Most extensive metabolic diversity is found in $:-$

$A$ heterocyst constitutes a:

Statement $X$: Methanogens are responsible for the production of methane.
Statement $Y$: They can survive in acidic and high-temperature conditions.

Which of the following microorganisms possess the property of both photosynthesis and nitrogen fixation?

Mycoplasma is a

What are bacteria that absorb the Gram stain called?