Coral reef declines have been recorded for all major tropical ocean basins since the 1980s, averaging approximately 30%-50% reductions in reef cover globally. These losses are a result of numerous problems, including habitat destruction, pollution, overfishing, disease, and climate change. Greenhouse gas emissions and the associated increases in ocean temperature and carbon dioxide (CO2) concentrations have been implicated in increased reports of coral bleaching, disease outbreaks, and ocean acidification (OA). Mass coral bleaching events from 2014 to 2017 have resulted in dramatic coral die-offs. For the hundreds of millions of people who depend on reefs for food or livelihoods, the thousands of communities that depend on reefs for wave protection, the people whose cultural practices are tied to reef resources, and the many economies that depend on reefs for fisheries or tourism, the health and maintenance of this major global ecosystem is crucial.
While abatement of local and regional stressors will continue to be critical to coral reef persistence, these efforts on their own will not be sufficient to address the impacts of climate change. The recent pan-tropical bleaching events showed that remote coral reefs under minimal influence from human activities bleached as severely as reefs exposed to multiple stressors such as pollution and overfishing. Reduction and mitigation of carbon emissions will be required for successful global management of marine ecosystems. But even with such reductions, committed warming from the current accumulation of greenhouse gases is expected to expose the majority of the world’s reefs to bleaching conditions annually by 2050.
In the face of these predictions, a growing body of research on coral physiology, ecology, molecular biology, and responses to stress has revealed potential tools to increase coral resilience. Some of this knowledge is poised to provide practical interventions in the short-term, whereas other discoveries are poised to facilitate research that may later open the doors to additional interventions.
This committee has been tasked with reviewing the state of science on genetic, ecological, and environmental interventions meant to enhance the persistence and resilience of coral reefs. The complex nature of corals and their associated microbiome (the holobiont; including symbiotic algal, prokaryotic, fungal, and viral components) lends itself to a wide range of possible approaches. In this first report, the committee provides a summary of currently available information on the range of interventions present in the scientific literature. This report provides a basis for the remainder of the committee’s task to be covered in the final report. Specifically, the task in this report is to (the full task can be found in Box 1.2):
Review and summarize scientific research on a range of intervention strategies, either designed specifically for coral or with the potential to be applied to coral, including evaluation of the state of readiness. Strategies of interest include, but are not limited to, stress-hardening, translocation of non-native coral stocks or species, manipulation of symbiotic partnerships within the coral holobiont, managed selection, genetic modification, and to the extent possible, proposed engineering solutions to promote reef persistence, such as shading/cooling during bleaching events.
Resilience refers to the overall ability of individuals, populations, or communities to respond positively after disturbance, restoring some part of their original state. As a concept, resilience can be applied to different levels of ecosystems. For example, individual organisms can show physiological resilience via survival, sustained growth, and/or reproduction (fitness). Populations can show resilience through the ability to recruit new individuals after a disturbance. Communities can show resilience in ecosystem traits such as productivity, diversity, trophic linkages, or sustained biomass through shifts in species composition. This report is structured to address the interventions that have the potential to increase resilience at each of these scales. The report also includes consideration of interventions that could promote persistence of coral reefs although they may not improve resilience, particularly those that reduce exposure to environmental stress, as an important part of the toolkit of responses to deteriorating environmental conditions. For each intervention, its attributes, current feasibility, potential scale, limitations, and risks are reviewed. Strong attention has been paid to similar efforts under way in other countries that are home to extensive reefs and strong research capacity, particularly in Australia.
Attitudes about the need for novel interventions are coalescing among managers and scientists, and the core technologies needed to enact such interventions are quickly advancing. As such, this report is a benchmark that reflects current research, identifying efforts that range from those potentially feasible now to those that offer promise on a decadal time scale. Even with these interventions, reefs at the end of this century will not look like the reefs at the beginning. The goal has been to lay out the toolbox that might allow coral reefs to persist, stabilizing the value of these ecosystems to human well-being, national economies, and future wonder.
SUMMARY OF INTERVENTIONS TO INCREASE PERSISTENCE AND RESILIENCE
Genetic and Reproductive Interventions
Managed selection is the detection of corals with above average stress tolerance and their use in subsequent interventions. The intervention builds on the fact that coral reefs exist along a range of environmental gradients, including temperature and other stressors, reflecting the ability of individuals to acclimate, of populations and communities to adapt via selection of resilient phenotypes, and of species to adapt to one or multiple environmental pressures. These corals can be identified experimentally, by their presence in chronic extreme conditions, or by their survival after acute stresses such as mass bleaching events. Differential tolerance of corals to environmental stressors often has both a genetic component and an acclimation component. Multiple “omics” approaches (genomics, transcriptomics, proteomics, and metabolomics) help identify if phenotypic differences in corals collected from different reefs are due to fixed features.
