Sponges (Phylum Porifera) are among the most ancient animals and an important part of the benthic ecosystem. They form stable associations with a diversity of bacterial symbionts, which can make up to 33% of the sponge volume. Over the last decade several bacterial adaptations for the life inside sponge hosts have been revealed such as the presence of ankyrin-like domains and leucin-rich regions. The presence of archaea in sponges was first discovered in 1996. Since then many sponges have been found to have a stable associations with archaea, some of which are transmitted via the larvae. Based on the presence of ammonium monooxygenase A genes it has been suggested that the archaeal symbionts play a role in the nitrogen cycling within the host sponge by removing the host waste product ammonia. So far, only one archaeal symbiont genome, from Crenarchaeum symbiosum, the archaeal symbiont of the Pacific Ocean sponge Axinella mexicana, has been reported. In this study, we compare this genome to two novel draft genomes obtained here from sponge-associated archaea as well as to published genomes of free-living archaea.
