Answer:
They caused some certain diseases in humans.
Methanogens a type of archaebacteria are source of natural gas.
They are found in sewage for sewage treatment.
Step-by-step explanation:
Archaebacteria are microscopic and single celled organisms that lack nuclei. They don't have peptidoglylcan in their cell walls. They have tRNA and ribosomal RNA. They were formerly classified as bacteria.