The incorrect statement about methanogens among the options provided is:

They grow aerobically on cellulosic material.

Methanogens are anaerobic microorganisms that thrive in oxygen-deprived environments. They are known for producing methane gas as a metabolic byproduct in anaerobic conditions. Methanogens are commonly found in environments such as the rumen of cattle (a part of stomach), anaerobic sludge during sewage treatment, and various other anaerobic environments. They do not grow aerobically (in the presence of oxygen) but rather in anaerobic conditions.

Methanogen bacteria, which grow anaerobically on cellulosic material, produce large amount of methane along with CO₂ and H₂ and one such common bacterium is Methanobacterium. A lot of cellulosic material present in the food of cattle is also present in the rumen. In rumen, these bacteria help in the breakdown of cellulose and play an important role in the nutrition of cattle.