Prominence of environmental and anthropogenic agents on the occurrence of coral reef bleaching syndrome and coral diseases

Coral reefs are the most productive ecosystems on Earth. They ensure the conservation of biodiversity and are a live habitat for 25% of all marine organisms. The main relationship on the coral reef is the symbiosis between corals and algae from the genus Symbiodinium (commonly called zooxanthellae). The authors of this publication have characterized and described the factors limiting the occurrence of coral reefs, including: water temperature, salinity, access to sunlight, contamination, physicochemical and hydromechanical parameters of water. Moreover anthropogenic threats to coral reefs have been specified, including diving tourism, ecological disasters (e.g. oil spills) and the development of marine aquaristics. Rapid changes in the basic living conditions are dangerous for corals and their symbionts and may cause the unsuitability of the new environment resulting in diseases such as coral bleaching. Corals bleaching is a disease associated with the break of the coral and algae relationship which results in a coral reef death on a global scale. Awareness of these negative factors, often related to human activity, may allow us to better understand the ecological processes that are the basis of reef functioning and might enable us to prevent and oppose to the changes and ecological recessions of coral reefs.

Coral diseases of mushroom coral (Fungiidae) in Pari Island, Kepulauan Seribu, Jakarta

Coral disease is one of the causes of the decline in the condition of coral reef ecosystems. This study aims to measure coral health based on the abundance and prevalence of coral health categories. The research was conducted in the Pari Island Cluster, Seribu Islands at four stations. The Belt Transect method with 2 × 100 meters was used to calculate coral health and a 30 m Line Intercept Transect (LIT) with three replications to determine substrate cover. The condition of coral reefs can be categorized as moderate to good based on this percentage value. The study results found five genera from the Fungiidae, namely Fungia, Ctenactis, Herpolitha, Heliofungia, and Sandalolitha. The most commonly found genus is the genus Fungia. The health condition of Fungiidae corals in Pari Island is divided into two categories, namely 35% healthy and 65% unhealthy, consisting of changes in tissue color - white (coral bleaching), changes in tissue color - not white (yellow band disease), and compromised health (damage by sedimentation). Yellow band disease is only found in the genus ˆ and is not found in other genera.

Characterization of Symbiodinium-associated viruses and implications for coral health

<p>Coral reefs are in decline worldwide. Much of this decline is attributable to mass coral bleaching events and disease outbreaks, both of which are linked to anthropogenic climate change. Despite increased research effort, much remains unknown about these phenomena, especially the causative agents of many coral diseases. In particular, coral-associated viruses have received little attention, and their potential roles in coral diseases are largely unknown. This study aimed to address this lack of information by characterising the viruses associated with reef-building corals and Symbiodinium (dinoflagellates that can form symbioses with corals). Symbiodinium virus screening experiments revealed the presence of numerous and varied virus-like particles (VLPs) inside cells. Of the 49 Symbiodinium cultures screened, approximately one third contained putative latent viral infections that could be induced to enter their lytic cycle by UV irradiation. Electron microscope examination revealed VLPs closely resembling viruses previously found in dinoflagellates and other microalgae. Three cultures that showed evidence of latent viral infections were chosen for whole transcriptome sequencing, which revealed the presence of viral genes that were expressed in several different types of Symbiodinium. The relationship between the detected genes and known viral gene sequences suggested that the cells were infected with double-stranded DNA (dsDNA) viruses. In order to determine how the host cell responds to stress-induced viral infection, the expression levels of genes associated with stress response and viral infection were measured. The expression levels of many genes were unchanged following UV stress, and expression of genes that were predicted to be upregulated following stress, such as those encoding antioxidant enzymes, in fact showed lower expression levels. Despite this, several groups of genes involved in viral infection and host cell response were upregulated following stress, providing further evidence for stress-induced latent or chronic viral infections. In addition to the research carried out on Symbiodinium cell cultures, viruses associated with three coral diseases were studied using electron microscopy. Virus-like particles were present in coral and Symbiodinium cells from all three diseases, but viral abundance was correlated with disease state in only one: white patch syndrome (WPS) of Porites australiensis. The locations and morphologies of the VLPs associated with WPS suggested the presence of dsDNA and single-stranded RNA (ssRNA) viruses infecting both the coral animal and Symbiodinium cells. DNA sequences obtained from WPS-affected corals matched closely with sequences obtained from VLP-containing Symbiodinium cells. Based on the evidence gathered from Symbiodinium cell cultures and coral tissues, I propose a theoretical model of viral infection in WPS. In this model, the coral animal cells are routinely subject to chronic viral infections, and Symbiodinium cells harbour two types of chronic or latent infections – a dsDNA and an ssRNA virus – that can be induced via stress, resulting in cell lysis or loss of the cells from the coral host. In addition to detection and rudimentary identification of viruses infecting Symbiodinium cells, this study generated the largest dinoflagellate transcriptomic dataset to date. These data will prove valuable for future research into Symbiodinium, both in terms of viral infections and more generally.</p>

