Which of the following statements regarding methanogens is incorrect?

Methanogens grow aerobically and breakdown cellulose-rich food.

The correct answer is Option (3) - Methanogens grow aerobically and breakdown cellulose-rich food.

(1) Methanogens can be utilized to produce biogas: This statement is correct. Methanogens are microorganisms that are capable of producing methane gas. Methanogens are commonly used in anaerobic digesters or biogas plants to break down organic materials, such as animal manure, sewage, or agricultural waste, and produce biogas, which is primarily composed of methane.

(2) Methanogens are present in the rumen of cattle and their excreta: This statement is also correct. Methanogens are naturally present in the rumen, which is a part of the stomach in ruminant animals like cattle. Methanogens play a crucial role in the digestion process of ruminants by breaking down complex carbohydrates, such as cellulose and hemicellulose, present in the food.

(3) Methanogens grow aerobically and breakdown cellulose-rich food: This statement is incorrect. Methanogens are strict anaerobes, meaning they require an oxygen-free environment to grow and function. They cannot survive or grow in the presence of oxygen.

(4) Methanogens produce methane gas: This statement is correct. Methanogens are known for their ability to produce methane gas as a metabolic byproduct. Methane is a potent greenhouse gas and a significant component of biogas.

In conclusion, statement (3) "They grow aerobically and breakdown cellulose-rich food" is the incorrect statement. Methanogens are strict anaerobes and collaborate with other microorganisms to break down cellulose-rich food.