Will coral reefs survive by adaptive bleaching?

Some reef-building corals form symbioses with multiple algal partners that differ in ecologically important traits like heat tolerance. Coral bleaching and recovery can drive symbiont community turnover toward more heat-tolerant partners, and this ‘adaptive bleaching’ response can increase future bleaching thresholds by 1–2°C, aiding survival in warming oceans. However, this mechanism of rapid acclimatization only occurs in corals that are compatible with multiple symbionts, and only when the disturbance regime and competitive dynamics among symbionts are sufficient to bring about community turnover. The full scope of coral taxa and ecological scenarios in which symbiont shuffling occurs remains poorly understood, though its prevalence is likely to increase as warming oceans boost the competitive advantage of heat-tolerant symbionts, increase the frequency of bleaching events, and strengthen metacommunity feedbacks. Still, the constraints, limitations, and potential tradeoffs of symbiont shuffling suggest it will not save coral reef ecosystems; however, it may significantly improve the survival trajectories of some, or perhaps many, coral species. Interventions to manipulate coral symbionts and symbiont communities may expand the scope of their adaptive potential, which may boost coral survival until climate change is addressed.

Characterization of the Coral Disease 'Porites Bleaching with Tissue Loss' (PBTL) From Hawaii

<p>Coral reefs around the world are facing many threats and have sustained severe losses in coral cover over the past few decades. Coral bleaching and disease outbreaks have contributed substantially to this reef decline, however our understanding of factors contributing to the increase in coral disease prevalence are poorly understood. Information on the disease dynamics of different diseases affecting a reef system is essential for the development of effective management strategies. The aim of this research was to characterise and build a case study of a bleaching response affecting Porites compressa in Kaneohoe Bay, Oahu, Hawaii. It manifests as a localised, discrete area on the coral colony with a bleached coenenchyme and pigmented polyps, giving the affected area a “speckled” appearance. A disease by definition is any interruption, cessation or disorder of body functions, systems or organs. Results of this study showed that this localised bleaching causes tissue loss and a reduction in the number of gametes, and hence harm to the host. It was therefore classified as a disease and named Porites bleaching with tissue loss (PBTL). In addition, PBTL does not appear to represent a common thermal bleaching response as it was present throughout the year during times when seawater temperature was well within the coral’s thermal threshold. Symbiodinium cell density in PBTL-affected areas of the coral colony was reduced by 65%, and examination of affected host tissue using light microscopy showed fragmentation and necrosis. However, no potential pathogen was observed. Transmission electron microscopy (TEM) revealed a high occurrence of potential apoptotic Symbiodinium cells and a potential increase in the abundance of virus-like particles (VLPs) in PBTL-affected tissue. However a causal relationship remains to be established. Long-term monitoring showed spatio-temporal variations in PBTL prevalence. Temporal variations in prevalence reflected a seasonal trend with a peak during the summer months, linked to increasing seawater temperature. Spatial variations in disease prevalence were correlated with parrotfish density, turbidity and water motion. Of these, a negative correlation with variability (SD) in turbidity explained most of the variability in PBTL prevalence (12.8%). A positive correlation with water motion explained 9% and a positive correlation with the variability in parrotfish density explained 4.4%. Overall, only a relatively small proportion of variability in PBTL prevalence could be explained by these three factors (26.2%), suggesting that other factors, not investigated in this study, play a more important role in explaining PBTL patterns or that temporal variation in temperature is the overall major driving force. Monitoring of individually tagged P. compressa colonies showed that >80% of affected colonies sustained partial colony mortality (tissue loss) within two months; on average, one third of the colony is lost. The amount of tissue loss sustained was correlated to lesion size but not colony size. Case fatality (total mortality) was low (2.6%), however this disease can affect the same colonies repeatedly, suggesting a potential for progressive damage which could cause increased tissue loss over time. PBTL was not transmissible through direct contact or the water column in controlled aquaria experiments, suggesting that this disease might not be caused by a pathogen, is not highly infectious, or perhaps requires a vector for transmission. At present, PBTL has only been observed within Kaneohe Bay. An investigation of the potential role of host and Symbiodinium genetics in disease susceptibility revealed the same Symbiodinium sub-clade (C15) in healthy and PBTL-affected colonies, suggesting no involvement of Symbiodinium type in disease etiology. Results regarding host genetics remained inconclusive; however a difference in allele frequency at one microsatellite locus was observed between healthy and diseased samples. This difference could, however, be due to a lower amplification of PBTL-affected samples at this locus and needs to be regarded with some caution. The results of this study provide a case definition of PBTL which can be used as a baseline in further studies. P. compressa is the main framework building species in Kaneohe Bay, and the information gathered here on disease dynamics and virulence suggests that PBTL has the potential to negatively impact the resilience of reefs within the bay. Further research into the etiology of PBTL is necessary to fully understand the impact that this disease could have on coral reefs in Hawaii.</p>

