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Information and IdeasMediumSupporting a claim about desert varnish microbes

Information and Ideas practice question

Desert varnish — the dark sheen on exposed rock in arid lands — was long attributed to purely chemical deposition. Some geomicrobiologists now claim that microorganisms actively contribute to building the coating. ____

Which finding, if true, would most directly support the researchers' claim?

  1. A. Varnish layers contained DNA from living microbial communities arranged in bands matching the varnish's growth layers.Correct
  2. B. Desert varnish forms more quickly on south-facing rock surfaces.
  3. C. Varnish coatings are rich in manganese and iron oxides.
  4. D. Ancient petroglyphs were carved by scraping varnish from rock faces.

Answer: A. Varnish layers contained DNA from living microbial communities arranged in bands matching the varnish's growth layers.

The claim is that microbes help build the varnish; living communities layered in step with the varnish's own growth bands ties the organisms to its formation.

Why the other answers are wrong

B. Desert varnish forms more quickly on south-facing rock surfaces.
Faster growth on sunny faces could have purely chemical causes.
C. Varnish coatings are rich in manganese and iron oxides.
The coating's mineral content is what needs explaining — it does not show microbes produced it.
D. Ancient petroglyphs were carved by scraping varnish from rock faces.
Petroglyph carving concerns human use of varnish, not its origin.

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