Managed breeding is the maintenance and restoration of diverse coral reef populations through artificial propagation to achieve increased population sizes and fitness. It may take the form of supportive breeding within populations, outcrossing between populations, and hybridization between species. Supportive breeding within a population seeks to maintain or rebuild population diversity by augmenting local genotypes and increasing population size. The intent of performing crosses within or between species is to introduce additional genetic diversity, resulting in individuals that would have higher fitness than the parental populations or species. Managed breeding relies on the sexual propagation of corals under controlled laboratory conditions, via gamete and larval collection from the field, or through a combination of both. Success relies on high survivorship after reintroduction, and depending on management goals, demonstration of recruitment following outplanting.
Gamete and larval capture and seeding seeks to enhance the natural processes of sexual reproduction in corals by using natural spawning events to supply gametes for future use or larvae for settlement and population re-establishment or replenishment. These tools augment the reproductive strategies of other approaches, particularly managed breeding. Gametes collected in the field can be outcrossed in situ or in the laboratory, providing an opportunity to enhance levels of fertilization and target desirable genotypes or crosses. Larvae can be used to create chimeric colonies or hybrids. As described in the section on algal symbiont manipulation, larvae devoid of symbionts can be infected with types that convey resilience.
Coral cryopreservation is the process by which gametes, embryos, or other living materials are frozen in such a way that they remain viable after being thawed. Much of the effort for corals has focused on gamete cryopreservation, particularly sperm. However, there have been some efforts to test methods to cryopreserve embryonic material, adult tissues, and algal symbionts. Cryopreserved material can be used to increase genetic variation in critically endangered species, and it allows for fertilization between species that in nature do not live close together or that spawn at different times. Creation of viable embryos through fertilization of eggs with cryopreserved sperm is currently feasible, but other approaches are still in the development stage.
Genetic manipulation refers to the direct alteration of the genome of an organism, which may be the coral or a symbiont. Current interest in genetic manipulation is fueled by developments in CRISPR/Cas9-based genome editing, and transcriptome editing, that can be applied to a wide variety of organisms to generate loss-of-function mutations or to modify existing genes. With CRISPR/Cas9 methods, it may be possible to maintain the standing genetic variation at nontarget loci while propagating desirable traits into the population. There is also interest in using gene drives, which create a biased system of inheritance by enhancing passage of a selected genotype to offspring, to spread a desired alteration rapidly through the coral population. The basic mechanism of gene manipulation with CRISPR/Cas9 has been demonstrated in corals. However, there has been no demonstration of altered phenotypes from manipulation and no demonstration of incorporation of manipulated genes into an adult coral. Feasibility for enhancing coral resilience will be dependent on the identification of clear gene targets hypothesized to be able to alter coral resilience through genetic changes. In addition, the long generation time of corals will significantly lengthen the time from research to deployment. In the near term, genetic manipulation also provides an approach to experimentally identify the genetic causes of variation in stress tolerance.
Pre-exposure is the deliberate exposure of an organism to conditions that might confer some degree of additional tolerance to subsequent re-exposure of the organism (and, potentially, its progeny) to the same or similar conditions. Evidence that pre-exposure has a beneficial effect (whether or not the specific mechanism is known) is widespread. The response of a coral to environmental stress is inherently physiological and might involve a shift in basic metabolism, cellular function, energy balance, and relationships with internal symbionts or the microbiome. Mechanisms by which these changes occur include acclimatory and adaptive changes in gene expression, epigenetic modifications, and shifts in algal symbiont communities and/or the microbiome. These responses vary in their longevity, ranging from short term (hours to days) to longer term (months to years), with certain responses potentially lasting for the entire lifespan of the coral colony (typically decades) and even being transgenerational (i.e., passed along to offspring).
Algal symbiont manipulation refers to mechanisms by which algal symbiont communities (family Symbiodiniaceae) are changed in favor of types that enhance the stress tolerance of the coral host. Although corals can often experience changes in symbiont communities following episodes of severe bleaching in the field, directed manipulations of adult corals in favor of more thermotolerant symbionts have to date only been achieved in the laboratory by duplicating these conditions. However, because the majority of corals produce gametes that do not contain algal symbionts, there are also opportunities to introduce algal symbionts during early coral life stages. A potential tradeoff in selecting symbionts that are naturally more heat tolerant, such as some members of the symbiont genus Durusdinium, is that their coral hosts may grow more slowly. These approaches (both at the adult and recruit stage) may also be limited by the availability of preferred symbionts in the region of interest, the specificity of symbionts to their hosts, and the longevity of the manipulated association.
Microbiome manipulation may alter the phenotype of the coral host and subsequently its fitness in response to environmental change. The microbiome in this case refers to the fungal, prokaryotic (bacteria and archaea), and viral components of the microbiome, as opposed to the algal symbiont. The microbiome can influence host coral health through facilitation of enhanced nutrient cycling; production of antibiotics; protection against stressor agents; and supply of essential trace nutrients, metals, and vitamins. The microbiome may be manipulated by shifting abundance through inoculations, adding beneficial bacteria to the holobiont,
subjecting the holobiont to stress to select for adaptive microbiome members, and genetic modification. However, because very little is known about the functional attributes of the coral microbiome, targeted actions are difficult to design without further basic research.
