Soru

Desert varnish, a thin, dark red to black coating found on rock surfaces in arid regions, has long intrigued geologists. Composed primarily of clay minerals, iron, and manganese oxides, this micro-crust forms over thousands of years. Early researchers hypothesized that the varnish was an abiotic precipitate, formed when mineral-rich moisture evaporated from the rock's interior. However, recent microscopic analyses have revealed that the primary agent in the creation of desert varnish is actually rock-dwelling bacteria, specifically mixotrophic species like *Metallogenium*.

Because these bacteria possess a sheath that concentrates trace elements from the atmosphere, they are uniquely suited to survive the harsh desert sun. The bacteria oxidize manganese, which is relatively rare in the environment, converting it into an insoluble black oxide. This chemical conversion acts as a protective shield for the bacteria, shielding them from lethal ultraviolet radiation. As a consequence of this biological activity, the manganese becomes highly concentrated in the varnish—often up to a hundred times its concentration in the surrounding soil. The clay minerals, which are carried by the wind and adhere to the sticky bacterial secretions, provide a physical matrix that cements the oxidized minerals to the rock face, stabilizing the varnish against wind erosion.

According to the passage, the rock-dwelling bacteria are shielded from lethal ultraviolet radiation as a direct result of which of the following?