Why is archaea anaerobic?
This is related with primordial atmosphere. Primordial atmosphere was without oxygen.
Life began on earth when there was no oxygen. The earliest organisms use to get energy from methane, carbon dioxide, and sulphur compounds.
Archea origin was with such environment. There cells are without membrane bound organelles.
Naturally these were anaerobic .
When cyanobacteria developed oxygen was released in atmosphere by photosynthesis.
This was beginning of aerobic organisms.