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Abstract Sea cucumbers have been overharvested world-wide, making assessments of their ecological effects challenging, but recent research demonstrated that sea cucumbers increase coral survival via disease suppression and were therefore important for facilitating reef health. The mechanisms underpinning the sea cucumber-coral interaction therefore are not well understood but are likely mediated through sea cucumber grazing of microbes from reef sediments. We explored how sea cucumber grazing alters the sediment microbiome by leveraging a healthy sea cucumber population on a reef in French Polynesia. We used quantitative PCR, 16S rRNA gene sequencing, and shotgun metagenomics to compare the sediment microbiome in cages placed in situ with or without sea cucumbers. We hypothesized that grazing would lower microbial biomass, change sediment microbiome composition, and deplete sediment metagenomes of anaerobic metabolisms, likely due to aeration of the sediments. Sea cucumber grazing resulted in a 75% reduction in 16S rRNA gene abundances and reshaped microbiome composition, causing a significant decrease of cyanobacteria and other phototrophs relative to ungrazed sediments. Grazing also resulted in a depletion of genes associated with cyanotoxin synthesis, suggesting a potential link to coral health. In contrast to expectations, grazed sediment metagenomes were enriched with marker genes of diverse anaerobic or microaerophilic metabolisms, including those encoding high oxygen affinity cytochrome oxidases. This enrichment differs from patterns linked to other bioturbating invertebrates. We hypothesize that grazing enriches anaerobic processes in sediment microbiomes through removal of oxygen-producing autotrophs, fecal deposition of sea cucumber gut-associated anaerobes, or modification of sediment diffusibility. These results suggest that sea cucumber harvesting influences biogeochemical processes in reef sediments, potentially mediating coral survival by altering the sediment microbiome and its production of coral-influencing metabolites. 

Abstract Hundreds of studies now document positive relationships between biodiversity and critical ecosystem processes, but as ecological communities worldwide shift toward new species configurations, less is known regarding how the biodiversity of undesirable species will shape the functioning of ecosystems or foundation species. We manipulated macroalgal species richness in experimental field plots to test whether and how the identity and diversity of competing macroalgae affected the growth, survival, and microbiome of a common coral in Mo'orea, French Polynesia. Compared to controls without algal competitors, coral growth was significantly suppressed across three macroalgal monocultures, a polyculture of the same three macroalgae, and plots containing inert seaweed mimics; coral mortality was limited and did not differ significantly among treatments. One macroalga suppressed coral growth significantly less than the other two, but none differed from the inert mimic in terms of coral suppression. The composition, dispersion, and diversity of coral microbiomes in treatments with live macroalgae or inert plastic mimics did not differ from controls experiencing no competition. Microbiome composition differed between two macroalgal monocultures and a monoculture versus plastic mimics, but no other microbiome differences were observed among macroalgal or mimic treatments. Together, these findings suggest that algal diversity does not alter harmful impacts of macroalgae on coral performance, which could be accounted for by physical structure alone in these field experiments. While enhancing biodiversity is a recognized strategy for promoting desirable species, it would be worrisome if biodiversity also enhanced the negative impacts of undesirable species. We documented no such effects in this investigation. 

Abstract Coral reefs are in global decline with coral diseases playing a significant role. This is especially true for Acroporid corals that represent ~25% of all Pacific coral species and generate much of the topographic complexity supporting reef biodiversity. Coral diseases are commonly sediment-associated and could be exacerbated by overharvest of sea cucumber detritivores that clean reef sediments and may suppress microbial pathogens as they feed. Here we show, via field manipulations in both French Polynesia and Palmyra Atoll, that historically overharvested sea cucumbers strongly suppress disease among corals in contact with benthic sediments. Sea cucumber removal increased tissue mortality ofAcropora pulchraby ~370% and colony mortality by ~1500%. Additionally, farmerfish that killAcropora pulchrabases to culture their algal gardens further suppress disease by separating corals from contact with the disease-causing sediment—functioning as mutualists rather than parasites despite killing coral bases. Historic overharvesting of sea cucumbers increases coral disease and threatens the persistence of tropical reefs. Enhancing sea cucumbers may enhance reef resilience by suppressing disease. 

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. 

As coral reefs degrade worldwide, researchers and managers need to determine whether corals can acclimatize to altered local conditions or whether their fixed phenotypes prevent coral persistence under these new environmental conditions. Fixed phenotypes could produce environmental mismatches that reduce population connectivity and exacerbate decline in the near-term, but a capacity for acclimatization could be harnessed in both passive and proactive coral restoration efforts. Here, we conducted a reciprocal transplant experiment in Mo‘orea, French Polynesia, to test how intraspecific performance of 2 common coral species (Acropora hyacinthusandPocillopora verrucosa) varied between a neighboring forereef and backreef that differed dramatically in trajectories of coral loss, resilience over decadal time scales, and cover of corals versus competing macroalgae. We also tested how corals responded to 2 common stressors—corallivory and macroalgal competition—and how this varied as a function of transplant location and the area of origin. Growth and survival of both coral species were affected by macroalgal competition, corallivory, transplant location, or some combination thereof, but we found limited evidence that the habitat of origin significantly impacted intraspecific performance. These results suggest that acclimatization capacity may outweigh local adaptation for these common reef-building species and could be leveraged to facilitate coral restoration. 

Tropical reefs are commonly transitioning from coral to macroalgal dominance, but the role of macroalgae in coral decline remains inadequately understood. A growing body of research suggests that algae may harm corals via disruptions to the homeostasis of the coral holobiont, including resident microbial communities, but the processes that mediate these potential microbial effects and the spatial scales at which they operate are uncertain. Resolving the relative importance and context dependencies of microbially-mediated algal-coral competition is critical for understanding and predicting coral dynamics as reefs further degrade. In this review, we examine the current state of knowledge surrounding algal impacts on corals via disruption of their microbiomes, with a particular focus on the mechanisms hypothesized to mediate microbial effects, the scales at which they are thought to operate, and the evidence from laboratory- and field-based studies for their existence and ecological relevance in the wild. Lastly, we highlight challenges for further advancing the field. 

A mass sea urchin die-off in the Caribbean Sea in the 1980s may have resulted from a single-cell protist called a scuticociliate.