Exploring the effect of salinity changes on the levels of Hsp60 in the tropical coral Seriatopora caliendrum

Coral Restoration in the Omics Era: Development of Point-of-Care Tools for Monitoring Disease, Reproduction, and Thermal Stress

Coral reef degradation has captured global attention from governments, conservationists, and researchers, who are making concerted efforts to develop sustainable solutions to support reef resilience in the face of environmental degradation. The goal is to empower local community efforts for effective marine resource management. However, one of the major barriers to coral conservation is the lack of timely and affordable population-level health data, which can delay effective management responses. Although progress has been made in understanding the molecular basis of coral health outcomes, more translational work is needed to develop cost-effective, point-of-care (POC) diagnostic tools for real-time monitoring. This review assesses the current state of translational omics-based research for coral health monitoring, focusing on highlighting key gaps and actionable next steps to guide the implementation of effective, field-ready tools for monitoring coral disease, reproduction, and thermal stress. These advancements can be used to advance urgent conservation needs and promote reef management by local communities.

Coral microbiome in estuary coral community of Pearl River Estuary: insights into variation in coral holobiont adaptability to low-salinity conditions

BACKGROUND

Low salinity is a crucial environmental stressor that affects estuarine coral ecosystems considerably. However, few studies have focused on the effects of low-salinity conditions on coral-associated microorganisms and the adaptability of coral holobionts.

METHODS

We explored the community structure of coral symbiotic Symbiodiniaceae and associated bacteria in low-salinity conditions using samples of six coral species from the Pearl River Estuary and analyzed the adaptability of coral holobionts in estuaries.

RESULTS

The symbiotic Symbiodiniaceae of all six studied coral species were dominated by Cladocopium, but, the Symbiodiniaceae subclades differed among these coral species. Some coral species (e.g., Acropora solitaryensis) had a high diversity of symbiotic Symbiodiniaceae but low Symbiodiniaceae density, with different adaptability to low-salinity stress in the Pearl River Estuary. Other coral species (e.g., Plesiastrea versipora) potentially increased their resistance by associating with specific Symbiodiniaceae subclades and with high Symbiodiniaceae density under low-salinity stress. The microbiome associated with the coral species were dominated by Proteobacteria, Chloroflexi, and Bacteroidetes; however, its diversity and composition varied among coral species. Some coral species (e.g., Acropora solitaryensis) had a high diversity of associated bacteria, with different adaptability owing to low-salinity stress. Other coral species (e.g., Plesiastrea versipora) potentially increased their resistance by having minority bacterial dominance under low-salinity stress.

CONCLUSIONS

High Symbiodiniaceae density and high bacterial diversity may be conducive to increase the tolerance of coral holobiont to low-salinity environments. Different coral species have distinct ways of adapting to low-salinity stress, and this difference is mainly through the dynamic regulation of the coral microbiome by corals.

Differential molecular biomarker expression in corals over a gradient of water quality stressors in Maunalua Bay, Hawaii

Coral reefs globally face unprecedented challenges from anthropogenic stressors, necessitating innovative approaches for effective assessment and management. Molecular biomarkers, particularly those related to protein expressions, provide a promising avenue for diagnosing coral health at the cellular level. This study employed enzyme-linked immunosorbent assays to evaluate stress responses in the coral Porites lobata along an environmental gradient in Maunalua Bay, Hawaii. The results revealed distinct protein expression patterns correlating with anthropogenic stressor levels across the bay. Some proteins, such as ubiquitin and Hsp70, emerged as sensitive biomarkers, displaying a linear decrease in response along the environmental gradient, emphasizing their potential as indicators of stress. Our findings highlighted the feasibility of using protein biomarkers for real-time assessment of coral health and the identification of stressors. The identified biomarkers can aid in establishing stress thresholds and evaluating the efficacy of management interventions. Additionally, we assessed sediment and water quality from the inshore areas in the bay and identified organic contaminants, including polycyclic aromatic hydrocarbons and pesticides, in bay sediments and waters.