Characterization of Symbiodinium-associated viruses and implications for coral health

<p>Coral reefs are in decline worldwide. Much of this decline is attributable to mass coral bleaching events and disease outbreaks, both of which are linked to anthropogenic climate change. Despite increased research effort, much remains unknown about these phenomena, especially the causative agents of many coral diseases. In particular, coral-associated viruses have received little attention, and their potential roles in coral diseases are largely unknown. This study aimed to address this lack of information by characterising the viruses associated with reef-building corals and Symbiodinium (dinoflagellates that can form symbioses with corals). Symbiodinium virus screening experiments revealed the presence of numerous and varied virus-like particles (VLPs) inside cells. Of the 49 Symbiodinium cultures screened, approximately one third contained putative latent viral infections that could be induced to enter their lytic cycle by UV irradiation. Electron microscope examination revealed VLPs closely resembling viruses previously found in dinoflagellates and other microalgae. Three cultures that showed evidence of latent viral infections were chosen for whole transcriptome sequencing, which revealed the presence of viral genes that were expressed in several different types of Symbiodinium. The relationship between the detected genes and known viral gene sequences suggested that the cells were infected with double-stranded DNA (dsDNA) viruses. In order to determine how the host cell responds to stress-induced viral infection, the expression levels of genes associated with stress response and viral infection were measured. The expression levels of many genes were unchanged following UV stress, and expression of genes that were predicted to be upregulated following stress, such as those encoding antioxidant enzymes, in fact showed lower expression levels. Despite this, several groups of genes involved in viral infection and host cell response were upregulated following stress, providing further evidence for stress-induced latent or chronic viral infections. In addition to the research carried out on Symbiodinium cell cultures, viruses associated with three coral diseases were studied using electron microscopy. Virus-like particles were present in coral and Symbiodinium cells from all three diseases, but viral abundance was correlated with disease state in only one: white patch syndrome (WPS) of Porites australiensis. The locations and morphologies of the VLPs associated with WPS suggested the presence of dsDNA and single-stranded RNA (ssRNA) viruses infecting both the coral animal and Symbiodinium cells. DNA sequences obtained from WPS-affected corals matched closely with sequences obtained from VLP-containing Symbiodinium cells. Based on the evidence gathered from Symbiodinium cell cultures and coral tissues, I propose a theoretical model of viral infection in WPS. In this model, the coral animal cells are routinely subject to chronic viral infections, and Symbiodinium cells harbour two types of chronic or latent infections – a dsDNA and an ssRNA virus – that can be induced via stress, resulting in cell lysis or loss of the cells from the coral host. In addition to detection and rudimentary identification of viruses infecting Symbiodinium cells, this study generated the largest dinoflagellate transcriptomic dataset to date. These data will prove valuable for future research into Symbiodinium, both in terms of viral infections and more generally.</p>

