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1. Nutrient and sediment loading affect multiple facets of coral functionality in a tropical branching coral

   https://doi.org/10.1242/jeb.225045  

   Becker, Danielle M. ; Silbiger, Nyssa J. ( November 2020 , Journal of Experimental Biology)

   null (Ed.)

   Coral reefs, one of the most diverse ecosystems in the world, face increasing pressures from global and local anthropogenic stressors. Therefore, a better understanding of the ecological ramifications of warming and land-based inputs (e.g., sedimentation and nutrient loading) on coral reef ecosystems is necessary. In this study, we measured how a natural nutrient and sedimentation gradient affected multiple facets of coral functionality, including endosymbiont and coral host response variables, holobiont metabolic responses, and percent cover of Pocillopora acuta colonies in Mo'orea, French Polynesia. We used thermal performance curves to quantify the relationship between metabolic rates and temperature along the environmental gradient. We found that algal endosymbiont % nitrogen content, endosymbiont densities, and total chlorophyll a content increased with nutrient input, while endosymbiont nitrogen content cell−1 decreased, likely representing competition among the algal endosymbionts. Nutrient and sediment loading decreased coral metabolic responses to thermal stress in terms of their thermal performance and metabolic rate processes. The acute thermal optimum for dark respiration decreased, along with the maximal performance for gross photosynthetic and calcification rates. Gross photosynthetic and calcification rates normalized to a reference temperature (26.8 °C) decreased along the gradient. Lastly, percent cover of P. acuta colonies decreased by nearly two orders of magnitude along the nutrient gradient. These findings illustrate that nutrient and sediment loading affect multiple levels of coral functionality. Understanding how local-scale anthropogenic stressors influence the responses of corals to temperature can inform coral reef management, particularly on the mediation of land-based inputs into coastal coral reef ecosystems. 

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2. Microbiomes of a disease-resistant genotype of Acropora cervicornis are resistant to acute, but not chronic, nutrient enrichment

   https://doi.org/10.1038/s41598-023-30615-x  

   Klinges, J. Grace ; Patel, Shalvi H. ; Duke, William C. ; Muller, Erinn M. ; Vega Thurber, Rebecca L. ( March 2023 , Scientific Reports)

   Chronically high levels of inorganic nutrients have been documented in Florida’s coral reefs and are linked to increased prevalence and severity of coral bleaching and disease. Naturally disease-resistant genotypes of the staghorn coralAcropora cervicornisare rare, and it is unknown whether prolonged exposure to acute or chronic high nutrient levels will reduce the disease tolerance of these genotypes. Recently, the relative abundance of the bacterial genusAquarickettsiawas identified as a significant indicator of disease susceptibility inA. cervicornis, and the abundance of this bacterial species was previously found to increase under chronic and acute nutrient enrichment. We therefore examined the impact of common constituents of nutrient pollution (phosphate, nitrate, and ammonium) on microbial community structure in a disease-resistant genotype with naturally low abundances ofAquarickettsia.We found that although this putative parasite responded positively to nutrient enrichment in a disease-resistant host, relative abundances remained low (< 0.5%). Further, while microbial diversity was not altered significantly after 3 weeks of nutrient enrichment, 6 weeks of enrichment was sufficient to shift microbiome diversity and composition. Coral growth rates were also reduced by 6 weeks of nitrate treatment compared to untreated conditions. Together these data suggest that the microbiomes of disease-resistantA. cervicornismay be initially resistant to shifts in microbial community structure, but succumb to compositional and diversity alterations after more sustained environmental pressure. As the maintenance of disease-resistant genotypes is critical for coral population management and restoration, a complete understanding of how these genotypes respond to environmental stressors is necessary to predict their longevity.

    

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3. Phosphate enrichment induces increased dominance of the parasite Aquarickettsia in the coral Acropora cervicornis

   https://doi.org/10.1093/femsec/fiac013  

   Klinges, J Grace ; Patel, Shalvi H ; Duke, William C ; Muller, Erinn M ; Vega Thurber, Rebecca L ( February 2022 , FEMS Microbiology Ecology)

   Abstract Nutrient pollution is linked to coral disease susceptibility and severity, but the mechanism behind this effect remains underexplored. A recently identified bacterial species, ‘Ca. Aquarickettsia rohweri,’ is hypothesized to parasitize the Caribbean staghorn coral, Acropora cervicornis, leading to reduced coral growth and increased disease susceptibility. Aquarickettsia rohweri is hypothesized to assimilate host metabolites and ATP and was previously demonstrated to be highly nutrient-responsive. As nutrient enrichment is a pervasive issue in the Caribbean, this study examined the effects of common nutrient pollutants (nitrate, ammonium, and phosphate) on a disease-susceptible genotype of A. cervicornis. Microbial diversity was found to decline over the course of the experiment in phosphate-, nitrate-, and combined-treated samples, and quantitative PCR indicated that Aquarickettsia abundance increased significantly across all treatments. Only treatments amended with phosphate, however, exhibited a significant shift in Aquarickettsia abundance relative to other taxa. Furthermore, corals exposed to phosphate had significantly lower linear extension than untreated or nitrate-treated corals after 3 weeks of nutrient exposure. Together these data suggest that while experimental tank conditions, with an elevated nutrient regime associated with coastal waters, increased total bacterial abundance, only the addition of phosphate significantly altered the ratios of Aquarickettsia compared to other members of the microbiome. 

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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. Positive genetic associations among fitness traits support evolvability of a reef‐building coral under multiple stressors

   https://doi.org/10.1111/gcb.14764  

   Wright, Rachel M. ; Mera, Hanaka ; Kenkel, Carly D. ; Nayfa, Maria ; Bay, Line K. ; Matz, Mikhail V. ( August 2019 , Global Change Biology)

   Climate change threatens organisms in a variety of interactive ways that requires simultaneous adaptation of multiple traits. Predicting evolutionary responses requires an understanding of the potential for interactions among stressors and the genetic variance and covariance among fitness‐related traits that may reinforce or constrain an adaptive response. Here we investigate the capacity ofAcropora millepora, a reef‐building coral, to adapt to multiple environmental stressors: rising sea surface temperature, ocean acidification, and increased prevalence of infectious diseases. We measured growth rates (weight gain), coral color (a proxy for Symbiodiniaceae density), and survival, in addition to nine physiological indicators of coral and algal health in 40 coral genets exposed to each of these three stressors singly and combined. Individual stressors resulted in predicted responses (e.g., corals developed lesions after bacterial challenge and bleached under thermal stress). However, corals did not suffer substantially more when all three stressors were combined. Nor were trade‐offs observed between tolerances to different stressors; instead, individuals performing well under one stressor also tended to perform well under every other stressor. An analysis of genetic correlations between traits revealed positive covariances, suggesting that selection to multiple stressors will reinforce rather than constrain the simultaneous evolution of traits related to holobiont health (e.g., weight gain and algal density). These findings support the potential for rapid coral adaptation under climate change and emphasize the importance of accounting for corals’ adaptive capacity when predicting the future of coral reefs.

    

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