Bioconcentration and cellular effects of emerging contaminants in sponges from Maldivian coral reefs: A managing tool for sustainable tourism

Tourism is the main income source for the Maldives, but concurrently, it represents a growing threat to its marine ecosystem. Here, we monitored the bioaccumulation of 15 emerging contaminants (ECs) in the Maldivian reef sponges Spheciospongia vagabunda collected in two resort islands (Athuruga and Thudufushi, Ari Atoll) and an inhabited island (Magoodhoo, Faafu Atoll), and we analysed their impact on different sponge cellular stress biomarkers. Caffeine and the insect repellent DEET were detected in sponges of all the islands, whereas the antibiotic erythromycin and the UV filter 4-methylbenzylidene camphor were found in resort islands only. Although concentrations were approximately a few ng/g d.w., we quantified various induced cellular effects, in particular an increase of the levels of the enzyme glutathione S-transferase involved in cell detoxification. Our results highlight the importance to increase awareness on ECs pollution, promoting the use of more environmental friendly products to achieving the sustainable development goals.

Physiological responses (Hsps 60 and 32, caspase 3, H2O2 scavenging, and photosynthetic activity) of the coral Pocillopora damicornis under thermal and high nitrate stresses

This study explored the physiological responses of the coral Pocillopora damicornis to high nitrate concentrations and thermal stresses. The expression of heat shock proteins Hsp60 and Hsp32, Symbiodiniaceae density, Chl a concentration, Fv/Fm, H₂O₂ scavenging, and caspase 3 activity varied during 60 h incubations at 28 °C or 32 °C, ambient or high nitrate (~10 μM) concentrations, and their combinations. In combined stresses, corals showed a rapid and high oxidation level negatively affecting the Symbiodiniaceae density and Chl a concentration at 12 h, followed by caspase 3 and Hsps upregulations that induced apoptosis, bleaching and tissue detachment. Corals under thermal stress showed the highest oxidation and upregulation of Hsps and caspase 3 resulting in coral discoloration. High nitrate treatment alone did not seriously affect the coral function. Results showed that combined stress treatment severely affected coral physiology and, judging from the condition of detached tissues, these corals might have lower chances to recover.

Local acclimatisation-driven differential gene and protein expression patterns of Hsp70 in Acropora muricata: Implications for coral tolerance to bleaching

Corals show spatial acclimatisation to local environment conditions. However, the various cellular mechanisms involved in local acclimatisation and variable bleaching patterns in corals remain to be thoroughly understood. In this study, the modulation of a protein implicated in cellular heat stress tolerance, the heat shock protein 70, was compared at both gene (hsp70) and protein (Hsp70) expression level in bleaching tolerant near-coast Acropora muricata colonies and bleaching susceptible reef colonies, in the lagoon of Belle Mare (Mauritius). The relative Hsp70 levels varied significantly between colonies from the two different locations, colonies having different health conditions and the year of collection. Before the bleaching event of 2016, near-coast colonies had higher basal levels of both Hsp70 gene and protein compared to reef colonies. During the bleaching event, the near-coast colonies did not bleach and had significantly higher relative levels of both Hsp70 gene and protein compared to bleached reef colonies. No significant genetic differentiation between the two studied coral populations was observed and all the colonies analysed were associated with Symbiodiniaceae of the genus Symbiodinium (Clade A) irrespective of location and sampling period. These findings provide further evidence of the involvement of Hsp70 in conferring bleaching tolerance to corals. Moreover, the consistent expression differences of Hsp70 gene and protein between the near-coast and reef coral populations in a natural setting indicate that the modulation of this Hsp is involved in local acclimatisation of corals to their environments.

