In situ feeding and element removal in the symbiont-bearing sponge Theonella swinhoei:: Bulk DOC is the major source for carbon

The vast majority of organic matter in the world ocean is found in the dissolved pool. However, no evidence has been demonstrated for direct uptake of bulk dissolved organic matter (DOM) by organisms other than bacteria and some invertebrate larvae. The total organic carbon (TOC) is 10-30% higher in coral reefs than in adjacent open waters. The dissolved organic carbon (DOC) accounts for >90% of the TOC. Using a new in situ technique for clean sampling of the seawater inhaled and exhaled by benthic suspension feeders, we measured directly the removal of DOC in the symbiont-bearing reef sponge Theonella swinhoei. The sponge removed up to 26% (mean +/- SD: 12% +/- 8%) of the TOC (dissolved and particulate) from the water it filtered during a single passage through its filtration system. Most of the carbon gained by the sponge was from the dissolved pool (10 +/- 7 mumol C L-1), an order of magnitude greater than the carbon gained from the total living cells (phytoplankton and bacteria) the sponge removed (2 +/- 1 mumol C L-1). In T. swinhoei, over two-thirds of the sponge biomass consists of symbiotic bacteria, which likely play an important role in DOC uptake. Our findings indicate that the role of DOC in metazoan nutrition and the role of metazoans in DOC cycling may have been grossly underestimated.