Major bulk dissolved organic carbon (DOC) removal by encrusting coral reef cavity sponges

We studied the removal of dissolved organic carbon (DOC) and bacterioplankton by the encrusting spongesHalisarca caerulea,Mycale microsigmatosaandMerlia normaniin coral reefs along Curaçao, Netherlands Antilles. Sponge specimens were collected from coral reef cavities and incubations were done on the fore-reef slope at 12 m depth. The concentrations of DOC and bacterioplankton carbon (BC) were monitoredin situ, using incubation chambers with sponges and without sponges (incubations with coral rock or ambient reef water only). Average (±SD) DOC removal rates (in μmol C cm–3sponge h–1) amounted to 13.1 ± 2.5, 15.2 ± 0.9 and 13.6 ± 2.4 forH. caerula,M. microsigmatosaandM. normani, respectively. The DOC removal rates by the 3 sponges were on average 2 orders of magnitude higher than BC removal rates and accounted for more than 90% of the total organic carbon removal. Total organic carbon removal rates presented here were the highest ever reported for sponges. In an additional experiment withH. caerulea, the fate of organic carbon was reconstructed by measuring dissolved oxygen (O₂) removal and dissolved inorganic carbon (DIC) release in a laminar flow chamber.H. caerulearespired 39 to 45% of the organic carbon removed. The remaining 55 to 61% of carbon is expected to be assimilated. We argue thatH. caeruleamay have a rapid turnover of matter. All 3 sponge species contained associated bacteria, but it is unclear to what extent the associated bacteria are involved in the nutrition of the sponge. We conclude that the 3 sponge–microbe associations are (related to the availability of dissolved and particulate carbon sources in the ambient water) ‘dissolved organic matter (DOM)-feeders’ and encrusting sponges are of quantitative importance in the removal of DOC in coral reef cavities.