archaebacterium vs archaebacteria

archaebacterium and archaebacteria both are nouns.

Word	Noun	Adjective	Verb	Adverb
archaebacterium	Yes	No	No	No
archaebacteria	Yes	No	No	No

As nouns, archaebacterium and archaebacteria are synonyms defined as:

archaebacterium and archaebacteria: considered ancient life forms that evolved separately from bacteria and blue-green algae

Other synonyms of archaebacterium include archaeobacteria, archeobacteria.

archaebacterium (noun)	archaebacteria (noun)
considered ancient life forms that evolved separately from bacteria and blue-green algae	considered ancient life forms that evolved separately from bacteria and blue-green algae