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1. Species‐specific responses to climate change and community composition determine future calcification rates of Florida Keys reefs

   https://doi.org/10.1111/gcb.13481  

   Okazaki, Remy R. ; Towle, Erica K. ; van Hooidonk, Ruben ; Mor, Carolina ; Winter, Rivah N. ; Piggot, Alan M. ; Cunning, Ross ; Baker, Andrew C. ; Klaus, James S. ; Swart, Peter K. ; et al ( September 2016 , Global Change Biology)

   Anthropogenic climate change compromises reef growth as a result of increasing temperatures and ocean acidification. Scleractinian corals vary in their sensitivity to these variables, suggesting species composition will influence how reef communities respond to future climate change. Because data are lacking for many species, most studies that model future reef growth rely on uniform scleractinian calcification sensitivities to temperature and ocean acidification. To address this knowledge gap, calcification of twelve common and understudied Caribbean coral species was measured for two months under crossed temperatures (27, 30.3 °C) andCO₂partial pressures (pCO₂) (400, 900, 1300 μatm). Mixed‐effects models of calcification for each species were then used to project community‐level scleractinian calcification using Florida Keys reef composition data andIPCC AR5 ensemble climate model data. Three of the four most abundant species,Orbicella faveolata, Montastraea cavernosa,andPorites astreoides, had negative calcification responses to both elevated temperature andpCO₂. In the business‐as‐usualCO₂emissions scenario, reefs with high abundances of these species had projected end‐of‐century declines in scleractinian calcification of >50% relative to present‐day rates.Siderastrea siderea, the other most common species, was insensitive to both temperature andpCO₂within the levels tested here. Reefs dominated by this species had the most stable end‐of‐century growth. Under more optimistic scenarios of reducedCO₂emissions, calcification rates throughout the Florida Keys declined <20% by 2100. Under the most extreme emissions scenario, projected declines were highly variable among reefs, ranging 10–100%. Without considering bleaching, reef growth will likely decline on most reefs, especially where resistant species likeS. sidereaare not already dominant. This study demonstrates how species composition influences reef community responses to climate change and how reducedCO₂emissions can limit future declines in reef calcification.

    

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2. Shifts in the coral microbiome in response to in situ experimental deoxygenation

   https://doi.org/10.1128/aem.00577-23  

   Howard, Rachel D. ; Schul, Monica D. ; Rodriguez Bravo, Lucia M. ; Altieri, Andrew H. ; Meyer, Julie L. ( November 2023 , Applied and Environmental Microbiology)

   Biddle, Jennifer F. (Ed.)

   Global climate change impacts marine ecosystems through rising surface temperatures, ocean acidification, and deoxygenation. While the response of the coral holobiont to the first two effects has been relatively well studied, less is known about the response of the coral microbiome to deoxygenation. In this study, we investigated the response of the microbiome to hypoxia in two coral species that differ in their tolerance to hypoxia. We conductedin situoxygen manipulations on a coral reef in Bahía Almirante on the Caribbean coast of Panama, which has previously experienced documented episodes of hypoxia. Naïve coral colonies (previously unexposed to hypoxia) ofSiderastrea sidereaandAgaricia lamarckiwere transplanted to a reef and either enclosed in chambers that created hypoxic conditions or left at ambient oxygen levels. We collected samples of surface mucus and tissue after 48 hours of exposure and characterized the microbiome by sequencing 16S rRNA genes. We found that the microbiomes of the two coral species were distinct from one another and remained so after exhibiting similar shifts in microbiome composition in response to hypoxia. There was an increase in both abundance and number of taxa of anaerobic microbes after exposure to hypoxia. Some of these taxa may play beneficial roles in the coral holobiont by detoxifying the surrounding environment during hypoxic stress or may represent opportunists exploiting host stress. This work describes the first characterization of the coral microbiome under hypoxia and is an initial step toward identifying potential beneficial bacteria for corals facing this environmental stressor.

   Marine hypoxia is a threat for corals but has remained understudied in tropical regions where coral reefs are abundant. Though microbial symbioses can alleviate the effects of ecological stress, we do not yet understand the taxonomic or functional response of the coral microbiome to hypoxia. In this study, we experimentally lowered oxygen levels aroundSiderastrea sidereaandAgaricia lamarckicoloniesin situto observe changes in the coral microbiome in response to deoxygenation. Our results show that hypoxia triggers a stochastic change of the microbiome overall, with some bacterial families changing deterministically after just 48 hours of exposure. These families represent an increase in anaerobic and opportunistic taxa in the microbiomes of both coral species. Thus, marine deoxygenation destabilizes the coral microbiome and increases bacterial opportunism. This work provides novel and fundamental knowledge of the microbial response in coral during hypoxia and may provide insight into holobiont function during stress.

