Dark CO2 fixation into phospholipid-derived fatty acids by the cold-water coral associated sponge Hymedesmia (Stylopus) coriacea (Tisler Reef, NE Skagerrak)

Many cold-water sponges harbour microorganisms of which the role in the sponge host remains enigmatic. Here, we show a transfer of fixed inorganic carbon by sponge-associated microbes to its host, the cold-water coral encrusting sponge Hymedesmia (Stylopus) coriacea. Sponge were collected at approx. 100 m depth and incubated for 1.5–2.5 days with 13C labelled dissolved inorganic carbon (DIC) as tracer. Total DIC fixation rates ranged from 0.03–0.11 mmol C × mmol Csponge × d−1. 13C-tracer was recovered in bacterial-specific (i.e. short and branched) and sponge-specific (very long-chained) phospholipid-derived fatty acids (PLFA's), but was not incorporated into archaeal lipids. 13C-incorporation in biomarkers such as C16:1w7c and C18:1w7c indicated that nitrifying and/or sulphur-oxidizing bacteria (chemoautotrophs) were likely active in the sponge. Trophic transfer of microbially-fixed carbon to the sponge host was confirmed by recovery of label in very long chain fatty acids (VLCFA's) including C26:2 and C26:3. Tracer accumulation into several VLCFA's continued after removal of 13C-DIC, while tracer in most bacteria-specific PLFA's declined, indicating a transfer and elongation of bacterial-specific PLFA's to sponge-specific PLFA's. This implies that PLFA precursors released from chemo- as well as heterotrophic microbes in sponges contributed to the synthesis of VLCFA's, identifying sponge-associated bacteria as symbionts of the sponge

DOC concentrations across a depth-dependent light gradient on a Caribbean coral reef

Photosynthates released by benthic primary producers (BPP), such as reef algae and scleractinian corals, fuel the dissolved organic carbon (DOC) production on tropical coral reefs. DOC concentrations near BPP have repeatedly been observed to be elevated compared to those in the surrounding water column. As the DOC release of BPP increases with increasing light availability, elevated DOC concentrations near them will, in part, also depend on light availability. Consequently, DOC concentrations are likely to be higher on the shallow, well-lit reef terrace than in deeper sections on the fore reef slope. We measured in situ DOC concentrations and light intensity in close proximity to the reef alga Dictyota sp. and the scleractinian coral Orbicella faveolata along a depth-dependent light gradient from 5 to 20 m depth and compared these to background concentrations in the water column. At 10 m (intermediate light),DOC concentrations near Dictyota sp. were elevated by 15 mmol C L􀀀1 comparedto background concentrations in the water column, but not at 5 and 20 m (highand low light, respectively), or near O. faveolata at any of the tested depths. DOCconcentrations did not differ between depths and thereby light environments forany of the tested water types. However, water type and depth appear to jointlyaffect in situ DOC concentrations across the tested depth-dependent light gradient. Corroborative ex situ measurements of excitation pressure on photosystem II suggest that photoinhibition in Dictyota sp. is likely to occur at light intensities that are commonly present on Curacaoan coral reefs under high light levels at 5 m depth during midday. Photoinhibition may have thereby reduced the DOC release of Dictyota sp. and DOC concentrations in its close proximity. Our results indicate that the occurrence of elevated DOC concentrations did not follow a natural light gradiënt across depth. Instead, a combination of multiple factors, such as water type, light availability (including the restriction by photoinhibition), and water movement are proposed to interactively determine the DOC concentrations in the close vicinity ofBPP

Spatio-temporal variation in stable isotope signatures (δ¹³C and δ¹⁵N) of sponges on the Saba Bank

Sponges are ubiquitous on coral reefs, mostly long lived and therefore adaptive to changing environmental conditions. They feed on organic matter withdrawn from the passing water and they may harbor microorganisms (endosymbionts), which contribute to their nutrition. Their diets and stable isotope (SI) fractionation determine the SI signature of the sponge holobiont. Little is known of spatio- temporal variations in SI signatures of δ13C and δ15N in tropical sponges and whether they reflect variations in the environment. We investigated the SI signatures of seven common sponge species with different functional traits and their potential food sources between 15 and 32 m depth along the S-SE and E-NE side of the Saba Bank, Eastern Caribbean, in October 2011 and October 2013. SI signatures differed significantly between most sponge species, both in mean values and in variation, indicating different food preferences and/or fractionation, inferring sponge species-specific isotopic niche spaces. In 2011, all sponge species at the S-SE side were enriched in δ13C compared to the E-NE side. In 2013, SI signatures of sponges did not differ between the two sides and were overall lighter in δ13C and δ15N than in 2011. Observed spatio-temporal changes in SI in sponges could not be attributed to changes in the SI signatures of their potential food sources, which remained stable with different SI signatures of pelagic (particulate organic matter (POM): δ13C -24.9‰, δ15N +4.3‰) and benthic-derived food (macroalgae: δ13C -15.4‰, δ15N +0.8‰). Enriched δ13C signatures in sponges at the S-SE side in 2011 are proposed to be attributed to predominantly feeding on benthic-derived C. This interpretation was supported by significant differences in water mass constituents between sides in October 2011. Elevated NO3 and dissolved organic matter concentrations point toward a stronger reef signal in reef overlying water at the S-SE than N-NE side of the Bank in 2011. The depletions of δ13C and δ15N in sponges in October 2013 compared to October 2011 concurred with significantly elevated POM concentrations. The contemporaneous decrease in δ15N suggests that sponges obtain their N mostly from benthic-derived food with a lower δ15N than pelagic food. Average proportional feeding on available sources varied between sponge species and ranged from 20% to 50% for benthic and 50% to 80% for pelagic-derived food, assuming trophic enrichment factors of 0.5‰± sd 0.5 for δ13C and 3‰ ± sd 0.5 for δ15N for sponges. We suggest that observed variation of SI in sponges between sides and years were the result of shifts in the proportion of ingested benthic- and pelagic-derived organic matter driven by environmental changes. We show that sponge SI signatures reflect environmental variability in space and time on the Saba Bank and that SI of sponges irrespective of their species-specific traits move in a similar direction in response to these environmental changes.</p

