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JOURNAL ARTICLE
Rock Varnish
Ronald I. Dorn
American Scientist
Vol. 79, No. 6 (November-December 1991), pp. 542-553
Published by: Sigma Xi, The Scientific Research Honor Society
https://www.jstor.org/stable/29774522
Page Count: 12
Over thousands of years, a thin coating of clay, cemented to rocks by manganese and iron, records the history of landscape development and ancient cultures.
In July of 1799, Alexander von Humboldt, the German geographer, arrived in Cumana in northeastern Venezuela. A hundred miles away, at the mouth of the Orinoco River, he found granite boulders that appeared “smooth, black, and as if coated with plumbago." Indian legends explained that these rocks had been burnt by the hot tropical sun, and that they were dangerous to one’s health. While investigating the origin of the rocks, von Humboldt and his crew scoffed at the local legends, but each night they retreated to white beaches, distant from the black boulders.
Von Humboldt's boulders and similar rocks found elsewhere generated a scientific mystery that continued for nearly two centuries. Charles Darwin found dark coated rocks at Bahia in Brazil. Others found such coatings on rocks in the rain forest, underneath glaciers in the Alps and even at the apex of the Pyramid of Cheops. Hypotheses about the origin of the black coating ranged from deposits left by ancient oceans to residues from decomposing organic matter, such as pollen. The most popular hypothesis attributed the coating to a process called sweating. According to the sweating hypothesis, water sweats out of the rocks under the hot sun, and precipitates are deposited as a black coating on the surface when the water evaporates. But was the sun hot enough to induce sweating of solutions from rocks in the rain forest or underneath glaciers?
The black coating has been known by various names, such as wustenlacken (desert varnish) and patina; only recently has the scientific community settled on a term that seems sufficiently broad and descriptive: rock varnish. Even while the origin of the rock varnish remained enigmatic, its physical and chemical structure came to be understood. Rock varnish consists of a thin layer, less titan half a millimeter deep, on a rock‘s surface. The coating is typically composed of about 60 percent clay minerals, 20 to 30 percent oxides of manganese and iron, and trace amounts of more than 30 minor compounds, such as copper and zinc oxides. It is the amount of manganese oxide that determines the color of the varnish. Often, the varnish concentrates manganese oxide to levels up to 100 times that found in surrounding rocks; such high concentrations of manganese oxide make the varnish black. Some varnish lacks manganese altogether and appears bright orange from the abundant iron oxides; this is particularly prevalent on the bottom of rocks and inside rock crevices. In other places, such as the hyperarid Peru Desert, the level of manganese oxide is intermediate, leaving the varnish a brownish orange.
The decisive clue to the mystery of rock varnish was a biological one. In 1981, T. M. Oberlander of the University of California at Berkeley and l suggested that bacteria concentrate manganese in rock varnish. We developed this hypothesis on strong circumstantial evidence. First, manganese-rich varnish often forms where water intermittently flows over rocks. The moisture provides a hospitable environment for microorganisms. Second, varnish develops well on porous surfaces that are easily flushed and, hence, are poor in nutrients. In such a harsh environment manganese-oxidizing mixotrophs (which derive some of their energy from inorganic manganese) are able to live; but faster growing heterotrophic organisms (which rely entirely on organic nutrients) are unable to survive.
A third observation in support of the bacterial hypothesis is that dark varnish grows on rocks with a nearly neutral pH. Nonbiological mechanisms for oxidizing manganese operate only in an alkaline environment, where the pH exceeds 9. Thus, manganese-oxidizing bacteria are one plausible candidate. This line of argument is further supported by findings that varnish is orange when the local pH is too high to support manganese-oxidizing bacteria.
Finally, there are many documented interactions between clay, the primary constituent of varnish, and bacteria. Clay particles are often adsorbed onto the surface of bacteria, and vice versa. Clay concentrates nutrients, a useful property in a nutrient-poor environment, and stimulates bacterial metabolism. And a coating of clay can protect bacteria against desiccation and high temperatures. In combination, these factors supported the possibility that bacteria produce manganese—rich rock varnish.
With these ideas in mind, Oberlander and I examined rock varnish for the presence of bacteria. By searching though varnish with the aid of scanning electron microscopy; we found them—manganese-concentrating bacteria of the genus Metallogenium and other species. Fred Palmer and his colleagues at the University of Washington found bacteria of the genus Arthrobacter in rock varnish. Next, we isolated living bacteria from natural rock varnish; when we grew these bacteria in laboratory cultures, they produced varnish that was morphologically similar to the natural product.
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发表于 2018-8-16 10:10:06
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