The impact of short-term exposure to near shore stressors on the early life stages of the reef building coral Montipora capitata

Successful reproduction and survival are crucial to the continuation and resilience of corals globally. As reef waters warm due to climate change, episodic largescale tropical storms are becoming more frequent, drastically altering the near shore water quality for short periods of time. Therefore, it is critical that we understand the effects warming waters, fresh water input, and run-off have on sexual reproduction of coral. To better understand the effects of these near shore stressors on Hawaiian coral, laboratory experiments were conducted at the Institute of Marine Biology to determine the independent effects of suspended sediment concentrations (100 mg l^-1 and 200 mg l^-1), lowered salinity (28‰), and elevated temperature (31 °C) on the successful fertilization, larval survival, and settlement of the scleractinian coral Montipora capitata. In the present study, early developmental stages of coral were exposed to one of three near shore stressors for a period of 24 h and the immediate (fertilization) and latent effects (larval survival and settlement) were observed and measured. Fertilization success and settlement were not affected by any of the treatments; however, larval survival was negatively affected by all of the treatments by 50% or greater (p > 0.05). These data show that early life stages of M. capitata may be impacted by near shore stressors associated with warming and more frequent storm events.

Different calcification responses of two hermatypic corals to CO2-driven ocean acidification

Understanding how calcification is influenced by the enhanced dissolution of CO₂ in the oceans is the key to evaluating the effects of ocean acidification (OA) on coral reefs. In this study, two branching hermatypic corals widely distributed in the South China Sea, Pocillopora damicornis and Seriatopora caliendrum, were used to study the calcification responses to CO₂-driven OA (7.77 ± 0.07 vs. 8.15 ± 0.12). Our results showed that the calcification rate (0.17 ± 0.04%/day to 0.21 ± 0.12%/day) in P. damicornis remained unchanged in the acidified seawaters, but that in S. caliendrum decreased significantly (0.62 ± 0.21%/day to 0.44 ± 0.11%/day). Our results suggested that reef corals with high calcification rates may be more susceptible to the enhanced dissolution of CO₂. Differential calcified response to elevated CO₂ may be closely attributed to coralline capacity of the upregulation at their site of calcification in acidified seawater.

Influence of salinity and sedimentation on Vibrio infection of the Hawaiian coral Montipora capitata

Environmental cofactors alter host-pathogen interactions and influence disease dynamics by impairing host resistance and/or increasing pathogen virulence. Terrestrial runoff is recognized as a major threat to coral reef health. However, the direct links between runoff and coral disease are not clear. Montipora white syndrome (MWS) is a coral disease that occurs in the Hawaiian archipelago, can be caused by various bacterial pathogens, including Vibrio species, and is linked to conditions associated with heavy rainfall and runoff. The objective of this study was to determine whether a short-term hyposalinity stress (20 ppt for 24 h) or sedimentation stress (1000 g m-2 d-1) would influence bacterial infection of the coral Montipora capitata. Hyposalinity increased M. capitata susceptibility to infection by 2 MWS pathogens, Vibrio coralliilyticus strain OCN008 and Vibrio owensii strain OCN002. Specifically, hyposalinity allowed OCN008 to infect at lower doses (106 CFU ml-1 compared with 108 CFU ml-1) and reduced the amount of time before onset of OCN002 infection at high doses (108 CFU ml-1). In contrast, short-term sedimentation stress did not affect M. capitata infection by either of these 2 pathogens. Although several studies have found a correlation between runoff and increased coral disease prevalence in field studies, this is the first study to show that one aspect of runoff (reduced salinity) enhances bacterial infection of coral using manipulative experiments.

The cellular stress response of the scleractinian coral Goniopora columna during the progression of the black band disease

Black band disease (BBD) is a widespread coral pathology caused by a microbial consortium dominated by cyanobacteria, which is significantly contributing to the loss of coral cover and diversity worldwide. Since the effects of the BBD pathogens on the physiology and cellular stress response of coral polyps appear almost unknown, the expression of some molecular biomarkers, such as Hsp70, Hsp60, HO-1, and MnSOD, was analyzed in the apparently healthy tissues of Goniopora columna located at different distances from the infection and during two disease development stages. All the biomarkers displayed different levels of expression between healthy and diseased colonies. In the healthy corals, low basal levels were found stable over time in different parts of the same colony. On the contrary, in the diseased colonies, a strong up-regulation of all the biomarkers was observed in all the tissues surrounding the infection, which suffered an oxidative stress probably generated by the alternation, at the progression front of the disease, of conditions of oxygen supersaturation and hypoxia/anoxia, and by the production of the cyanotoxin microcystin by the BBD cyanobacteria. Furthermore, in the infected colonies, the expression of all the biomarkers appeared significantly affected by the development stage of the disease. In conclusion, our approach may constitute a useful diagnostic tool, since the cellular stress response of corals is activated before the pathogens colonize the tissues, and expands the current knowledge of the mechanisms controlling the host responses to infection in corals.

