Coral-excavating sponges are the most important bioeroders on Caribbean reefs and increase in abundance throughout the region. This increase is commonly attributed to a concomitant increase in food availability due to eutrophication and pollution. We therefore investigated the uptake of organic matter by the two coral-excavating sponges Siphonodictyon sp. and Cliona delitrix and tested whether they are capable of consuming dissolved organic carbon (DOC) as part of their diet. A device for simultaneous sampling of water inhaled and exhaled by the sponges was used to directly measure the removal of DOC and bacteria in situ. During a single passage through their filtration system 14% and 13% respectively of the total organic carbon (TOC) in the inhaled water was removed by the sponges. 82% (Siphonodictyon sp.; mean±SD; 13±17 μmol L⁻¹) and 76% (C. delitrix; 10±12 μmol L⁻¹) of the carbon removed was taken up in form of DOC, whereas the remainder was taken up in the form of particulate organic carbon (POC; bacteria and phytoplankton) despite high bacteria retention efficiency (72±15% and 87±10%). Siphonodictyon sp. and C. delitrix removed DOC at a rate of 461±773 and 354±562 μmol C h⁻¹ respectively. Bacteria removal was 1.8±0.9×10¹⁰ and 1.7±0.6×10¹⁰ cells h⁻¹, which equals a carbon uptake of 46.0±21.2 and 42.5±14.0 μmol C h⁻¹ respectively. Therefore, DOC represents 83 and 81% of the TOC taken up by Siphonodictyon sp. and C. delitrix per hour. These findings suggest that similar to various reef sponges coral-excavating sponges also mainly rely on DOC to meet their carbon demand. We hypothesize that excavating sponges may also benefit from an increasing production of more labile algal-derived DOC (as compared to coral-derived DOC) on reefs as a result of the ongoing coral-algal phase shift.

Turf algae increasingly dominate benthic communities on coral reefs. Given their abundance and high dissolved organic carbon (DOC) release rates, turf algae are considered important contributors to the DOC pool on modern reefs. The release of photosynthetically fixed carbon as DOC generally, but not always, increases with increased light availability. Nutrient availability was proposed as an additional factor to explain these conflicting observations. To address this proposed but untested hypothesis, we documented the interactive contributions of light and nutrient availability on the release of DOC by turf algae. DOC release rates and oxygen production were quantified in incubation experiments at two light levels (full and reduced light) and two nutrient treatments (natural seawater and enriched seawater). In natural seawater, DOC release at full light was four times higher than at reduced light. When nutrients were added, DOC release rates at both light levels were similar to the natural seawater treatment at full light. Our results therefore show that low light in combination with low nutrient availability reduces the release of DOC by turf algae and that light and nutrient availability interactively determine DOC release rates by this important component of Caribbean reef communities.

Sponges are ubiquitous components of various deep‐sea habitats, including cold water coral reefs, and form deep‐sea sponge grounds. Although the deep sea is generally considered to be a food‐limited environment, these ecosystems are known to be hotspots of biodiversity and carbon cycling. To assess the role of sponges in the carbon cycling of deep‐sea ecosystems, we studied the carbon budgets of six dominant deep‐sea sponges of different phylogenetic origin, with various growth forms and hosting distinct associated microbial communities, in an ex situ aquarium setup. Additionally, we determined biomass metrics—planar surface area, volume, wet weight, dry weight (DW), ash‐free dry weight, and organic carbon (C) content—and conversion factors for all species. Oxygen (O₂) removal rates averaged 3.3 ± 2.8 μmol O₂ g DW_sponge h⁻¹ (mean ± SD), live particulate (bacterio‐ and phytoplankton) organic carbon removal rates averaged 0.30 ± 0.39 μmol C g DW_sponge h⁻¹ and dissolved organic carbon (DOC) removal rates averaged 18.70 ± 25.02 μmol C g DW_sponge h⁻¹. Carbon mass balances were calculated for four species and revealed that the sponges acquired 1.3–6.6 times the amount of carbon needed to sustain their minimal respiratory demands. These results indicate that irrespective of taxonomic class, growth form, and abundance of microbial symbionts, DOC is responsible for over 90% of the total net organic carbon removal of deep‐sea sponges and allows them to sustain themselves in otherwise food‐limited environments on the ocean floor.

Reef-building corals are endangered animals with a complex colonial organization. Physiological mechanisms connecting multiple polyps and integrating them into a coral colony are still enigmatic. Using live imaging, particle tracking, and mathematical modeling, we reveal how corals connect individual polyps and form integrated polyp groups via species-specific, complex, and stable networks of currents at their surface. These currents involve surface mucus of different concentrations, which regulate joint feeding of the colony. Inside the coral, within the gastrovascular system, we expose the complexity of bidirectional branching streams that connect individual polyps. This system of canals extends the surface area by 4-fold and might improve communication, nutrient supply, and symbiont transfer. Thus, individual polyps integrate via complex liquid dynamics on the surface and inside the colony.

