Which of the following statements regarding methanogens is incorrect?

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

Methanogens are bacteria that grow without oxygen (anaerobically) on cellulose-rich material and give out a lot of methane. Methanobacterium is a common example. The question asks for the incorrect statement.

Option (1) Methanogens can be utilised to produce biogas. — A correct statement. Cattle dung is rich in methanogens, and it is used to make biogas (gobar gas).

Option (2) Methanogens are present in the rumen of cattle and their excreta. — A correct statement. Methanogens live in the rumen, a part of the cattle stomach, and help break down cellulose there. So the dung of cattle is also rich in them.

Option (3) Methanogens grow aerobically and break down cellulose-rich food. — The incorrect statement. Methanogens break down cellulose, but they grow anaerobically, not aerobically. They are found in places without oxygen, such as the rumen and the anaerobic sludge of sewage plants.

Option (4) Methanogens produce methane gas. — A correct statement. They produce a large amount of methane, along with CO₂ and H₂.

NCERT Extract:
Microbes in Production of Biogas – “… However, certain bacteria, which grow anaerobically on cellulosic material, produce large amount of methane along with CO₂ and H₂. These bacteria are collectively called methanogens, and one such common bacterium is Methanobacterium. These bacteria are commonly found in the anaerobic sludge during sewage treatment. These bacteria are also present in the rumen (a part of stomach) of cattle. … Thus, the excreta (dung) of cattle, commonly called gobar, is rich in these bacteria. Dung can be used for generation of biogas, commonly called gobar gas.”