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1. Amino acid carbon isotope fingerprints are unique among eukaryotic microalgal taxonomic groups

   https://doi.org/10.1002/lno.12350  

   Stahl, Angela R. ; Rynearson, Tatiana A. ; McMahon, Kelton W. ( April 2023 , Limnology and Oceanography)

   Eukaryotic microalgae play critical roles in the structure and function of marine food webs. The contribution of microalgae to food webs can be tracked using compound‐specific isotope analysis of amino acids (CSIA‐AA). Previous CSIA‐AA studies have defined eukaryotic microalgae as a single functional group in food web mixing models, despite their vast taxonomic and ecological diversity. Using controlled cultures, this work characterizes the amino acidδ¹³C (δ¹³C_AA) fingerprints—a multivariate metric of amino acid carbon isotope values—of four major groups of eukaryotic microalgae: diatoms, dinoflagellates, raphidophytes, and prasinophytes. We found excellent separation of essential amino acidδ¹³C (δ¹³C_EAA) fingerprints among four microalgal groups (mean posterior probability reclassification of 99.2 ± 2.9%). We also quantified temperature effects, a primary driver of microalgal bulk carbon isotope variability, on the fidelity ofδ¹³C_AAfingerprints. A 10°C range in temperature conditions did not have significant impacts on variance inδ¹³C_AAvalues or the diagnostic microalgalδ¹³C_EAAfingerprints. Theseδ¹³C_EAAfingerprints were used to identify primary producers at the base of food webs supporting consumers in two contrasting systems: (1) penguins feeding in a diatom‐based food web and (2) mixotrophic corals receiving amino acids directly from autotrophic endosymbiotic dinoflagellates and indirectly from water column diatoms, prasinophytes, and cyanobacteria, likely via heterotrophic feeding on zooplankton. The increased taxonomic specificity of CSIA‐AA fingerprints developed here will greatly improve future efforts to reconstruct the contribution of diverse eukaryotic microalgae to the sources and cycling of organic matter in food web dynamics and biogeochemical cycling studies.

    

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2. Trophic ecology of Caribbean sponges in the mesophotic zone

   https://doi.org/10.1002/lno.11668  

   Macartney, Keir J. ; Slattery, Marc ; Lesser, Michael P. ( December 2020 , Limnology and Oceanography)

   Sponges are a crucial component of Caribbean coral reef ecosystem structure and function. In the Caribbean, many sponges show a predictable increase in percent cover or abundance as depth increases from shallow (< 30 m) to mesophotic (30–150 m) depths. Given that sponge abundances are predicted to increase in the Caribbean as coral cover declines, understanding ecological factors that control their distribution is critical. Here we assess if sponge cover increases as depth increases into the mesophotic zone for three common Caribbean reef sponges,Xestospongia muta,Agelas tubulata, andPlakortis angulospiculatus, and use stable isotope analyses to determine whether shifts in trophic resource utilization along a shallow to mesophotic gradient occurred. Ecological surveys show that all target sponges significantly increase in percent cover as depth increases. Using bulk stable isotope analysis, we show that as depth increases there are increases in the δ¹³C and δ¹⁵N values, reflecting that all sponges consumed more heterotrophic picoplankton, with low C:N ratios in the mesophotic zone. However, compound‐specific isotope analysis of amino acids (CSIA‐AA) shows that there are species‐specific increases in δ¹³C_AAand δ¹⁵N_AAvalues.Xestospongia mutaandP. angulospiculatusshowed a reduced reliance on photoautotrophic resources as depth increased, whileA. tubulataappears to rely on heterotrophy at all depths. The δ¹³C_AAand δ¹⁵N_AAvalues of these sponges also reflect species‐specific patterns of host utilization of both POM and dissolved organic matter (DOM), its subsequent re‐synthesis, and translocation, by their microbiomes.

    

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3. Mesophotic corals in Hawai‘i maintain autotrophy to survive low-light conditions

   https://doi.org/10.1098/rspb.2023.1534  

   Backstrom, Callum H ; Padilla-Gamiño, Jacqueline L ; Spalding, Heather L ; Roth, Melissa S ; Smith, Celia M ; Gates, Ruth D ; Rodrigues, Lisa J ( February 2024 , Proceedings of the Royal Society B: Biological Sciences)