Seamounts represent ideal systems to study the influence and interdependency of environmental gradients at a single geographic location. These topographic features represent a prominent habitat for various forms of life, including microbiota and macrobiota, spanning benthic as well as pelagic organisms. While it is known that seamounts are globally abundant structures, it still remains unclear how and to which extent the complexity of the sea floor is intertwined with the local oceanographic mosaic, biogeochemistry, and microbiology of a seamount ecosystem. Along these lines, the present study aimed to explore whether and to what extent seamounts can have an imprint on the microbial community composition of seawater and of sessile benthic invertebrates, sponges. For our high-resolution sampling approach of microbial diversity (16S rRNA gene amplicon sequencing) along with measurements of inorganic nutrients and other biogeochemical parameters, we focused on the Schulz Bank seamount ecosystem, a sponge ground ecosystem which is located on the Arctic Mid-Ocean Ridge. Seawater samples were collected at two sampling depths (mid-water, MW, and near-bed water, BW) from a total of 19 sampling sites. With a clustering approach we defined microbial microhabitats within the pelagic realm at Schulz Bank, which were mapped onto the seamount's topography and related to various environmental parameters (such as suspended particulate matter, SPM; dissolved inorganic carbon, DIC; silicate, ${\mathrm{SiO}}_{\mathrm{4}}^{-}$ ; phosphate, ${\mathrm{PO}}_{\mathrm{4}}^{\mathrm{3}-}$ ; ammonia, ${\mathrm{NH}}_{\mathrm{4}}^{+}$ ; nitrate, ${\mathrm{NO}}_{\mathrm{3}}^{\mathrm{2}-}$ ; nitrite, ${\mathrm{NO}}_{\mathrm{2}}^{-}$ ; depth; and dissolved oxygen, O₂). The results of our study reveal a “seamount effect” (sensu stricto) on the microbial mid-water pelagic community at least 200 m above the sea floor. Further, we observed a strong spatial heterogeneity in the pelagic microbial landscape across the seamount, with planktonic microbial communities reflecting oscillatory and circulatory water movements, as well as processes of bentho-pelagic coupling. Depth, ${\mathrm{NO}}_{\mathrm{3}}^{\mathrm{2}-}$ , ${\mathrm{SiO}}_{\mathrm{4}}^{-}$ , and O₂ concentrations differed significantly between the determined pelagic microbial clusters close to the sea floor (BW), suggesting that these parameters were presumably linked to changes in microbial community structures. Secondly, we assessed the associated microbial community compositions of three sponge species along a depth gradient of the seamount. While sponge-associated microbial communities were found to be mainly species-specific, we also detected significant intra-specific differences between individuals, depending on the pelagic near-bed cluster they originated from. The variable microbial phyla (i.e. phyla which showed significant differences across varying depth, ${\mathrm{NO}}_{\mathrm{3}}^{\mathrm{2}-}$ , ${\mathrm{SiO}}_{\mathrm{4}}^{-}$ , O₂ concentrations, and different from local seawater communities) were distinct for every sponge species when considering average abundances per species. Variable microbial phyla included representatives of both those taxa traditionally counted for the variable community fraction and taxa counted traditionally for the core community fraction. Microbial co-occurrence patterns for the three examined sponge species Geodia hentscheli, Lissodendoryx complicata, and Schaudinnia rosea were distinct from each other. Over all, this study shows that topographic structures such as the Schulz Bank seamount can have an imprint (seamount effect sensu lato) on both the microbial community composition of seawater and sessile benthic invertebrates such as sponges by an interplay between the geology, physical oceanography, biogeochemistry, and microbiology of seamounts.

The increased release of dissolved organic matter (DOM) by algae has been associated with the fast but inefficient growth of opportunistic microbial pathogens and the ongoing degradation of coral reefs. Turf algae (consortia of microalgae and macroalgae commonly including cyanobacteria) dominate benthic communities on many reefs worldwide. Opposite to other reef algae that predominantly release DOM during the day, turf algae containing cyanobacteria may additionally release large amounts of DOM at night. However, this night‐DOM release and its potential contribution to the microbialization of reefs remains to be investigated. We first tested the occurrence of hypoxic conditions at the turf algae–water interface, as a lack of oxygen will facilitate the production and release of fermentation intermediates as night‐time DOM. Second, the dissolved organic carbon (DOC) release by turf algae was quantified during day time and nighttime, and the quality of day and night exudates as food for bacterioplankton was tested. Finally, DOC release rates of turf algae were combined with estimates of DOC release based on benthic community composition in 1973 and 2013 to explore how changes in benthic community composition affected the contribution of night‐DOC to the reef‐wide DOC production. A rapid shift from supersaturated to hypoxic conditions at the turf algae–water interface occurred immediately after the onset of darkness, resulting in night‐DOC release rates similar to those during daytime. Bioassays revealed major differences in the quality between day and night exudates: Night‐DOC was utilized by bacterioplankton two times faster than day‐DOC, but yielded a four times lower growth efficiency. Changes in benthic community composition were estimated to have resulted in a doubling of DOC release since 1973, due to an increasing abundance of benthic cyanobacterial mats (BCMs), with night‐DOC release by BCMs and turf algae accounting for >50% of the total release over a diurnal cycle. Night‐DOC released by BCMs and turf algae is likely an important driver in the microbialization of reefs by stimulating microbial respiration at the expense of energy and nutrient transfer to higher trophic levels via the microbial loop, thereby threatening the productivity and biodiversity of these unique ecosystems. Read the free Plain Language Summary for this article on the Journal blog.