The associated microbial community in the mesohyl of the Arctic deep-water sponge Tentorium semisuberites Schmidt, 1870 (Hadromerida, Demo-spongiae) is dominated by Archaea. This is the result of an integral approach applying analyses of microbial lipid biomarkers as well as microscopic investigations using differential fluorescence in situ hybridisation with universal probes and counterstaining with 4',6'-diami-dino-2-phenylindole (DAPI) on sponge sections based on samples collected in the Greenland Sea in 2001, 2002 and 2005. The distribution of isoprenoidal C_(40) hydrocarbons of the biphytane series suggests that affiliates of both major archaeal kingdoms, the Crenarchaeota and the Euryarchaeota, are present in the choanosome of T. semisuberites. Positive signals using the oligonu-cleotide probe ARCH915 indicate high numbers of Archaea in the mesohyl of this sponge. Based on optical estimations 70-90% of all microbial DAPI signals accounted for archaeal cells. Archaea in these high proportions have never been described in an Arctic deep-sea hadromerid sponge, nor in any other demo-sponge species. Similar observations in specimens collected over a time scale of 4 years suggest permanent sponge-Archaea associations.
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