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1. Nitrogen isotope ratios across the Bermuda coral reef: implications for coral nitrogen sources and the coral-bound nitrogen isotope proxy

   https://doi.org/10.3389/fmars.2025.1554418  

   Luu, Victoria H; Ryu, Yeongjun; Darling, Wren S; Oleynik, Sergey; de_Putron, Samantha J; Cohen, Anne L; Wang, Xingchen Tony; Sigman, Daniel M (March 2025, Frontiers in Marine Science)

   Hajime, Kayanne (Ed.)

   The nitrogen (N) isotopic composition of coral tissue provides insight into N sources and cycling on reefs, and coral skeleton-bound organic matter (CS-δ¹⁵N) can extend these insights into the past. Across the Bermuda platform, we measured the δ¹⁵N of four coral species and their potential N sources, as well as an asymbiotic filter feeder as a comparative heterotroph and benthic macroalgae as a comparative autotroph. Organisms and organic N pools from the coral reefs exhibit a δ¹⁵N increase toward the Bermuda coast, likely due to anthropogenic N inputs. At all sites, the δ¹⁵N of bulk coral tissue is consistent with corals feeding dominantly on zooplankton-sized organic matter and some smaller suspended particulate N. The corals lack the trophic δ¹⁵N elevation that characterizes serpulids; this is consistent with internal recycling and retention of low-δ¹⁵N metabolic N by symbiont-bearing corals. The data are inconsistent with corals’ reliance on the dissolved inorganic N used by macroalgae at the same sites. Among coral species, two species with smaller polyps (1-2 mm) have ~1‰ lower bulk tissue δ¹⁵N than two counterparts with larger polyps (5-10 mm), perhaps due to differences in food source. Taxon-specific δ¹⁵N differences are also observed between coral tissue and skeleton-bound N, with larger differences in the two small-polyp species. In net, however, CS-δ¹⁵N mean values and spatial gradients were similar in the four species studied. 

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2. Symbiosis maintenance in the facultative coral, Oculina arbuscula, relies on nitrogen cycling, cell cycle modulation, and immunity

   https://doi.org/10.1038/s41598-021-00697-6  

   Rivera, H. E.; Davies, S. W. (December 2021, Scientific Reports)

   Abstract Symbiosis with unicellular algae in the family Symbiodiniaceae is common across tropical marine invertebrates. Reef-building corals offer a clear example of cellular dysfunction leading to a dysbiosis that disrupts entire ecosystems in a process termed coral bleaching. Due to their obligate symbiotic relationship, understanding the molecular underpinnings that sustain this symbiosis in tropical reef-building corals is challenging, as any aposymbiotic state is inherently coupled with severe physiological stress. Here, we leverage the subtropical, facultatively symbiotic and calcifying coral Oculina arbuscula to investigate gene expression differences between aposymbiotic and symbiotic branches within the same colonies under baseline conditions. We further compare gene ontology (GO) and KOG enrichment in gene expression patterns from O. arbuscula with prior work in the sea anemone Exaiptasia pallida (Aiptasia) and the salamander Ambystoma maculatum —both of which exhibit endophotosymbiosis with unicellular algae. We identify nitrogen cycling, cell cycle control, and immune responses as key pathways involved in the maintenance of symbiosis under baseline conditions. Understanding the mechanisms that sustain a healthy symbiosis between corals and Symbiodiniaceae algae is of urgent importance given the vulnerability of these partnerships to changing environmental conditions and their role in the continued functioning of critical and highly diverse marine ecosystems. 

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3. Stable isotopes disentangle niche partitioning and co‐occurrence in a multi‐species marine mutualism

   https://doi.org/10.1111/oik.10553  

   Titus, Benjamin M; Froehlich, Catheline_Y M; Vondriska, Clayton; Baker, Ronald; Caves, Eleanor M (February 2025, Oikos)