Coral Disease and the Environment in the Pacific Ocean

<p>Coral diseases are a major threat to coral reef health and functioning worldwide. Little is known about how coral disease prevalence relates to multiple interacting changes in host densities, abiotic stressors, and levels of human impact. In particular, almost nothing is known about coral disease dynamics under changing abiotic conditions in the absence of direct anthropogenic stressors. Understanding how disease dynamics change relative to shifts in environmental conditions is crucial for the successful management and future survival of coral reefs. With the use of existing and novel field data and statistical modeling I examined the associations (abiotic and biotic) of multiple coral disease states across a variety of spatial scales encompassing a wide range of environmental conditions. Biomedical techniques were then used to relate these environmental associations to potential disease etiology. Study sites included areas with high levels of anthropogenic impact (e.g. Oahu, main Hawaiian Islands); to extremely remote quasi-pristine reefs removed from direct human influence (e.g. Palmyra Atoll National Wildlife Refuge). Over small spatial scales (100s m) at a marine reserve in the main Hawaiian Islands I modelled the spatial patterns of four coral diseases (Porites growth anomalies, Porites tissue loss, Porites trematodiasis and Montipora white syndrome). While Porites tissue loss and Montipora white syndrome were positively associated with poor environmental conditions (poor water quality, low coral cover), Porites growth anomalies and Porites trematodiasis were more prevalent in areas considered to be of superior quality (clearer water, increased host abundance, higher numbers of fish). At Palmyra Atoll, fatal tissue loss diseases were largely absent and although coral growth anomalies were present their prevalence was extremely low. Patterns of growth anomaly prevalence at Palmyra were positively associated with host abundance across four coral genera (Acropora, Astreopora, Montipora and Porites) and generally negatively associated with algal cover. Growth anomalies, although progressive and detrimental to the hosts, were most prevalent in the "healthiest" regions (the highest coral cover regions) of Palmyra. I hypothesised that differences seen in the types and prevalence of coral diseases between heavily populated parts of Hawaii and remote uninhabited locations such as Palmyra Atoll, could be a result of differing levels of either direct (e.g. pollution) or indirect (e.g. pollution leading to loss of key hosts) human stressors, in addition to natural changes in the environment. To begin disentangling the confounding effects of natural variability and human stressors on coral disease prevalence patterns I modelled two diseases (Acropora and Porites growth anomalies) across hundreds of sites throughout the Indo-Pacific Ocean (1000s km). Predictors included host densities, human population numbers, frequency of sea surface temperature anomalies, and input of ultra-violet radiation. Porites growth anomaly prevalence was positively associated with human population density (and to a lesser extent host density), while the prevalence of Acropora growth anomalies was strongly host density dependent. The positive association between the prevalence of Porites growth anomalies and human density suggests the presence and prevalence of the disease are related, directly or indirectly, to some environmental co-factor associated with increased human density at regional spatial scales. Although this association has been widely posited, this is one of the first wide scale studies unambiguously linking a coral disease with human population size. In summary, the types of coral diseases observed, their prevalence, and spatial patterns of distribution within reef systems are the result of multiple abiotic and biotic factors and stressors interacting, in some cases synergistically. Statistical modelling, in conjunction with biomedical techniques and field observations, proved essential to the understanding of coral disease ecology within single reefs and atolls to patterns across entire oceans.</p>

Similarities and Differences Between Two Deadly Caribbean Coral Diseases: White Plague and Stony Coral Tissue Loss Disease

For several decades, white plagues (WPDs: WPD-I, II and III) and more recently, stony coral tissue loss disease (SCTLD) have significantly impacted Caribbean corals. These diseases are often difficult to separate in the field as they produce similar gross signs. Here we aimed to compare what we know about WPD and SCTLD in terms of: (1) pathology, (2) etiology, and (3) epizootiology. We reviewed over 114 peer-reviewed publications from 1973 to 2021. Overall, WPD and SCTLD resemble each other macroscopically, mainly due to the rapid tissue loss they produce in their hosts, however, SCTLD has a more concise case definition. Multiple-coalescent lesions are often observed in colonies with SCTLD and rarely in WPD. A unique diagnostic sign of SCTLD is the presence of bleached circular areas when SCTLD lesions are first appearing in the colony. The paucity of histopathologic archives for WPDs for multiple species across geographies makes it impossible to tell if WPD is the same as SCTLD. Both diseases alter the coral microbiome. WPD is controversially regarded as a bacterial infection and more recently a viral infection, whereas for SCTLD the etiology has not been identified, but the putative pathogen, likely to be a virus, has not been confirmed yet. Most striking differences between WPD and SCTLD have been related to duration and phases of epizootic events and mortality rates. While both diseases may become highly prevalent on reefs, SCTLD seems to be more persistent even throughout years. Both transmit directly (contact) and horizontally (waterborne), but organism-mediated transmission is only proven for WPD-II. Given the differences and similarities between these diseases, more detailed information is needed for a better comparison. Specifically, it is important to focus on: (1) tagging colonies to look at disease progression and tissue mortality rates, (2) tracking the fate of the epizootic event by looking at initial coral species affected, the features of lesions and how they spread over colonies and to a wider range of hosts, (3) persistence across years, and (4) repetitive sampling to look at changes in the microbiome as the disease progresses. Our review shows that WPDs and SCTLD are the major causes of coral tissue loss recorded in the Caribbean.

