Carbon & Sulphur Cycle - Microbiology Questions and Answers

This set of Microbiology Multiple Choice Questions & Answers (MCQs) focuses on “Soil Microbiology – Carbon Cycle & Sulphur Cycle”.

1. Which among the following are the most important agents for carbon dioxide fixation?
a) bacteria
b) fungi
c) algae
d) protozoa
View Answer

Answer: c
Explanation: Green plants and algae are the most important agents of carbon dioxide fixation.

2. Fresh air contains approximately ______ percent carbon dioxide by volume.
a) 0.01
b) 2
c) 5
d) 0.03
View Answer

Answer: d
Explanation: Fresh air contains approximately 0.03 percent carbon dioxide by volume. The end product, carbon dioxide, from the degradation of organic carbon compounds are released into the air and soil.

3. Cellulose is degraded to cellobiose by the enzyme __________________
a) cellulase
b) beta-glucosidase
c) hexokinase
d) cellulose dehydrogenase
View Answer

Answer: a
Explanation: The most abundant organic material in plants is cellulose. The initial enzymatic at-tack is by cellulase which splits this long-chain polymer of glucose to cellobiose which contains two glucose units.

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4. The amount of organic material in the forest soil remains approximately the same from year to year.
a)True
b) False
View Answer

Answer: a
Explanation: Under most natural systems of vegetation, e.g., forests, the amount of organic material in the soil remains approximately the same from year to year. This results from a balance established between the annual litterfall and death of the plants and the capacity of microorganisms to degrade these tissues.

5. In carbon cycle flow of energy is _________________
a) bidirectional
b) linear
c) cyclic
d) irreversible
View Answer

Answer: b
Explanation: In carbon cycle flow of energy is linear in unidirectional way through carbon dioxide in the atmosphere to organic forms of carbon in soil.

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6. Cellobiose is metabolized directly by many microorganisms.
a) True
b) False
View Answer

Answer: b
Explanation: Cellobiose is split into glucose by the enzyme beta-glucosidase; glucose is metabolized directly by microorganisms.

7. Cysteine breaks down in presence of cysteine desulfurase to give _________________
a) oxaloacetic acid
b) sulphuric acid
c) pyruvic acid
d) glyoxalate
View Answer

Answer: c
Explanation: Degradation of proteins liberates amino acids, some of which contain sulphur released by the enzymatic activity of many heterotrophic bacteria. Cysteine breaks down in presence of cysteine desulfurase to give pyruvic acid, hydrogen sulphide and ammonia.

8. Which of the following processes is performed by Thiobacillus thiooxidans?
a) converting sulphur to sulphates
b) converting sulphur to sulphides
c) converting sulphur to sulphites
d) converting organic sulphur to inorganic sulphur
View Answer

Answer: a
Explanation: Thiobacillus thiooxidans, an autotroph, is capable of oxidizing elemental sulphur to sulphates.

9. Sulphates are reduced to hydrogen sulphide by _____________________
a) Desulfotomaculum sp.
b) Thiobacillus thiooxidans
c) Photosynthetic sulfur bacteria
d) Rhodospirillum
View Answer

Answer: a
Explanation: Sulphates are reduced to hydrogen sulphide by soil microorganisms like Desulfotomaculum species. Like calcium sulphate gives us hydrogen sulphide and calcium hydroxide.

10. The Winogradsky column experiment is done in the dark.
a) True
b) False
View Answer

Answer: b
Explanation: A laboratory technique which facilitates isolation of various sulfur-metabolilizing bacteria is the Winogradsky column. The column contains mud, CaSO₄, plant tissue, and water. It is exposed to daylight and incubated at room temperature.

11. The reduction of sulphates and sulphites to hydrogen sulphide is done by which group of bacteria?
a) aerobic sulfate-reducing bacteria
b) photosynthetic sulphur bacteria
c) anaerobic sulphate-reducing bacteria
d) heterotrophic bacteria
View Answer

Answer: c
Explanation: Organic acids serve as the electron donors for the reduction of sulfates to sulfites to hydrogen sulfide by anaerobic sulfate-reducing bacteria.

12. Which among the following develop in the upper portion of the Winogradsky column?
a) Sulfate-reducing bacteria
b) Green-sulfur bacteria
c) Purple-sulfur bacteria
d) Thiobacilli
View Answer

Answer: d
Explanation: The aerobic sulfur-metabolizing bacteria, Thiobacillus sp.., develop in the upper portion of the column and oxidize reduced sulfur compounds.

13. Purple and green sulfur bacteria use ___________________ as the electron donor to reduce carbon dioxide.
a) S^2-
b) SO₄^2-
c) H²S
d) Organic acids
View Answer

Answer: c
Explanation: Photosynthetic microorganisms such as the purple and green sulfur bacteria use hydrogen sulphide as the electron donor to reduce carbon dioxide.

14. Which among the following is a non-sulfur purple bacteria?
a) Rhodomicrobium
b) Thiobacillus
c) Chromatium
d) Chlorobium
View Answer

Answer: a
Explanation: The non-sulfur bacteria purple bacteria like Rhodomicrobium, Rhodospirillum. Rhodopseudomonas are facultative phototrophs and can utilize sulfide at low levels.

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