Magazine article Natural History

Directory of Hard Rock Habitats

Magazine article Natural History

Directory of Hard Rock Habitats

Article excerpt

In a land scorched by sun and wind, exposed rock would seem an unlikely refugium for life of any kind. Yet even in the most hostile of climates, a single rock can provide a variety of living situations for microbiota adapted to life at the extremes. In addition to the environment beneath translucent stones, both the surface and subsurface of porous rocks often support a surprising diversity of microorganisms.

Most conspicuous of the surface rock-dwellers in hot deserts are the crustose lichens, which have an extraordinary capacity to endure months of almost complete dehydration and then resume growth quickly when wet. They are also able to continue photosynthesizing beyond the temperature thresholds at which most plants shut down. Resilient as they are, however, crustose lichens have their limits, and where rock surfaces become too inhospitable for the lichens, they are often replaced by bacteria.

Commonly referred to as "desert varnish," the dark, often shiny surfaces of sun-baked rock (favored sites of some prehistoric artists) found in numerous desert environments are actually large colonies of bacteria. Able to obtain energy from inorganic as well as organic substances, these hardy colonists adsorb submicroscopic bits of wind-transported clay to their cellular surfaces to build a thin layer of protection from direct sunlight. Minute quantities of manganese and iron, also collected from atmospheric dust and subsequently oxidized by the bacteria, combine with the clay minerals to form a tough coating of dark manganese oxide or reddish iron oxide.

Once established, the bacterial colony grows outwardly from scattered points, often starting in tiny depressions that capture the wind-transported minerals and retain moisture from dew or runoff (the bacteria themselves may have been similarly "trapped"). The colony may eventually cover entire cliff faces in arid regions. …

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