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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.

[1] Water column stratification increased at climatic transitions from cold to warm periods during the late Quaternary and led to anoxic conditions and sapropel formation in the deep eastern Mediterranean basins. Highresolution data sets on sea-surface temperatures (SST) (estimated from U 37 O seawater depletion of eastern Mediterranean surface waters at the transition is between 0.5 and 3.0%, and in all but one case exceeded the depletion seen in a western Mediterranean core. The depletion in d 18O seawater is most pronounced at sapropel bases, in agreement with an initial sudden input of monsoon-derived freshwater. Most sapropels coincide with warming trends of SST. The density decrease by initial freshwater input and continued warming of the sea surface pooled fresh water in the surface layer and prohibited deep convection down to ageing deep water emplaced during cold and arid glacial conditions. An exception to this pattern is ''glacial'' sapropel S6; its largest d 18 O seawater depletion (3%) is almost matched by the depletion in the western Mediterranean Sea, and it is accompanied by surface water cooling following an initially rapid warming phase. A second period of significant isotopic depletion is in isotope stage 6 at the 150 kyr insolation maximum. While not expressed as a sapropel due to cold SST, it is in accord with a strengthened monsoon in the southern catchment.

Summary Corals and macroalgae release large quantities of dissolved organic matter (DOM), one of the largest sources of organic matter produced on coral reefs. By rapidly taking up DOM and transforming it into particulate detritus, coral reef sponges are proposed to play a key role in transferring the energy and nutrients in DOM to higher trophic levels via the recently discovered sponge loop. DOM released by corals and algae differs in quality and composition, but the influence of these different DOM sources on recycling by the sponge loop has not been investigated. Here, we used stable isotope pulse‐chase experiments to compare the processing of naturally sourced coral‐ and algal‐derived DOM by three Red Sea coral reef sponge species: Chondrilla sacciformis, Hemimycale arabica and Mycale fistulifera. Incubation experiments were conducted to trace 13C‐ and 15N‐enriched coral‐ and algal‐derived DOM into the sponge tissue and detritus. Incorporation of 13C into specific phospholipid‐derived fatty acids (PLFAs) was used to differentiate DOM assimilation within the sponge holobiont (i.e. the sponge host vs. its associated bacteria). All sponges assimilated both coral‐ and algal‐derived DOM, but incorporation rates were significantly higher for algal‐derived DOM. The two DOM sources were also processed differently by the sponge holobiont. Algal‐derived DOM was incorporated into bacteria‐specific PLFAs at a higher rate while coral‐derived DOM was more readily incorporated into sponge‐specific PLFAs. A substantial fraction of the dissolved organic carbon (C) and nitrogen (N) assimilated by the sponges was subsequently converted into and released as particulate detritus (15–24% C and 27–49% N). However, algal‐derived DOM was released as detritus at a higher rate. The higher uptake and transformation rates of algal‐ compared with coral‐derived DOM suggest that reef community phase shifts from coral to algal dominance may stimulate DOM cycling through the sponge loop with potential consequences for coral reef biogeochemical cycles and food webs.

Bottom-water oxygen supply is a key factor governing the biogeochemistry and community composition of marine sediments. Whether it also determines carbon burial rates remains controversial. We investigated the effect of varying oxygen concentrations (170 to 0 mM O 2 ) on microbial remineralization of organic matter in seafloor sediments and on community diversity of the northwestern Crimean shelf break. This study shows that 50% more organic matter is preserved in surface sediments exposed to hypoxia compared to oxic bottom waters. Hypoxic conditions inhibit bioturbation and decreased remineralization rates even within short periods of a few days. These conditions led to the accumulation of threefold more phytodetritus pigments within 40 years compared to the oxic zone. Bacterial community structure also differed between oxic, hypoxic, and anoxic zones. Functional groups relevant in the degradation of particulate organic matter, such as Flavobacteriia, Gammaproteobacteria, and Deltaproteobacteria, changed with decreasing oxygenation, and the microbial community of the hypoxic zone took longer to degrade similar amounts of deposited reactive matter. We conclude that hypoxic bottom-water conditions-even on short time scales-substantially increase the preservation potential of organic matter because of the negative effects on benthic fauna and particle mixing and by favoring anaerobic processes, including sulfurization of matter. INTRODUCTIONMarine sediments preserve only <1% of the primary produced organic matter because of its efficient remineralization in the water column and on the seafloor by fauna and microorganisms (1). Over geological time scales, the burial rate of organic matter affects the global carbon and oxygen cycle; thus, key questions remain as to the environmental factors that alter faunal and microbial transformation of deposited organic matter. One such factor apparently controlling burial and efficiency of organic carbon degradation is bottom-water oxygen concentration (1-4). Low oxygen supply at the seafloor promotes the accumulation of organic matter in sediments, but the underlying mechanisms for this effect are still not fully elucidated. Previous investigations have compared the effects of oxygen on organic matter degradation rates by assessing oxic versus anoxic conditions or oscillations of both in the field and laboratory (5-10) and by global data syntheses and modeling [(3, 11) and references therein)]. Because of the increasing spread of hypoxia, it is important to understand and to quantify the consequences of low oxygen supply for marine life, ecosystem function, and biogeochemical cycles (12). Hypoxic conditions are defined as oxygen concentrations (<63 mM O 2 ) known to affect faunal physiology, community structure, and ecosystem function (10).The inhibition of faunal activity has been proposed as a key factor in hypoxia-induced organic matter accumulation (10). By dwelling in surface sediments, benthic fauna can actively mix oxygen and fresh organic deposits with deeper anoxic ...

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