Diversity and abundance of ammonia-oxidizing <i>Archaea</i> and <i>Bacteria</i> in tropical and cold-water coral reef sponges

We analysed the diversity and abundance of ammonia-oxidizing Archaea (AOA) and Bacteria (AOB) in the shallow warm-water sponge Halisarca caerulea and the deep cold-water sponges Higginsia thielei and Nodastrella nodastrella. The abundance of AOA and AOB was analysed using catalyzed reporter deposition-fluorescence in situ hybridization and (real-time) quantitative PCR (Q-PCR) targeting archaeal and bacterial amoA genes. Archaeal abundance was similar between sponge species, while bacterial abundance was higher in H. caerulea than in N. nodastrella and H. thielei. Q-PCR showed that AOA outnumbered AOB by a factor of 2 to 35, suggesting a larger role of AOA than of AOB in ammonia oxidation in sponges. PCR-denaturing gradient gel electrophoresis was performed to analyse the taxonomic affiliation of the microbial community associated with these sponges. Archaeal and bacterial amoA genes were found in all 3 sponges. The structure of the phylogenetic trees in relation to temperature and sponge species was analysed using all published amoA sequences retrieved from sponges. Temperature was an important factor influencing the distribution of nitrifiers in sponges. Both archaeal and bacterial amoA sponge sequences tended to cluster with sequences retrieved from habitats of similar temperature. This is the first time that similarity in AOB diversity is described between distantly related species (H. thielei belonging to the class Demospongiae, and N. nodastrella to Hexactinellida). The results described here support the idea of a relatively uniform microbial community between distantly related sponges and suggest that temperature (rather than phylogenetic distance) is determining the diversity of AOA and AOB in sponges