   Ecologists have long sought general explanations for the co‐occurrence of ecologically similar taxa. Niche theory explains co‐occurrence via functional differences among taxa that reduce competition and promote resource partitioning. Alternatively, the unified neutral theory of biodiversity and biogeography suggests that co‐occurrence can be attributed to stochastic processes, and thus, presupposes that ecologically similar species that occur in sympatry are functionally analogous. We test these hypotheses using the most diverse crustacean‐sea anemone symbiosis from coral reefs in the Tropical Western Atlantic. δ¹³C and δ¹⁵N stable isotope analyses of six crustacean symbionts that co‐occur around the host anemoneBartholomea annulataexhibit highly differentiated isotopic niche space spanning several trophic levels. As multiple crustacean species within the symbiosis have been documented as cleaners that remove parasites from reef fishes, we extended our investigation into the broader cleaner community. Our stable isotope analyses of cleaners shows that Pederson's cleaner shrimpAncylomenes pedersoniexhibits the highest δ¹⁵N isotopic values‐ significantly higher than all other putative cleaner species and consistent with expectations of a dedicated cleaning lifestyle. However, for other species previously described or observed to clean reef fishes, includingPericlimenes yucatanicus, Stenopus hispidusandStenorhynchus seticornis,δ¹⁵N isotopic values were substantially lower, raising questions about the degree to which these species rely on cleaning interactions to meet their nutritional requirements. Taken together, our data are consistent with the expectations of niche theory: co‐occurring symbiotic crustaceans have highly partitioned niche space with low levels of functional redundancy. 

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4. Sponge-derived matter is assimilated by coral holobionts

   https://doi.org/10.1038/s42003-024-05836-z  

   Reigel, Alicia M.; Easson, Cole G.; Apprill, Amy; Freeman, Christopher J.; Bartley, Michaela M.; Fiore, Cara L. (February 2024, Communications Biology)

   Abstract Coral reef biodiversity is maintained by a complex network of nutrient recycling among organisms. Sponges assimilate nutrients produced by other organisms like coral and algae, releasing them as particulate and dissolved matter, but to date, only a single trophic link between sponge-derived dissolved matter and a macroalgae has been identified. We sought to determine if sponge-coral nutrient exchange is reciprocal using a stable isotope ‘pulse-chase’ experiment to trace the uptake of¹³C and¹⁵N sponge-derived matter by the coral holobiont for three coral species (Acropora cervicornis, Orbicella faveolata, andEunicea flexuosa). Coral holobionts incorporated 2.3–26.8x more¹⁵N than¹³C from sponge-derived matter andA. cervicornisincorporated more of both C and N than the other corals. Differential isotopic incorporation among coral species aligns with their ecophysiological characteristics (e.g., morphology, Symbiodiniaceae density). Our results elucidate a recycling pathway on coral reefs that has implications for improving coral aquaculture and management approaches. 

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5. Stable isotope tracing reveals compartmentalized nitrogen assimilation in scleractinian corals

   https://doi.org/10.3389/fmars.2022.1035523  

   Chiles, Eric N.; Huffmyer, Ariana S.; Drury, Crawford; Putnam, Hollie M.; Bhattacharya, Debashish; Su, Xiaoyang (November 2022, Frontiers in Marine Science)

   Corals form symbiotic relationships with dinoflagellate algae of the family Symbiodiniaceae, bacteria, and other microbes. Central to that relationship is the regulation of nutrition flux between the animal host and the photosynthetic Symbiodiniaceae that it is reliant on for the majority of metabolic needs. Nitrogen availability controls the growth and density of Symbiodiniaceae within coral tissues and has been proposed to play a role in host derived symbiosis regulation. Warming ocean temperatures and subsequent increases in dissolved organic carbon can potentially increase nitrogen fixation and lead to bleaching. We investigated the importance of nitrogen metabolism in vivo with LC-MS based stable isotope tracing using nubbins from three species of Hawaiian coral, the more heat tolerant Montipora capitata and Porites compressa and the more heat sensitive Pocillopora acuta , that were collected from reefs in Kāne’ohe Bay, O’ahu. In addition to 15 N incorporation into nucleotides, amino acids, and urea cycle metabolites, we also observed significant isotopic labeling in dipeptides, supporting their previous identification as major heat stress response metabolites. Surprisingly, the dipeptides are highly enriched in 15 N compared to free amino acids, which are the biosynthetic precursors for dipeptides. This suggests that there is a high turnover of dipeptide pools and distinct biosynthetic mechanisms that separately mediate amino acid and dipeptide production. These preliminary data show that nitrogen assimilation in the coral holobiont is likely compartmentalized, with rapid assimilation and quick dipeptide turnover occurring in one region of the holobiont and slow turnover of other nitrogen containing metabolites in other region(s). 

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