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1. Complex interactions with nutrients and sediment alter the effects of predation on a reef‐building coral

   https://doi.org/10.1111/maec.12670  

   Rice, Mallory M. ; Baldwin, Daniel G. ; Fischer, Janine N. ; Fuchs, Corinne ; Burkepile, Deron E. ( June 2021 , Marine Ecology)

   Framework‐building corals create the three‐dimensional structure of coral reefs and are subject to predation from fishes, echinoderms, and gastropods. Anthropogenic stressors can magnify the effects of such top‐down pressure on foundation species. The gastropodCoralliophilaviolacea(Kiener, 1836) depletes coral energy reserves via predation, potentially increasing coral susceptibility to land‐based pollution (i.e., sediment accumulation and nutrient pollution). We hypothesized that sedimentation would worsen coral mortality, while nutrient enrichment would mitigate the harmful effects of sediment and predation on coral mortality by increasing the densities of algal symbionts. To test these hypotheses, we conducted in situ surveys of the fringing reefs in Mo'orea, French Polynesia to explore the relationships among massivePoritesspp. cover,C. violaceadensities, and sediment accumulation on coral colonies across low and high nutrient sites. We also conducted a factorial field experiment to test the interactions among these stressors on coral tissue mortality, symbiont densities, and chlorophyll. MassivePoritescolonies at higher nutrient sites hadC. violaceadensities 13 times higher than at low nutrient sites but there was no difference in the amount of live tissue on coral colonies with or without snails among these sites. In our experiment, there were interactions between predation and nutrients as well as nutrients and sediment that impacted coral mortality.more »Sedimentation and predation byC. violaceaincreased coral tissue mortality independently by ~20%. Nutrient enrichment reduced this effect in corals under sedimentation or predation pressure by lowering coral tissue mortality by 18% and increasing algal symbiont densities by ~28%. Our results indicate that sediment does not magnify top‐down pressure on this coral, and that moderate nutrient enrichment may interact with predation in complex, unexpected ways to alter the responses of corals to top‐down pressure.

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2. Differential resistance and acclimation of two coral species to chronic nutrient enrichment reflect life‐history traits

   https://doi.org/10.1111/1365-2435.13780  

   Fox, Michael D. ; Nelson, Craig E. ; Oliver, Thomas A. ; Quinlan, Zachary A. ; Remple, Kristina ; Glanz, Jess ; Smith, Jennifer E. ; Putnam, Hollie M. ; Fox, ed., Charles ( March 2021 , Functional Ecology)

   The effects of nutrient pollution on coral reef ecosystems are multifaceted. Numerous experiments have sought to identify the physiological effects of nutrient enrichment on reef‐building corals, but the results have been variable and sensitive to choices of nutrient quantity, chemical composition and exposure duration.

   To test the effects of chronic, ecologically relevant nutrient enrichment on coral growth and photophysiology, we conducted a 5‐week continuous dosing experiment on two Hawaiian coral species,Porites compressaandPocillopora acuta. We acclimated coral fragments to five nutrient concentrations (0.1–7 µMand 0.06–2.24 µM) with constant stoichiometry 2.5:1 nitrate to phosphate) bracketing in situ observations from reefs throughout the Pacific.

   Nutrient enrichment linearly increased photophysiological performance of both species within 3 weeks. The effect of nutrients onP. acutaphotochemical efficiency increased through time while a consistent response inP. compressaindicated acclimation to elevated nutrients within 5 weeks. Endosymbiont densities and total chlorophyll concentrations also increased proportionally with nutrient enrichment inP. acuta, but not inP. compressa, revealing contrasting patterns of host–symbiont acclimatization.

   The two species also exhibited contrasting effects of nutrient enrichment on skeletal growth. Calcification was enhanced at low nutrient enrichment (1 µM) inP. acuta, but comparable to the control at higher concentrations, whereas calcification was reduced inP. compressa(30%–35%) above 3 µMmore »title='urn:x-wiley:02698463:media:fec13780:fec13780-math-0004'/>.

