MEPS 358:137-150 (2008)  -  DOI: https://doi.org/10.3354/meps07370

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 × 10⁸ cm^–3 in H. thielei and 2.5 × 10⁸ 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 ³H-leucine and ¹⁴C-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 (NH₄)₂SO₄ 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 CO₂ 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.

KEY WORDS: Deep water coral reef · Sponge-associated prokaryotes · Leucine incorporation · Bicarbonate fixation · Nitrification