Characterization of the Coral Disease 'Porites Bleaching with Tissue Loss' (PBTL) From Hawaii

<p>Coral reefs around the world are facing many threats and have sustained severe losses in coral cover over the past few decades. Coral bleaching and disease outbreaks have contributed substantially to this reef decline, however our understanding of factors contributing to the increase in coral disease prevalence are poorly understood. Information on the disease dynamics of different diseases affecting a reef system is essential for the development of effective management strategies. The aim of this research was to characterise and build a case study of a bleaching response affecting Porites compressa in Kaneohoe Bay, Oahu, Hawaii. It manifests as a localised, discrete area on the coral colony with a bleached coenenchyme and pigmented polyps, giving the affected area a “speckled” appearance. A disease by definition is any interruption, cessation or disorder of body functions, systems or organs. Results of this study showed that this localised bleaching causes tissue loss and a reduction in the number of gametes, and hence harm to the host. It was therefore classified as a disease and named Porites bleaching with tissue loss (PBTL). In addition, PBTL does not appear to represent a common thermal bleaching response as it was present throughout the year during times when seawater temperature was well within the coral’s thermal threshold. Symbiodinium cell density in PBTL-affected areas of the coral colony was reduced by 65%, and examination of affected host tissue using light microscopy showed fragmentation and necrosis. However, no potential pathogen was observed. Transmission electron microscopy (TEM) revealed a high occurrence of potential apoptotic Symbiodinium cells and a potential increase in the abundance of virus-like particles (VLPs) in PBTL-affected tissue. However a causal relationship remains to be established. Long-term monitoring showed spatio-temporal variations in PBTL prevalence. Temporal variations in prevalence reflected a seasonal trend with a peak during the summer months, linked to increasing seawater temperature. Spatial variations in disease prevalence were correlated with parrotfish density, turbidity and water motion. Of these, a negative correlation with variability (SD) in turbidity explained most of the variability in PBTL prevalence (12.8%). A positive correlation with water motion explained 9% and a positive correlation with the variability in parrotfish density explained 4.4%. Overall, only a relatively small proportion of variability in PBTL prevalence could be explained by these three factors (26.2%), suggesting that other factors, not investigated in this study, play a more important role in explaining PBTL patterns or that temporal variation in temperature is the overall major driving force. Monitoring of individually tagged P. compressa colonies showed that >80% of affected colonies sustained partial colony mortality (tissue loss) within two months; on average, one third of the colony is lost. The amount of tissue loss sustained was correlated to lesion size but not colony size. Case fatality (total mortality) was low (2.6%), however this disease can affect the same colonies repeatedly, suggesting a potential for progressive damage which could cause increased tissue loss over time. PBTL was not transmissible through direct contact or the water column in controlled aquaria experiments, suggesting that this disease might not be caused by a pathogen, is not highly infectious, or perhaps requires a vector for transmission. At present, PBTL has only been observed within Kaneohe Bay. An investigation of the potential role of host and Symbiodinium genetics in disease susceptibility revealed the same Symbiodinium sub-clade (C15) in healthy and PBTL-affected colonies, suggesting no involvement of Symbiodinium type in disease etiology. Results regarding host genetics remained inconclusive; however a difference in allele frequency at one microsatellite locus was observed between healthy and diseased samples. This difference could, however, be due to a lower amplification of PBTL-affected samples at this locus and needs to be regarded with some caution. The results of this study provide a case definition of PBTL which can be used as a baseline in further studies. P. compressa is the main framework building species in Kaneohe Bay, and the information gathered here on disease dynamics and virulence suggests that PBTL has the potential to negatively impact the resilience of reefs within the bay. Further research into the etiology of PBTL is necessary to fully understand the impact that this disease could have on coral reefs in Hawaii.</p>