   Stable isotope analysis revealed species‐specific nitrogen uptake dynamics in the coral–algal symbiosis. The endosymbionts ofP. acutaexhibited increased nitrogen uptake (decreased δ¹⁵N) and incorporation (19%–31% decrease in C:N ratios) across treatments. In contrast,P. compressaendosymbionts maintained constant δ¹⁵N values and low levels of nitrogen incorporation (9%–11% decrease in C:N ratios). The inability ofP. acutato regulate endosymbiont nutrient uptake may indicate an emerging destabilization in the coral–algal symbiosis under nutrient enrichment that could compromise resistance to additional environmental stressors.

   Our results highlight species‐specific differences in the coral–algal symbiosis, which influence responses to chronic nutrient enrichment. These findings showcase how symbioses can vary among closely related taxa and underscore the importance of considering how life‐history traits modify species response to environmental change.

   A freePlain Language Summarycan be found within the Supporting Information of this article.

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3. Empirically derived thermal thresholds of four coral species along the Red Sea using a portable and standardized experimental approach

   https://doi.org/10.1007/s00338-022-02233-y  

   Evensen, Nicolas R. ; Voolstra, Christian R. ; Fine, Maoz ; Perna, Gabriela ; Buitrago-López, Carol ; Cárdenas, Anny ; Banc-Prandi, Guilhem ; Rowe, Katherine ; Barshis, Daniel J. ( March 2022 , Coral Reefs)

   Global warming is causing an unprecedented loss of species and habitats worldwide. This is particularly apparent for tropical coral reefs, with an increasing number of reefs experiencing mass bleaching and mortality on an annual basis. As such, there is a growing need for a standardized experimental approach to rapidly assess the thermal limits of corals and predict the survival of coral species across reefs and regions. Using a portable experimental system, the Coral Bleaching Automated Stress System (CBASS), we conducted standardized 18 h acute thermal stress assays to quantitively determine the upper thermal limits of four coral species across the length of the Red Sea coastline, from the Gulf of Aqaba (GoA) to Djibouti (~ 2100 km). We measured dark-acclimated photosynthetic efficiency (F_v/F_m), algal symbiont density, chlorophyll a, and visual bleaching intensity following heat stress.F_v/F_mwas the most precise response variable assessed, advancing theF_v/F_meffective dose 50 (ED50, i.e., the temperature at which 50% of the initialF_v/F_mis measured) as an empirically derived proxy for thermal tolerance. ED50 thermal thresholds from the central/southern Red Sea and Djibouti populations were consistently higher forAcropora hemprichii, Pocillopora verrucosa,andStylophora pistillata(0.1–1.8 °C above GoA corals, respectively), in line with prevailing warmer maximum monthly means (MMMs), though were lower than GoA coralsmore »relative to site MMMs (1.5–3.0 °C).P. verrucosahad the lowest thresholds overall. Despite coming from the hottest site, thresholds were lowest forPorites lobatain the southern Red Sea, suggesting long-term physiological damage or ongoing recovery from a severe, prior bleaching event. Altogether, the CBASS resolved historical, taxonomic, and possibly recent environmental drivers of variation in coral thermal thresholds, highlighting the potential for a standardized, short-term thermal assay as a universal approach for assessing ecological and evolutionary variation in the upper thermal limits of corals.

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4. Gene Expression of Endangered Coral (Orbicella spp.) in Flower Garden Banks National Marine Sanctuary After Hurricane Harvey

   https://doi.org/doi.org/10.3389/fmars.2019.00672  

   Wright, Rachel M. ; Correa, Adrienne M. ; Quigley, Lucinda A. ; Santiago-Vázquez, Lory Z. ; Shamberger, Kathryn E. ; Davies, Sarah W. ( November 2019 , Frontiers in marine science)

