The removal of dissolved organic matter by marine sponges is a function of its composition and concentration: An in situ seasonal study of four Mediterranean species

Abstract

Sponges are unique among metazoans in their ability to use dissolved organic matter (DOM), the largest pool of organic matter in the ocean, as a major food source. The effect of variations in DOM abundance and composition on its uptake by sponges has rarely been studied. We examined, in situ, the seasonal uptake of DOM by four sponges [2 species with high microbial abundance (HMA) and 2 with low microbial abundance (LMA)] in the northwestern Mediterranean. Dissolved organic carbon (DOC) showed a strong seasonality with 3-fold higher concentrations in fall-winter (256 ± 16 μmol L−1, mean ± SE) than in spring-summer (88 ± 3 μmol L−1). Dissolved organic nitrogen (DON) showed the opposite trend, with higher summer concentrations (8.9 ± 0.4 μmol L−1) and mean concentrations of 2.5–3.4 μmol L−1 in the other seasons. DOC removal by all sponge species increased linearly with its ambient concentration, but only above a DOC removal threshold that was threefold higher in fall-winter (198 μmol L−1) than in spring-summer (74 μmol L−1). All species showed a concentration-dependent DON removal, but LMA sponges removed more DON than HMA sponges. The DOC removal rate (normalized to sponge volume) was 2–3 times higher in fall-winter, when ambient DOC levels were high, than in spring-summer. Sponges efficiently removed clusters of the fluorescent DOM (FDOM) associated with protein-rich DOM, but not those associated with humic material. The clear threshold for DOC removal and the protein-like FDOM uptake pattern suggest that the quality and quantity of DOM control its removal and transformation by marine sponges. Our results indicate that marine sponges transform the composition of the coastal DOM pool, thereby affecting its fate. It is postulated that the DOM excreted by the sponges is more recalcitrant; consequently, sponge activity enhances carbon sequestration in benthic habitats in a similar fashion to that of the oceanic ‘microbial pump’.