons ${\rm m}\sp{-2}$ ${\rm s}\sp{-1}$ when wet. Because of the low light levels, it is expected that the ...
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