Antipathogenic Activity of Acroporid Bacterial Symbionts Against Brown Band Disease-Associated Bacteria

The coral reefs’ condition in most regions in Indonesia has been declining due to coral diseases, such as Brown Band Disease (BrBD). A treatment for BrBD involves the use of biological control agents that have antagonistic properties against disease-causing agents. This study aimed to isolate bacteria from healthy hard coral, those associated with BrBD, and those that had bioactivities against BrBD. Sampling and identification of corals and BrBD were carried out in March 2015 at the Marine National Park of Karimunjawa. Bacteria from healthy and infected corals were isolated and purified. The isolates were subjected to antipathogenic assay using overlay and agar diffusion methods. Finally, molecular identification of active bacteria was carried out using the 16S rRNA gene amplification. As many as 57 bacterial isolates were obtained from healthy coral, as well as four bacterial isolates from coral with BrBD symptoms. A total of 15 bacterial isolates (26%) showed antipathogenic activity against BrBD-associated bacteria. Three isolates with the strongest antipathogenic activities, i.e., GAMSH 3, KASH 6, and TAPSH 1 were identified by 16S rRNA gene sequences. The results showed that they were aligned to Virgibacillus marismortui (97%), Oceanobacillus iheyensis (97%), and Bacillus cereus (96%), respectively.

A comparative baseline of coral disease in three regions along the Saudi Arabian coast of the central Red Sea

Coral disease is a growing problem for coral reefs globally and diseases have been linked to thermal stress, excess nutrients, overfishing and other human impacts. The Red Sea is a unique environment for corals with a strong environmental gradient characterized by temperature extremes and high salinities, but minimal terrestrial runoff or riverine input and their associated pollution. Yet, relatively little is known about coral diseases in this region. Disease surveys were conducted at 22 reefs within three regions (Yanbu, Thuwal, Al Lith) in the central Red Sea along the Saudi Arabian coast. Surveys occurred in October 2015, which coincided with a hyperthermal-induced bleaching event. Our objectives were to 1) document types, prevalence, and distribution of coral diseases in a region with minimal terrestrial input, 2) compare regional differences in diseases and bleaching along a latitudinal gradient of environmental conditions, and 3) use histopathology to characterize disease lesions at the cellular level. Coral reefs of the central Red Sea had a widespread but a surprisingly low prevalence of disease (<0.5%), based on the examination of >75,750 colonies. Twenty diseases were recorded affecting 16 coral taxa and included black band disease, white syndromes, endolithic hypermycosis, skeletal eroding band, growth anomalies and focal bleached patches. The three most common diseases were Acropora white syndrome (59.1% of the survey sites), Porites growth anomalies (40.9%), and Porites white syndrome (31.8%). Sixteen out of 30 coral genera within transects had lesions and Acropora, Millepora and Lobophyllia were the most commonly affected. Cell-associated microbial aggregates were found in four coral genera including a first report in Stylophora. Differences in disease prevalence, coral cover, amount of heat stress as measured by degree heating weeks (DHW) and extent of bleaching was evident among sites. Disease prevalence was not explained by coral cover or DHW, and a negative relationship between coral bleaching and disease prevalence was found. The northern-most sites off the coast of Yanbu had the highest average disease prevalence and highest average DHW values but no bleaching. Our study provides a foundation and baseline data for coral disease prevalence in the central Red Sea, which is projected to increase as a consequence of increased frequency and severity of ocean warming.

Ranking 67 Florida Reefs for Survival of Acropora cervicornis Outplants

Over the past three decades, coral populations have declined across the tropical and subtropical oceans because of thermal stress, coral diseases, and pollution. Restoration programs are currently attempting to re-establish depauperate coral populations along the Florida reef tract. We took an integrated Bayesian approach to determine which Florida reefs ranked highest based on the survival of outplanted colonies of Acropora cervicornis from 2012 to 2018. Survival of A. cervicornis outplants was highly variable in the upper Florida Keys with some reefs showing the highest likelihood of survival (e.g., North Dry Rocks, Carysfort, Key Largo Dry Rocks, and Conch Reef), whereas some adjacent reefs showed the lowest likelihood of survival (e.g., an Unnamed Reef, Pickles Reef, and U47 Patch Reef). Similarly, survival was highly variable in the middle and lower Florida Keys and in the Broward-Miami subregions. Survival was high and less variable in Biscayne Bay and low and less variable in the Marquesas subregions. The reefs that ranked lowest for outplant survival were exposed to high wave energy. Partitioning out the spatial effects of reefs and subregions from the model, we detected spatial latent effects of low survival that were most evident in the middle and the upper Florida Keys, particularly between 2015 and 2017. The overall high spatial and temporal variability in survival among adjacent reefs highlights a need to outplant nursery-reared colonies strategically, in order to optimize coral-population recovery efforts in Florida.