Hsp60 expression profiles in the reef-building coral Seriatopora caliendrum subjected to heat and cold shock regimes

Climate changes have increased the intensity/frequency of extreme thermal events, which represent serious threats to the health of reef-building corals. Since the vulnerability of corals exposed to thermal stresses are related to their ability to regulate Heat shock proteins (Hsps), we have analyzed together the time related expression profiles of the mitochondrial Hsp60 and the associated changes in tissue pigmentation in Seriatopora caliendrum subjected to 48 h of heat and cold treatments characterized by moderate (±2 °C) and severe (±6 °C) shocks. For the first time, an Hsp60 response was observed in a scleractinian coral exposed to cold stresses. Furthermore, the Hsp60 modulations and the changes in the tissue coloration were found to be specific for each treatment. A strong down-regulation at the end of the treatments was observed following both the severe shocks, but only the severe heat stress led to bleaching in concert with the lowest levels of Hsp60, suggesting that a severe heat shock can be more deleterious than an exposure to a severe cold temperature. On the contrary, a moderate cold stress seems to be more harmful than a moderate temperature increase, which could allow coral acclimation. Our results can provide a potential framework for understanding the physiological tolerance of corals under possible future climate changes.

Long-term salinity tolerance is accompanied by major restructuring of the coral bacterial microbiome

Scleractinian corals are assumed to be stenohaline osmoconformers, although they are frequently subjected to variations in seawater salinity due to precipitation, freshwater run-off and other processes. Observed responses to altered salinity levels include differences in photosynthetic performance, respiration and increased bleaching and mortality of the coral host and its algal symbiont, but a study looking at bacterial community changes is lacking. Here, we exposed the coral Fungia granulosa to strongly increased salinity levels in short- and long-term experiments to disentangle temporal and compartment effects of the coral holobiont (i.e. coral host, symbiotic algae and associated bacteria). Our results show a significant reduction in calcification and photosynthesis, but a stable microbiome after short-term exposure to high-salinity levels. By comparison, long-term exposure yielded unchanged photosynthesis levels and visually healthy coral colonies indicating long-term acclimation to high-salinity levels that were accompanied by a major coral microbiome restructuring. Importantly, a bacterium in the family Rhodobacteraceae was succeeded by Pseudomonas veronii as the numerically most abundant taxon. Further, taxonomy-based functional profiling indicates a shift in the bacterial community towards increased osmolyte production, sulphur oxidation and nitrogen fixation. Our study highlights that bacterial community composition in corals can change within days to weeks under altered environmental conditions, where shifts in the microbiome may enable adjustment of the coral to a more advantageous holobiont composition.

Physiological response to elevated temperature and pCO2 varies across four Pacific coral species: Understanding the unique host+symbiont response

The physiological response to individual and combined stressors of elevated temperature and pCO₂ were measured over a 24-day period in four Pacific corals and their respective symbionts (Acropora millepora/Symbiodinium C21a, Pocillopora damicornis/Symbiodinium C1c-d-t, Montipora monasteriata/Symbiodinium C15, and Turbinaria reniformis/Symbiodinium trenchii). Multivariate analyses indicated that elevated temperature played a greater role in altering physiological response, with the greatest degree of change occurring within M. monasteriata and T. reniformis. Algal cellular volume, protein, and lipid content all increased for M. monasteriata. Likewise, S. trenchii volume and protein content in T. reniformis also increased with temperature. Despite decreases in maximal photochemical efficiency, few changes in biochemical composition (i.e. lipids, proteins, and carbohydrates) or cellular volume occurred at high temperature in the two thermally sensitive symbionts C21a and C1c-d-t. Intracellular carbonic anhydrase transcript abundance increased with temperature in A. millepora but not in P. damicornis, possibly reflecting differences in host mitigated carbon supply during thermal stress. Importantly, our results show that the host and symbiont response to climate change differs considerably across species and that greater physiological plasticity in response to elevated temperature may be an important strategy distinguishing thermally tolerant vs. thermally sensitive species.

