The aim of this study was to assess the potential for nitrification in Aplysina aerophoba Schmidt 1862 using a
combined physiological and molecular approach. Whole animals were incubated in experimental aquaria and the concentrations
of ammonia, nitrate and nitrite were determined in the incubation water using colorimetric assays. Nitrate excretion rates
reached values of up to 3.6 μmol g-1 fresh weight day-1 (equivalent to 830 nmol g-1 dry weight h-1) and were matched by
ammonia excretion rates of up to 0.56 ± 0.09 μmol g-1 fresh weight day-1. An accumulation of nitrite was not detected in any
of the experiments. Control experiments without sponges showed no variation in nitrogen species in the incubation water. A
slight increase in ammonia excretion was observed over 11 days of maintenance in holding tanks that were constantly supplied
with fresh, untreated Mediterranean seawater. Other sponges from the same habitat (Dysidea avara Schmidt 1862, Tethya sp.,
Chondrosia reniformis Nardo 1847) showed high rates of ammonia excretion but nitrate excretion was significantly reduced
or absent. Using specific PCR primers, 16S rRNA genes of the betaproteobacterial clade of the Nitrosospira cluster 1 were
recovered from A. aerophoba, D. avara and Tethya sp. tissues. In conclusion, this study provides physiological and molecular
evidence for the presence of nitrifying bacteria in A. aerophoba while the potential for nitrification in the other sponges
remains to be investigated
