Abstract
Shallow warm-water and deep-sea cold-water corals engineer the coral reef framework and fertilize reef communities by releasing coral mucus, a source of reef dissolved organic matter (DOM). By transforming DOM into particulate detritus, sponges play a key role in transferring the energy and nutrients in DOM to higher trophic levels on Caribbean reefs via the so-called sponge loop. Coral mucus may be a major DOM source for the sponge loop, but mucus uptake by sponges has not been demonstrated. Here we used laboratory stable isotope tracer experiments to show the transfer of coral mucus into the bulk tissue and phospholipid fatty acids of the warm-water sponge Mycale fistulifera and cold-water sponge Hymedesmia coriacea, demonstrating a direct trophic link between corals and reef sponges. Furthermore, 21–40% of the mucus carbon and 32–39% of the nitrogen assimilated by the sponges was subsequently released as detritus, confirming a sponge loop on Red Sea warm-water and north Atlantic cold-water coral reefs. The presence of a sponge loop in two vastly different reef environments suggests it is a ubiquitous feature of reef ecosystems contributing to the high biogeochemical cycling that may enable coral reefs to thrive in nutrient-limited (warm-water) and energy-limited (cold-water) environments.
Highlights
Scleractinian corals act as ecosystem engineers on warm-water (WW) and cold-water (CW) coral reefs by forming the complex 3D-reef framework and driving reef biogeochemical cycles[1,2]
The similar rate of assimilation of coral mucus into the bulk tissue of the two sponges indicates that WW and CW sponges may have a comparable capacity for mucus uptake
Based on natural abundance stable isotope signatures, coral mucus has been inferred to be a major component (48–73%) of the diet of Caribbean cavity sponges[42], but our findings provide unequivocal and quantitative evidence for the uptake and assimilation of coral mucus by sponges originating from shallow-water and deep-sea reef habitats
Summary
Scleractinian corals act as ecosystem engineers on warm-water (WW) and cold-water (CW) coral reefs by forming the complex 3D-reef framework and driving reef biogeochemical cycles[1,2]. CW reefs are typically restricted to oceanic regions with high surface primary production and enhanced vertical transport due to elevated currents[12,13,14] Despite these pronounced environmental differences (see Supplementary Table S1) both WW and CW reefs are considered hotspots of marine biodiversity and biological activity[3,5,15]. Scleractinian corals secrete a surface mucus layer that is continuously released into the water column in particulate and dissolved forms, thereby substantially contributing to reef organic matter pools[16,17,19]. The sponge loop plays a major role in organic matter cycling on Caribbean reefs, but has not yet been investigated in other oceanic regions. Assimilation of coral mucus C into sponge phospholipid-derived fatty acids (PLFAs) was measured to further examine the processing of coral mucus by the sponges and evaluate the potential role of sponge-associated bacteria in its uptake
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