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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)

   Abstract 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. Interactions between consumer access and nutrient enrichment have divergent impacts on two common Pacific corals

   https://doi.org/10.3354/meps14737  

   Altman-Kurosaki, NT; Hay, ME (December 2024, Marine Ecology Progress Series)

   As coral reefs face accelerating threats at global scales, examining how different coral species respond to local disruption from top-down and bottom-up forces can inform management at local scales. This may provide additional time for reducing global stressors. We conducted a full factorial experiment examining the effects of corallivory, herbivory, and fertilizer addition on 2 corals:Acropora pulchraandPorites rus, the former of which is faster-growing but more susceptible to predation, disease, and heat-induced mortality. Fertilizer addition enhanced corallivory but did not affect net coral growth. Consumer exclusion enhanced the net growth ofA. pulchraby 18.1 times regardless of fertilizer treatment, while the net growth ofP. rusdid not differ among caging and fertilizer treatments.A. pulchrawas rarely overgrown by algae regardless of treatment, while herbivore exclusion and fertilizer enrichment produced opposing effects on overgrowth ofP. rus. In uncaged treatments, fertilizer enrichment led to greater herbivory but also 1.8 times greater algal overgrowth ofP. rusrelative to unfertilized treatments. However, in caged treatments, algal overgrowth ofP. ruswas 1.9 times higher in unfertilized versus fertilized treatments. Our results suggest that interactions between corallivory, herbivory, and fertilizer enrichment can have alternative effects on different coral species, with a hardier coral experiencing more negative impacts of fertilizer enrichment than a more sensitive coral, which was, in turn, more strongly suppressed by predation. Local stressors that disrupt top-down and bottom-up processes may increase the vulnerability of even the most robust corals, and it is these corals that have been predicted to become more common under future ocean scenarios. 

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5. Effects of corallivory and coral colony density on coral growth and survival

   https://doi.org/10.1007/s00338-021-02076-z  

   Kopecky, Kai L.; Cook, Dana T.; Schmitt, Russell J.; Stier, Adrian C. (April 2021, Coral Reefs)

   null (Ed.)

   Abstract A suite of processes drive variation in coral populations in space and time, yet our understanding of how variation in coral density affects coral performance is limited. Theory predicts that reductions in density can send coral populations into a predator pit, where concentrated corallivory maintains corals at low densities. In reality, how variation in coral density alters corallivory rates is poorly resolved. Here, we experimentally quantified the effects of corallivory and coral density on growth and survival of small colonies of the staghorn coral Acropora pulchra . Our findings suggest that coral density and corallivory have strong but independent effects on coral performance. In the presence of corallivores, corals suffered high but density-independent mortality. When corallivores were excluded, however, vertical extension rates of colonies increased with increasing densities. While we found no evidence for a predator pit, our results suggest that spatio-temporal variation in corallivore and coral densities can fundamentally alter population dynamics via strong effects on juvenile corals. 

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