Physiological Differences in Bleaching Response of the Coral Porites astreoides Along the Florida Keys Reef Tract During High-Temperature Stress

The Florida Keys reef tract (FKRT) has a unique geological history wherein Holocene sea-level rise and bathymetry interacted, resulting in a reef-building system with notable spatial differences in reef development. Overprinted on this geologic history, recent global and local stressors have led to degraded reefs dominated by fleshy algae, soft corals, and sponges. Here, we assessed how coral physiology (calcification rate, tissue thickness, reproduction, symbiosis, and bleaching) varies seasonally (winter vs. summer) and geographically using 40 colonies of the mustard hill coral Porites astreoides from four sites across 350 km along the FKRT from 2015 to 2017. The study coincided with a high-temperature event in late summer 2015 that caused heterogeneous levels of coral bleaching across sites. Bleaching severity differed by site, with bleaching response more aligned with heat stress retroactively calculated from local degree heating weeks than those predicted by satellites. Despite differences in temperature profiles and bleaching severity, all colonies hosted Symbiodiniaceae of the same genus (formerly Clade A and subtypes). Overall, P. astreoides at Dry Tortugas National Park, the consistently coolest site, had the highest calcification rates, symbiont cell densities, and reproductive potential (all colonies were reproductive, with most planula larvae per polyp). Corals at Dry Tortugas and Fowey Rocks Light demonstrated strong seasonality in net calcification (higher in summer) and did not express visual or partial-mortality responses from the bleaching event; in contrast, colonies in the middle and southern part of the upper keys, Sombrero Key and Crocker Reef, demonstrated similar reduced fitness from bleaching, but differential recovery trajectories following the heat stress. Identifying reefs, such as Dry Tortugas and possibly Fowey Rocks Light that may serve as heat-stress refugia, is important in selecting candidate sites for adaptive reef-management strategies, such as selective propagation and assisted gene flow, to increase coral-species adaptation to ocean warming.

Prussian Blue and Carbon-Dot Hybrids for Enhanced Electrochromic Performance

In this study, Prussian blue@Carbon-dot (PB@C-dot) hybrids have been developed by one-step hydrothermal method. The incorporation of C-dots into Prussian blue thin film as a way of improving its electrochromic performance was investigated. The structure of the PB@C-dot hybrid was characterized through X-ray diffraction, Raman spectroscopy and scanning electron microscopy. The electrochromic properties showed that incorporation of 10 mL C-dots into the film showed higher optical contrast of 1.6 and superior coloration/bleaching response of 10 and 3 s. It is proposed that the C-dots component used in the construction of the PB@C-dot hybrid plays a key role to achieve superior electrochromic performance.

