Q.73 Assume that the average DNA content of a single microbial cell is 4 femtogram. A soil sample analyzed
for its microbial community DNA is found to contain 𝟎. 𝟑𝟐𝝁 𝐠 DNA per gram of the soil. The number of
microbial cells per milligram of the soil are ____

Calculating Microbial Cells in Soil from DNA Content

Determine the number of microbial cells per milligram of soil when each
microbial cell contains 4 femtograms (fg) of DNA and the soil contains
0.32 μg DNA per gram.

Problem Breakdown

The average DNA content per microbial cell is given as
4 femtograms (fg), which is equal to:

4 × 10⁻¹⁵ grams (g)

Soil analysis shows 0.32 micrograms (μg) of DNA per gram of soil, which is:

0.32 × 10⁻⁶ g DNA per g soil

The objective is to convert this information into the number of
microbial cells per milligram (mg) of soil, noting that:

1 g = 1000 mg

Step-by-Step Calculation

Step 1: Calculate microbial cells per gram of soil

Divide the total DNA per gram of soil by the DNA content per microbial cell:

(0.32 × 10⁻⁶ g DNA per g soil) ÷ (4 × 10⁻¹⁵ g DNA per cell)

= 8 × 10⁷ cells per gram of soil

Step 2: Convert cells per gram to cells per milligram

Since 1 gram = 1000 milligrams:

(8 × 10⁷) ÷ 1000 = 8 × 10⁴ cells per mg

= 80,000 microbial cells per milligram of soil

Final Answer

The number of microbial cells per milligram of soil is:

8 × 10⁴ cells/mg (80,000 cells per mg)

This calculation assumes that all extracted DNA originates from intact microbial cells,
which is a standard assumption in soil microbiology despite the possible presence of
extracellular DNA.

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  Integrated in title, introduction, and headers with variations such as
  soil microbial cell count from DNA.