Archaea vs. Bacteria. Archaea and bacteria are both single-celled organisms that reproduce asexually and do not have a nucleus. Archaea belong to a kingdom of single-celled microorganisms that do not have a nucleus or other organelles.In the past, archaea were classified as bacteria, but now they are known to be a separate type of organism. The word archaea is derived from the Greek archaios, meaning ancient.rchaea vs. Bacteria. Archaea and bacteria are both single-celled organisms that reproduce asexually and do not have a nucleus. Archaea belong to a kingdom of single-celled microorganisms that do not have a nucleus or other organelles.