Substituent-Adjusted Electrochromic Behavior of Symmetric Viologens

As a promising electrochromic material, viologens have attracted increasing attention due to their high redox activity and adjustable electrochromic capability. In order to investigate the effect of alkyl substituents on electrochromic behavior, four alkyl-substituted viologens and a benzyl-substituted viologen were synthesized, namely 1,1′-dioctyl-4,4′-bipyridinium dibromide (OV), 1,1′-didekyl-4,4′-bipyridinium dibromide (DeV), 1,1′-didodecyl-4,4′-bipyridinium dibromide (DoV), 1,1′-dihexadecyl-4,4′-bipyridinium dibromide (HV), and 1,1′-dibenzyl-4,4′-bipyridinium dibromide (BV). The different photophysical and electrochemical properties of these viologens were attributed to their deviation in spatial structure caused by different substituents. Compared with benzyl-substituted BV, a slight blueshift occurred for the absorption peaks of alkyl-substituted viologens from 262 to 257 nm with the increase in alkyl chain length. Moreover, the first redox couple increased positively, and the dimerization of the compound decreased gradually, accompanied by the decrease in optical contrast and distinct chromatic difference. A comparison of chromatic and optical contrasts indicated that OV had the longest coloring response time (RTc), while it was shortest for HV. The bleaching response time (RTb) of viologen films gradually decreased with the alkyl chain length, and the OV film had the shortest RTb. Furthermore, when increasing the length of the alkyl chain, the cycling stabilities of alkyl viologens increased gradually. In addition, the OV film exhibited the best contrast after 200 continuous cycles.

Coral husbandry for ocean futures: leveraging abiotic factors to increase survivorship, growth, and resilience in juvenile Montipora capitata

Reef restoration via direct outplanting of sexually propagated juvenile corals is a key strategy in preserving coral reef ecosystem function in the face of global and local stressors (e.g. ocean warming). To advance our capacity to scale and maximize the efficiency of restoration initiatives, we examined how abiotic conditions (i.e. larval rearing temperature, substrate condition, light intensity, and flow rate) interact to enhance post-settlement survival and growth of sexually propagated juvenile Montipora capitata. Larvae were reared at 3 temperatures (high: 28.9°C, ambient: 27.2°C, low: 24.5°C) for 72 h during larval development, and were subsequently settled on aragonite plugs conditioned in seawater (1 or 10 wk) and raised in different light and flow regimes. These juvenile corals underwent a natural bleaching event in Kāne‘ohe Bay, O‘ahu, Hawai‘i (USA), in summer 2019, allowing us to opportunistically measure bleaching response in addition to survivorship and growth. This study demonstrates how leveraging light and flow can increase the survivorship and growth of juvenile M. capitata. In contrast, larval preconditioning and substrate conditioning had little overall effect on survivorship, growth, or bleaching response. Importantly, there was no optimal combination of abiotic conditions that maximized survival and growth in addition to bleaching tolerances. This study highlights the ability to tailor sexual reproduction for specific restoration goals by addressing knowledge gaps and incorporating practices that could improve resilience in propagated stocks.

Metabolomic shifts associated with heat stress in coral holobionts

Understanding the response of the coral holobiont to environmental change is crucial to inform conservation efforts. The most pressing problem is “coral bleaching,” usually precipitated by prolonged thermal stress. We used untargeted, polar metabolite profiling to investigate the physiological response of the coral species Montipora capitata and Pocillopora acuta to heat stress. Our goal was to identify diagnostic markers present early in the bleaching response. From the untargeted UHPLC-MS data, a variety of co-regulated dipeptides were found that have the highest differential accumulation in both species. The structures of four dipeptides were determined and showed differential accumulation in symbiotic and aposymbiotic (alga-free) populations of the sea anemone Aiptasia (Exaiptasia pallida), suggesting the deep evolutionary origins of these dipeptides and their involvement in symbiosis. These and other metabolites may be used as diagnostic markers for thermal stress in wild coral.

Population genetics of the coral Acropora millepora: Toward genomic prediction of bleaching

Although reef-building corals are declining worldwide, responses to bleaching vary within and across species and are partly heritable. Toward predicting bleaching response from genomic data, we generated a chromosome-scale genome assembly for the coral Acropora millepora. We obtained whole-genome sequences for 237 phenotyped samples collected at 12 reefs along the Great Barrier Reef, among which we inferred little population structure. Scanning the genome for evidence of local adaptation, we detected signatures of long-term balancing selection in the heat-shock co-chaperone sacsin. We conducted a genome-wide association study of visual bleaching score for 213 samples, incorporating the polygenic score derived from it into a predictive model for bleaching in the wild. These results set the stage for genomics-based approaches in conservation strategies.