   In mesophotic coral ecosystems, reef-building corals and their photosynthetic symbionts can survive with less than 1% of surface irradiance. How depth-specialist corals rely upon autotrophically and heterotrophically derived energy sources across the mesophotic zone remains unclear. We analysed the stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values of aLeptoseriscommunity from the ‘Au‘au Channel, Maui, Hawai‘i (65–125 m) including four coral host species living symbiotically with three algal haplotypes. We characterized the isotope values of hosts and symbionts across species and depth to compare trophic strategies. Symbiontδ¹³C was consistently 0.5‰ higher than hostδ¹³C at all depths. Mean colony host and symbiontδ¹⁵N differed by up to 3.7‰ at shallow depths and converged at deeper depths. These results suggest that both heterotrophy and autotrophy remained integral to colony survival across depth. The increasing similarity between host and symbiontδ¹⁵N at deeper depths suggests that nitrogen is more efficiently shared between mesophotic coral hosts and their algal symbionts to sustain autotrophy. Isotopic trends across depth did not generally vary by host species or algal haplotype, suggesting that photosynthesis remains essential toLeptoserissurvival and growth despite low light availability in the mesophotic zone.

    

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4. Lipid normalization and stable isotope discrimination in Pacific walrus tissues

   https://doi.org/10.1038/s41598-019-42095-z  

   Clark, Casey T. ; Horstmann, Lara ; Misarti, Nicole ( April 2019 , Scientific Reports)

   Analysis of stable carbon and nitrogen isotope values (δ¹³C and δ¹⁵N) of animal tissues can provide important information about diet, physiology, and movements. Interpretation of δ¹³C and δ¹⁵N values, however, is influenced by factors such as sample lipid content, tissue-specific isotope discrimination, and tissue turnover rates, which are typically species- and tissue-specific. In this study, we generated lipid normalization models for δ¹³C and investigated the effects of chemical lipid extractions on δ¹³C and δ¹⁵N in Pacific walrus (Odobenus rosmarus divergens) muscle, liver, and skin. We also evaluated tissue-specific isotope discrimination in walrus muscle, liver, skin, and bone collagen. Mean δ¹³C_lipid-freeof skin and bone collagen were similar, as were mean δ¹⁵N of muscle and liver. All other tissues differed significantly for both isotopes. Differences in δ¹³C_lipid-freeand δ¹⁵N among tissues agreed with published estimates of marine mammal tissue-specific isotope discrimination factors, with the exception of skin. The results of this work will allow researchers to gain a clearer understanding of walrus diet and the structure of Arctic food webs, while also making it possible to directly compare the results of contemporary walrus isotope research with those of historic and paleoecological studies.

    

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5. Coral trophic pathways impact the allocation of carbon and nitrogen for egg development after bleaching

   https://doi.org/10.3389/fevo.2023.1251220  

   Jaffe, Melissa D ; Padilla-Gamiño, Jacqueline L ; Nunn, Brook L ; Rodrigues, Lisa J ( November 2023 , Frontiers in Ecology and Evolution)

   The persistence of coral reefs requires the survival of adult coral colonies and their continued sexual reproduction despite thermal stress. To assess the trophic pathway (i.e., autotrophy and/or heterotrophy) used to develop gametes following bleaching, we thermally stressedMontipora capitatafor one month at a time when corals in Hawai’i typically experience elevated seawater temperatures. After six and nine months of recovery, we pulse-chased non-bleached and previously bleached colonies using a dual-label design to compare the allocation of carbon and nitrogen at significant stages of gamete development. Dissolved inorganic carbon- (DI¹³C) and nitrogen- (DI¹⁵N) labelled seawater or¹³C- and¹⁵N-labelled rotifers were used to assess the autotrophic and heterotrophic pathways, respectively. At multiple time points for up to two years later, we collected adult coral fragments and isolated host tissue, Symbiodiniaceae cells, and developing eggs and captured gamete bundles to analyze their carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes. We found that the presence of Symbiodiniaceae was important for gametogenesis in both non-bleached and previously bleached colonies in two main ways. First, autotrophically-acquired carbon and nitrogen were both allocated to gametes during development, suggesting that recovery of photosynthesis after bleaching is critical for gametogenesis. Second, only heterotrophically-acquired nitrogen, not carbon, was incorporated into gametes and was readily recycled between host tissues and Symbiodiniaceae cells. This suggests that one of the purposes of heterotrophy following coral bleaching forM. capitatamay be to supplement the nitrogen pool, providing available nutrients for endosymbiotic algal growth. Allocation of carbon and nitrogen to eggs coincided with the period when vertical transmission of symbionts to gametes occurs, further supporting the important relationship between gametogenesis and availability of Symbiodiniaceae forM. capitata.

    

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