Desert varnish is a thin, dark coating found on rock surfaces in arid environments. It is … — ACT

Desert varnish is a thin, dark coating found on rock surfaces in arid environments. It is primarily composed of clay minerals, manganese ( $Mn$ ) oxides, and iron ( $Fe$ ) oxides. Two scientists discuss the mechanism behind its formation.

Scientist 1
Desert varnish is formed biochemically by manganese-oxidizing bacteria (such as the genus $\mathit{Metallogenium}$ ). These bacteria inhabit rock surfaces and utilize soluble divalent manganese ( $Mn^{2+}$ ) from windborne dust as an energy source, oxidizing it to insoluble tetravalent manganese ( $Mn^{4+}$ ) oxides. These oxides, along with clay particles, are cemented to the rock surface by bacterial extracellular polymeric substances (EPS). This biochemical process requires living microbial cells, organic carbon nutrients, and trace liquid water.

Scientist 2
Desert varnish forms through a purely inorganic chemical-physical process. During wet periods, dew or light rain dissolves amorphous silica ( $SiO_2$ ) and trace metals from windborne dust on rock surfaces. As the rock heats and dries, the silica precipitates, forming a silica-rich glaze. This glaze physically traps ambient, pre-oxidized manganese and iron oxides from the dust, cementing them to the rock. This process does not require living organisms, organic nutrients, or biological activity, and can occur in completely sterile environments.

Which of the following experimental procedures would provide the most definitive evidence to resolve the conflict between the two scientists' models?

Place sterile rock slabs and sterile, synthetic dust (containing $Mn^{2+}$ , $SiO_2$ , and no organic carbon or microbes) in a sealed chamber, subject them to repeated wetting and drying cycles under high heat and UV light, and analyze if a manganese-rich glaze forms.Answer
B
Collect desert varnish samples from various geographic regions and measure both the total organic carbon content and the ratio of $Mn^{4+}$ to $Mn^{2+}$ in the coating.
C
Inoculate a nutrient-rich agar plate containing soluble $Mn^{2+}$ with $\mathit{Metallogenium}$ bacteria, expose the plates to different temperatures, and measure the rate of bacterial colony growth.
D
Introduce $\mathit{Metallogenium}$ bacteria to rock samples in a humid, nutrient-rich environment, then vary the concentrations of both silica ( $SiO_2$ ) and iron ( $Fe$ ) oxides simultaneously to observe which component is deposited faster.

Answer

Place sterile rock slabs and sterile, synthetic dust (containing

Mn^{2+}

SiO_2

, and no organic carbon or microbes) in a sealed chamber, subject them to repeated wetting and drying cycles under high heat and UV light, and analyze if a manganese-rich glaze forms.

The correct answer describes a controlled experiment that isolates the biological variable (the presence of living bacteria and organic nutrients). By maintaining sterile and inorganic conditions, any formation of the manganese-rich glaze can be attributed purely to the physical-chemical processes proposed by Scientist 2. Conversely, if no varnish forms under these conditions but forms when bacteria are introduced, Scientist 1's model is supported.

Step-by-Step Solution

Identify the core point of disagreement between the two models.

Scientist 1 asserts that living manganese-oxidizing bacteria and organic nutrients are biochemically required to form the varnish, whereas Scientist 2 asserts that the process is entirely inorganic and abiotic.

Resolving the conflict requires isolating the presence of living organisms and biological activity as the independent variable.

Evaluate the experimental setup that isolates this independent variable.

Creating a completely sterile environment with synthetic dust lacking organic carbon and microbes ensures that any varnish formation cannot be attributed to biological processes.

This directly tests the physical-chemical pathway proposed by Scientist 2 while excluding the biological pathway proposed by Scientist 1.

Analyze how the outcomes of the selected procedure resolve the conflict.

If a manganese-rich glaze forms under sterile conditions, Scientist 2's abiotic model is supported. If a glaze only forms when bacteria are introduced, Scientist 1's biochemical model is supported.

A conclusive experiment must yield distinct, non-overlapping outcomes that support one hypothesis while invalidating the other.

Key Concept

Designing a controlled experiment to isolate a variable and resolve conflicting scientific hypotheses.

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