Dissolved carbon fixation by sponge–microbe consortia of deep water coral mounds in the northeastern Atlantic Ocean

Abstract

We studied dissolved organic and inorganic carbon fixation in 2 sponge species from deep water coral mounds, viz. the demosponge Higginsia thielei and the hexactinellid sponge Rossella nodastrella. Sponges were collected between 500 and 700 m depth on coral mounds in the Rockall Trough (NE Atlantic). Prokaryote densities in sponge associations were on average 2.0 10 8 cm -3 in H. thielei and 2.5 10 8 cm -3 in R. nodastrella (ca. 7 to 30% Archaea, 36 to 65% Bacteria, counted after DAPI staining). Sponge samples were incubated in ultra-filtered seawater with 3 Hleucine and 14 C-bicarbonate. Mean leucine-based carbon production was 4 nmol C cm -3 sponge d -1 for H. thielei and 2 to 4 nmol C cm -3 d -1 for R. nodastrella. Average bicarbonate fixation by box-cored H. thielei was 0.7 nmol C cm -3 sponge d -1 and up to 4.5 nmol C cm -3 d -1 by R. nodastrella. Bicarbonate fixation by sponges was enhanced by (NH 4 ) 2 SO 4 addition to the incubations. Net ammonia oxidation and nitrite production were established for both sponge species and indicated the presence of sponge-associated nitrifiers. Results suggest that sponge-associated chemoautotrophic/mixotrophic nitrifying prokaryotes may contribute to the observed CO 2 assimilation. On average, dissolved carbon was processed 100 to 150 times faster by sponge-microbe consortia of H. thielei and R. nodastrella than by planktonic microbes in ambient water. Preliminary estimates suggest that the assimilation of dissolved (in)organic carbon contributed up to 10% of total carbon assimilation by sponge-microbe consortia of H. thielei and R. nodastrella.