   About 190 km south of the Texas–Louisiana border, the East and West Flower Garden Banks (FGB) have maintained > 50% coral cover with infrequent and minor incidents of disease or bleaching since monitoring began in the 1970s. However, a mortality event, affecting 5.6 ha (2.6% of the area) of the East FGB, occurred in late July 2016 and coincided with storm-generated freshwater runoff extending offshore and over the reef system. To capture the immediate effects of storm-driven freshwater runoff on coral and symbiont physiology, we leveraged the heavy rainfall associated with Hurricane Harvey in late August 2017 by sampling FGB corals at two time points: September 2017, when surface water salinity was reduced (∼34 ppt); and 1 month later when salinity had returned to typical levels (∼36 ppt in October 2017). Tissue samples (N = 47) collected midday were immediately preserved for gene expression profiling from two congeneric coral species (Orbicella faveolata and Orbicella franksi) from the East and West FGB to determine the physiological consequences of storm-derived runoff. In the coral, differences between host species and sampling time points accounted for the majority of differentially expressed genes. Gene ontology enrichment for genes differentially expressed immediately after Hurricane Harvey indicatedmore »increases in cellular oxidative stress responses. Although tissue loss was not observed on FGB reefs following Hurricane Harvey, our results suggest that poor water quality following this storm caused FGB corals to experience sub-lethal stress. We also found dramatic expression differences across sampling time points in the coral’s algal symbiont, Breviolum minutum. Some of these differentially expressed genes may be involved in the symbionts’ response to changing environments, including a group of differentially expressed post-transcriptional RNA modification genes. In this study, we cannot disentangle the effects of reduced salinity from the collection time point, so these expression patterns could also be related to seasonality. These findings highlight the urgent need for continued monitoring of these reef systems to establish a baseline for gene expression of healthy corals in the FGB system across seasons, as well as the need for integrated solutions to manage stormwater runoff in the Gulf of Mexico.« less
5. Virulence as a Side Effect of Interspecies Interaction in Vibrio Coral Pathogens

   https://doi.org/10.1128/mBio.00201-20  

   Rubio-Portillo, Esther ; Martin-Cuadrado, Ana B. ; Caraballo-Rodríguez, Andrés M. ; Rohwer, Forest ; Dorrestein, Pieter C. ; Antón, Josefa ( August 2020 , mBio)

   Dubilier, Nicole (Ed.)

   ABSTRACT The increase in prevalence and severity of coral disease outbreaks produced by Vibrio pathogens, and related to global warming, has seriously impacted reef-building corals throughout the oceans. The coral Oculina patagonica has been used as a model system to study coral bleaching produced by Vibrio infection. Previous data demonstrated that when two coral pathogens ( Vibrio coralliilyticus and Vibrio mediterranei ) simultaneously infected the coral O. patagonica , their pathogenicity was greater than when each bacterium was infected separately. Here, to understand the mechanisms underlying this synergistic effect, transcriptomic analyses of monocultures and cocultures as well as experimental infection experiments were performed. Our results revealed that the interaction between the two vibrios under culture conditions overexpressed virulence factor genes (e.g., those encoding siderophores, the type VI secretion system, and toxins, among others). Moreover, under these conditions, vibrios were also more likely to form biofilms or become motile through induction of lateral flagella. All these changes that occur as a physiological response to the presence of a competing species could favor the colonization of the host when they are present in a mixed population. Additionally, during coral experimental infections, we showed that exposure of corals to molecules released during V.more »coralliilyticus and V. mediterranei coculture induced changes in the coral microbiome that favored damage to coral tissue and increased the production of lyso-platelet activating factor. Therefore, we propose that competition sensing, defined as the physiological response to detection of harm or to the presence of a competing Vibrio species, enhances the ability of Vibrio coral pathogens to invade their host and cause tissue necrosis. IMPORTANCE Vibrio coralliilyticus and Vibrio mediterranei are important coral pathogens capable of inducing serious coral damage, which increases severely when they infect the host simultaneously. This has consequences related to the dispersion of these pathogens among different locations that could enhance deleterious effects on coral reefs. However, the mechanisms underlying this synergistic interaction are unknown. The work described here provides a new perspective on the complex interactions among these two Vibrio coral pathogens, suggesting that coral infection could be a collateral effect of interspecific competition. Major implications of this work are that (i) Vibrio virulence mechanisms are activated in the absence of the host as a response to interspecific competition and (ii) release of molecules by Vibrio coral pathogens produces changes in the coral microbiome that favor the pathogenic potential of the entire Vibrio community. Thus, our results highlight that social cues and competition sensing are crucial determinants of development of coral diseases.« less