Modulation of Hsp60 in response to coral brown band disease

Brown band disease (BrB), a virulent coral disease characterized by a dense concentration of ciliates ingesting coral tissue, is responsible for ongoing coral losses on Indo-Pacific reefs. Although several efforts have been made to identify the microbial communities associated with BrB and study the disease ecology, less attention has been given to the effect of ciliate presence on coral physiology. Levels of the mitochondrial heat shock protein 60-kDa (Hsp60, a biomarker indicative of cellular stress) were analyzed in apparently healthy coral polyps located at different distances along the advancing front of infection in Acropora muricata colonies affected by BrB in a Maldivian reef. Different Hsp60 levels were found in different parts of the same colony. Starting from a basal protein level in the healthy control colonies, a down-regulation of Hsp60 expression was detected near the ciliate band, indicating that the Hsp60 defense activity was probably already compromised due to the rapid progression rate of the BrB ciliate on the diseased branches and/or to the etiology of the disease. Moving away from the band, the Hsp60 levels gradually returned to a state comparable to that found in the control, showing that cellular damage was confined to areas near the infection. In conclusion, we propose the analysis of Hsp60 modulation as a useful tool for examining physiological variations that are not detected at the morphological level in corals subjected to epizootic diseases, while providing new insights into the immune response of corals.

Hyposalinity stress compromises the fertilization of gametes more than the survival of coral larvae

The life cycle of coral is affected by natural and anthropogenic perturbations occurring in the marine environment. In the context of global changes, it is likely that rainfall events will be more intense and that coastal reefs will be exposed to sudden drops in salinity. Therefore, a better understanding of how corals-especially during the pelagic life stages-are able to deal with declines in salinity is crucial. To fill this knowledge gap, this work investigated how gametes and larva stages of two species of Acropora (Acropora cytherea and Acropora pulchra) from French Polynesia cope with drops in salinity. An analysis of collected results highlights that both Acropora coral gametes displayed the same resistance to salinity changes, with 4h30-ES50 (effective salinity that decrease by 50% the fertilization success after 4h30 exposure) of 26.6 ± 0.1 and 27.5 ± 0.3‰ for A. cytherea and A. pulchra, respectively. This study also revealed that coral gametes were more sensitive to decreases in salinity than larvae, for which significant changes are only observed at 26‰ for A. cytherea after 14 d of exposure. Although rising seawater temperatures and ocean acidification are often perceived as the main threat for the survival of coral reefs, our work indicates that 70% of the gametes could be killed during a single night of spawning by a rainfall event that decreases salinity to 26‰. This suggests that changes in the frequency and intensity of rainfall events associated with climate changes should be taken seriously in efforts to both preserve coral gametes and ensure the persistence and renewal of coral populations.

The susceptibility of corals to thermal stress by analyzing Hsp60 expression

Due to the increasing frequency and severity of the coral bleaching events in the context of global warming, there is an urgent need to improve our understanding of the susceptibility of corals to thermal stresses, particularly at the sub-cellular level. In this context, we examined the modulation of the polyp mitochondrial Hsp60 in three scleractinian coral species (Seriatopora hystrix, Montipora monasteriata and Acropora echinata) under simulated heat shock bleaching at 34 °C during a time course of 36 h. All three species displayed a similar initial increase of Hsp60 level which accompanies the increasing paleness of coral tissue. Afterwards, each of them showed a specific pattern of Hsp60 down-regulation which can be indicative of a different threshold of resistance, although it proceeded in synchrony with the complete bleaching of tissues. The finely branched S. hystrix was the species most susceptible to heat stress while the plating M. monasteriata was the most tolerant one, as its Hsp60 down-regulation was less rapid than the branching corals. On the whole, the Hsp60 modulation appears useful for providing information about the susceptibility of the different coral taxa to environmental disturbances.