Wave-swept coralliths of Saba Bank, Dutch Caribbean

During a recent reef coral survey at the submarine Saba Bank (Eastern Caribbean), an uncommon and diverse assemblage of unattached scleractinian corals (coralliths) was encountered, which has not been reported from the Atlantic before. Four different types of these free-living (unattached) corals were distinguished. They were observed on a relatively flat seafloor (15–20 m deep) with poor coral cover and full exposure to oceanic swell. Much of the substratum was not consolidated and consisted mainly of sand and fragments of branching coralline algae. One of the four types is the (1) anthocyathus stage in the life history of the free-living species Manicina areolata and Meandrina danae. The other three are coralliths formed as ecophenotypic varieties: (2) spheroidal–amoeboidal (= globular and (sub)massive) in Porites astreoides, Siderastrea radians, S. siderea, and Stephanocoenia intersepta; (3) tumbleweed-like (= globular and ramose) in Porites divaricata and P. furcata; and (4) discoidal (flat and circular with short branches) in Madracis decactis and possibly in M. cf. auretenra. This assemblage of free-living corals is likely related to a combination of abiotic factors consisting of wave exposure (swell), depths that waves can reach, a horizontal sea floor with little relief, an unconsolidated substratum, and low coral cover

Dark CO2 fixation into phospholipid-derived fatty acids by the cold-water coral associated sponge Hymedesmia (Stylopus) coriacea (Tisler Reef, NE Skagerrak)

Many cold-water sponges harbour microorganisms of which the role in the sponge host remains enigmatic. Here, we show a transfer of fixed inorganic carbon by sponge-associated microbes to its host, the cold-water coral encrusting sponge Hymedesmia (Stylopus) coriacea. Sponge were collected at approx. 100 m depth and incubated for 1.5–2.5 days with 13C labelled dissolved inorganic carbon (DIC) as tracer. Total DIC fixation rates ranged from 0.03–0.11 mmol C × mmol Csponge × d−1. 13C-tracer was recovered in bacterial-specific (i.e. short and branched) and sponge-specific (very long-chained) phospholipid-derived fatty acids (PLFA's), but was not incorporated into archaeal lipids. 13C-incorporation in biomarkers such as C16:1w7c and C18:1w7c indicated that nitrifying and/or sulphur-oxidizing bacteria (chemoautotrophs) were likely active in the sponge. Trophic transfer of microbially-fixed carbon to the sponge host was confirmed by recovery of label in very long chain fatty acids (VLCFA's) including C26:2 and C26:3. Tracer accumulation into several VLCFA's continued after removal of 13C-DIC, while tracer in most bacteria-specific PLFA's declined, indicating a transfer and elongation of bacterial-specific PLFA's to sponge-specific PLFA's. This implies that PLFA precursors released from chemo- as well as heterotrophic microbes in sponges contributed to the synthesis of VLCFA's, identifying sponge-associated bacteria as symbionts of the sponge

Niche overlap between a cold-water coral and an associated sponge for isotopically-enriched particulate food sources

The cold-water coral Lophelia pertusa is an ecosystem engineer that builds reef structures on the seafloor. The interaction of the reef topography with hydrodynamics is known to enhance the supply of suspended food sources to the reef communities. However, the reef framework is also a substrate for other organisms that may compete for the very same suspended food sources. Here, we used the passive suspension feeder Lophelia pertusa and the active suspension feeding sponge Hymedesmia coriacea as model organisms to study niche overlap using isotopically-enriched algae and bacteria as suspended food sources. The coral and the sponge were fed with a combination of 13C-enriched bacteria/15N-enriched algae or 15N-enriched bacteria/13C-enriched algae, which was subsequently traced into bulk tissue, coral skeleton and dissolved inorganic carbon (i.e. respiration). Both the coral and the sponge assimilated and respired the suspended bacteria and algae, indicating niche overlap between these species. The assimilation rates of C and N into bulk tissue of specimens incubated separately were not significantly different from assimilation rates during incubations with co-occurring corals and sponges. Hence, no evidence for exploitative resource competition was found, but this is likely due to the saturating experimental food concentration that was used. We do not rule out that exploitative competition occurs in nature during periods of low food concentrations. Food assimilation and respiration rates of the sponge were almost an order of magnitude higher than those of the cold-water coral. We hypothesize that the active suspension feeding mode of the sponge explains the observed differences in resource uptake as opposed to the passive suspension feeding mode of the cold-water coral. These feeding mode differences may set constraints on suitable habitats for cold-water corals and sponges in their natural habitats

Corrigendum: Long-term Shifts in Coral Communities On Shallow to Deep Reef Slopes of Curaçao and Bonaire: Are There Any Winners?

<p>In the original article ([Front. Mar. Sci., 3, (2016) (247)] doi: 10.3389/fmars.2016.00247) a correction is described taking into account a possible decrease in carbonate production with decreasing light intensities at greater depths. This correction was eventually not applied to the actual data and does not feature in the results or conclusions. The authors sincerely apologize for this unfortunate error and stress that it does not change the scientific conclusions of this article in any way.</p