Biogenic rock varnishes of the negev desert (Israel) an ecological study of iron and manganese transformation by cyanobacteria and fungi

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Abstract
The ecology of the microflora, which produces rock varnishes in the Negev is described. It is shown that biogenic rock varnishes may form within relatively short periods (1967–1981) on places where pre-existing varnishes were eliminated. Rock varnishes are thin coatings, mainly composed of Fe and Mn hydroxides and clay material. Biogenic rock varnishes form at places where “microbial weathering fronts”, which destroy the rock substrate, advance extremely slowly or come to stillstand, thus enabling the development of biogenic “protective coatings”. Rock varnish is mainly produced by the activity of often lichenised epi- and endolithic cyanobacteria, chemoorganotrophic bacteria, and fungi, which are sometimes associated with the still debatable Metallogenium symbioticum. In cases, where “microbial weathering fronts” reach harder bedrocks during their progress, the then developing rock varnish plays a protective role for the microflora beneath the varnish in formation. This microflora otherwise would be directly exposed to the harsh desert conditions and could not survive. Biogenic rock varnishes are characteristic examples of a microbial ecosystem, which adapted itself to one of the most extreme environments on this planet, i.e. high irradiation, extremely low water activity, no chances of deplacement upwards or downwards and in addition the highest daily changes in temperature and irradiation and humidity one may observe in natural environments. It seems, that the “solution front community” which is trapped on increasingly harder and resistant rocks has evolved the capacity to protect itself from the harsh environmental conditions by the creation of rock varnish as a kind of armour shielding it from the extremes of environmental stress.