    

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3. Ocean acidification effects on in situ coral reef metabolism

   https://doi.org/10.1038/s41598-019-48407-7  

   Doo, Steve S. ; Edmunds, Peter J. ; Carpenter, Robert C. ( August 2019 , Scientific Reports)

   The Anthropocene climate has largely been defined by a rapid increase in atmospheric CO_2,causing global climate change (warming) and ocean acidification (OA, a reduction in oceanic pH). OA is of particular concern for coral reefs, as the associated reduction in carbonate ion availability impairs biogenic calcification and promotes dissolution of carbonate substrata. While these trends ultimately affect ecosystem calcification, scaling experimental analyses of the response of organisms to OA to consider the response of ecosystems to OA has proved difficult. The benchmark of ecosystem-level experiments to study the effects of OA is provided through Free Ocean CO₂Enrichment (FOCE), which we use in the present analyses for a 21-d experiment on the back reef of Mo’orea, French Polynesia. Two natural coral reef communities were incubatedin situ, with one exposed to ambient pCO₂(393 µatm), and one to high pCO₂(949 µatm). Our results show a decrease in 24-h net community calcification (NCC) under high pCO₂, and a reduction in nighttime NCC that attenuated and eventually reversed over 21-d. This effect was not observed in daytime NCC, and it occurred without any effect of high pCO₂on net community production (NCP). These results contribute to previous studies on ecosystem-level responses of coral reefs to the OA conditions projected for the end of the century, and they highlight potential attenuation of high pCO₂effects on nighttime net community calcification.

    

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4. 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 µM.

   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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5. Variable Species Responses to Experimental Stony Coral Tissue Loss Disease (SCTLD) Exposure

   https://doi.org/10.3389/fmars.2021.670829  

   Meiling, Sonora S. ; Muller, Erinn M. ; Lasseigne, Danielle ; Rossin, Ashley ; Veglia, Alex J. ; MacKnight, Nicholas ; Dimos, Bradford ; Huntley, Naomi ; Correa, Adrienne M. ; Smith, Tyler Burton ; et al ( April 2021 , Frontiers in Marine Science)

   null (Ed.)

   Stony coral tissue loss disease (SCTLD) was initially documented in Florida in 2014 and outbreaks with similar characteristics have since appeared in disparate areas throughout the northern Caribbean, causing significant declines in coral communities. SCTLD is characterized by focal or multifocal lesions of denuded skeleton caused by rapid tissue loss and affects at least 22 reef-building species of Caribbean corals. A tissue-loss disease consistent with the case definition of SCTLD was first observed in the U.S. Virgin Islands (USVI) in January of 2019 off the south shore of St. Thomas at Flat Cay. The objective of the present study was to characterize species susceptibility to the disease present in St. Thomas in a controlled laboratory transmission experiment. Fragments of six species of corals ( Colpophyllia natans , Montastraea cavernosa , Orbicella annularis , Porites astreoides , Pseudodiploria strigosa , and Siderastrea siderea ) were simultaneously incubated with (but did not physically contact) SCTLD-affected colonies of Diploria labyrinthiformis and monitored for lesion appearance over an 8 day experimental period. Paired fragments from each corresponding coral genotype were equivalently exposed to apparently healthy colonies of D. labyrinthiformis to serve as controls; none of these fragments developed lesions throughout the experiment. When tissue-loss lesions appeared and progressed in a disease treatment, the affected coral fragment, and its corresponding control genet, were removed and preserved for future analysis. Based on measures including disease prevalence and incidence, relative risk of lesion development, and lesion progression rates, O. annularis, C. natans , and S. siderea showed the greatest susceptibility to SCTLD in the USVI. These species exhibited earlier average development of lesions, higher relative risk of lesion development, greater lesion prevalence, and faster lesion progression rates compared with the other species, some of which are considered to be more susceptible based on field observations (e.g., P. strigosa ). The average transmission rate in the present study was comparable to tank studies in Florida, even though disease donor species differed. Our findings suggest that the tissue loss disease affecting reefs of the USVI has a similar epizootiology to that observed in other regions, particularly Florida. 

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