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1. Locally Adapted Coral Species Withstand a 2-Week Hypoxic Event

   https://doi.org/10.3390/oceans6010005  

   Lucey, Noelle; César-Ávila, Carolina; Eckert, Alaina; Veintimilla, Paul; Collin, Rachel (March 2025, Oceans)

   NA (Ed.)

   One approach to improve long-term coral restoration success utilizes naturally stress-tolerant corals from the wild. While the focus has primarily been on thermal stress, low oxygen is a growing threat to coral reefs and restoration efforts should also consider hypoxia tolerance. Here we determine if Siderastrea siderea and Agaricia tenuifolia populations from a reef with a historical record of low oxygen exhibit evidence of local adaptation to hypoxic events, compared to populations from a reference reef. We employed a laboratory-based reciprocal transplant experiment mimicking a severe 14-night hypoxic event and monitored bleaching responses, photo-physiology, metabolic rates, and survival of all four populations during, and for two weeks following the event. In both species, we found the populations from the hypoxic reef either fully persisted or recovered within 3 days of the event. In contrast, the conspecific naïve populations from the well-oxygenated reference reef experienced bleaching and death. This showcases the vulnerability of naïve corals exposed to low oxygen but also suggests that corals from the hypoxic reef locally adapted to survive severe episodic hypoxia. Other reefs with past episodic low oxygen may also be home to corals with adaptation signatures to hypoxia and may be useful for restoration efforts. 

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2. Hypoxia threatens coral and sea anemone early life stages

   https://doi.org/10.1002/lno.12798  

   Glass, Benjamin H; Barott, Katie L (March 2025, Limnology and Oceanography)

   Seawater hypoxia is increasing globally and can drive declines in organismal performance across a wide range of marine taxa. However, the effects of hypoxia on early life stages (e.g., larvae and juveniles) are largely unknown, and it is unclear how evolutionary and life histories may influence these outcomes. Here, we addressed this question by comparing hypoxia responses across early life stages of three cnidarian species representing a range of life histories: the reef‐building coralGalaxea fascicularis, a broadcast spawner with horizontal transmission of endosymbiotic algae (family Symbiodiniaceae); the reef‐building coralPorites astreoides, a brooder with vertical endosymbiont transmission; and the estuarine sea anemoneNematostella vectensis, a non‐symbiotic broadcast spawner. Transient exposure of larvae to hypoxia (dissolved oxygen < 2 mg L⁻¹for 6 h) led to decreased larval swimming and growth for all three species, which resulted in impaired settlement for the corals. Coral‐specific responses also included larval swelling, depressed respiration rates, and decreases in symbiont densities and function. These results indicate both immediate and latent negative effects of hypoxia on cnidarian physiology and coral–algal mutualisms specifically. In addition,G. fascicularisandP. astreoideswere sensitized to heat stress following hypoxia exposure, suggesting that the combinatorial nature of climate stressors will lead to declining performance for corals. However, sensitization to heat stress was not observed inN. vectensisexposed to hypoxia, suggesting that this species may be more resilient to combined stressors. Overall, these results emphasize the importance of reducing anthropogenic carbon emissions to limit further ocean deoxygenation and warming. 

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3. 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.)

   ABSTRACT 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. IMPORTANCEMarine 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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4. Hypoxia disrupts metabolism in coral and sea anemone larvae

   https://doi.org/10.1242/jeb.250372  

   Glass, Benjamin H; Barott, Katie L (June 2025, Journal of Experimental Biology)

   Anthropogenic pollution is driving an increase in the frequency and severity of seawater hypoxic events in coastal marine ecosystems. Although hypoxia decreases physiological performance in coral and sea anemone (phylum Cnidaria) larvae, the underlying cellular mechanisms remain unexplored. Here, larvae of the reef-building corals Galaxea fascicularis and Porites astreoides and the estuarine sea anemone Nematostella vectensis were exposed to normoxia or a simulated hypoxic event (6 h at <2 mg dissolved O2 l−1), and their metabolomic response was quantified at the end of the exposure period using targeted liquid chromatography-mass spectrometry. Baseline metabolite profiles (81 amino acids, acylcarnitines, organic acids and nucleotides) were broadly divergent between the three species, with the corals displaying a reliance on nitrogen cycling through amino acid metabolism, whereas N. vectensis relied on nucleotide metabolism. By contrast, several changes in metabolite abundances under hypoxia were shared (e.g. increases in lactate) and suggest the upregulation of glycolysis, lactic acid fermentation and fatty acid β-oxidation as conserved mechanisms for energy production under hypoxia. Changes in these pathways were correlated with adverse physiological outcomes, including conserved declines in swimming behavior and growth. Importantly, life history traits affecting metabolism influenced hypoxia responses. For example, P. astreoides larvae, which possess algal endosymbionts, displayed the least severe metabolic response to hypoxia among these species, possibly owing to symbiont resources. Overall, these findings demonstrate that hypoxia disrupts metabolic performance in coral and sea anemone larvae through conserved and divergent pathways, emphasizing the need to limit drivers of ocean deoxygenation. 

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5. Escaping the benthos with Coral Reef Arks: effects on coral translocation and fish biomass

   https://doi.org/10.7717/peerj.17640  

   Carilli, Jessica; Baer, Jason; Aquino, Jenna Marie; Little, Mark; Chadwick, Bart; Rohwer, Forest; Rosen, Gunther; van_der_Geer, Anneke; Sánchez-Quinto, Andrés; Ballard, Ashton; et al (January 2024, PeerJ)

   Anthropogenic stressors like overfishing, land based runoff, and increasing temperatures cause the degradation of coral reefs, leading to the loss of corals and other calcifiers, increases in competitive fleshy algae, and increases in microbial pathogen abundance and hypoxia. To test the hypothesis that corals would be healthier by moving them off the benthos, a common garden experiment was conducted in which corals were translocated to midwater geodesic spheres (hereafter called Coral Reef Arks or Arks). Coral fragments translocated to the Arks survived significantly longer than equivalent coral fragments translocated to Control sites (i.e., benthos at the same depth). Over time, average living coral surface area and volume were higher on the Arks than the Control sites. The abundance and biomass of fish were also generally higher on the Arks compared to the Control sites, with more piscivorous fish on the Arks. The addition of Autonomous Reef Monitoring Structures (ARMS), which served as habitat for sessile and motile reef-associated organisms, also generally significantly increased fish associated with the Arks. Overall, the Arks increased translocated coral survivorship and growth, and exhibited knock-on effects such as higher fish abundance. 

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