Antibiotics can be highly effective in the prevention and treatment of bacterial (and some protozoan) diseases. Improvement in the condition of corals might thereafter increase their resilience to environmental stress. There is experience applying antibiotics at the aquarium scale, but the risk and technical limitations of applying antibiotics on a large scale inhibits readiness for broad implementation. Specifically, there is a lack of information to guide the specificity and effective dosages that are necessary to reduce the risk of antibiotic resistance. Antibiotic treatment may also affect a range of other commensal and potentially beneficial microbes, especially because lack of specificity necessitates the use of broad-spectrum antibiotics.
Phage therapy is the isolation, identification, and application of viruses that specifically target and infect bacteria. These “bacteriophages” are highly specific to the target bacterial strains, making it unlikely that other symbiotic microbes are affected. In theory, because bacteriophages are self-generating entities, one application rather than multiple applications over time may be sufficient. However, the specific dynamics and practical requirements of temporal applications have not been assessed sufficiently for corals and reefs in general. The application of large numbers of a single bacteriophage to an open reef system presents risks of uncontrolled and unintentional gene transfer events, which may have negative effects on both microbial and macroorganism dynamics. Bacteriophages also have the potential to spread virulence traits across target and nontarget hosts.
Antioxidants may be used to deplete the reactive oxygen species that are produced as a result of exposure of corals to high incident light levels, which are linked to degradation and loss of symbionts. Antioxidants would be applied during early and peak periods of environmental stress, and potentially even following the stress events to help coral recovery. The understanding of the effectiveness of this approach, although promising in early studies, is rudimentary. The risks are currently unknown; while many of the antioxidants are naturally produced compounds, application of high concentrations may have detrimental impacts on organismal function.
Nutritional supplementation of corals with carbon and other essential nutritional elements during periodic stress events can provide increased
resilience, particularly by compensating for lost energy resulting from algal symbiont dysfunction caused by bleaching events. The coral aquarium trade, research facilities, and hobby aquarists routinely supplement the coral diet with a range of commercial feeds that include phytoplankton, rotifers, krill, and even pieces of shrimp, squid, or clams. However, there is currently no dedicated or robust assessment of an optimized coral diet for supplementing nutrition and building coral resilience. The addition of excess labile carbon, nitrogen, and phosphate into the reef environment may promote growth of species that may outcompete the corals or disrupt the symbiosis between corals and their algal partners.
Coral Population and Community Interventions
Managed relocation is the movement of species, populations, genotypes, or phenotypes from a source area to locations beyond their historical distribution, sometimes with different environmental parameters. There are varying goals associated with managed relocation at different scales. Assisted gene flow is the movement of genotypes within a population’s range to support the proliferation of selected genotypes with higher stress tolerance. Assisted migration is the movement of individuals beyond a species’ range to support movement to more favorable conditions, which is particularly valuable when natural dispersal is limited. Introduction to new areas is the introduction of stress-tolerant individuals to an area in order to maintain a coral reef community in the area. Managed relocation results in alterations to community diversity, and the cost and risk generally increase as the scale of movement increases. A key risk for all managed relocation types is the introduction of non-native pathogens, parasites, algae, microbes, commensal invertebrates, and coral predators. There is also the possibility that corals themselves become invasive. Key knowledge gaps for managed relocation generally concern what drives species distributions, species responses to novel environmental conditions, local-scale impacts of climate change, natural scales of long-distance dispersal, and the scale of local adaptation.
Shading of coral reefs reduces their exposure to high solar irradiance, lowering peak sea-surface temperatures during warm summer months and reducing light stress, which is a co-factor in the coral bleaching response. Shading interventions may occur in the atmosphere or in the water over a reef. Clouds and aerosols can be introduced in the atmosphere to absorb and scatter solar radiation. Techniques that have been suggested for use in the water include induced turbidity, polymer surface
layers, and microbubble plumes. A risk from shading is the reductions or cessation of photosynthesis, which will depend on the duration and extent of the light reduction. Consequences of aerosol injection in the atmosphere include the impact of settling aerosol (salt) particles and changes in precipitation in terrestrial or freshwater environments. Interventions that apply shading are largely limited by uncertainty in their effectiveness, control, and technical aspects of scaling up the effects.
Cool water mixing onto coral reefs is a way to reduce thermal stress by replacing or diluting warm water. Specific methods include pumps or processes that promote artificial upwelling using pipes, air lifts, or fans to partly or fully displace warm surface water with cooler water from deeper layers. While it is technically feasible to create mechanisms for artificial upwelling, these approaches are still at their testing stage, with particular questions regarding the ability to scale up. A consequence of artificial upwelling is that nutrient- and CO2-enriched water can be introduced, leading to enhanced algal growth and acidification effects. The efficacy of artificial water mixing to reduce coral bleaching risks depends on the reef setting, geomorphology, flow direction, prevailing winds, and the oceanography and bathymetry of surrounding waters.
Abiotic ocean acidification interventions at the local reef scale alter the carbon chemistry of the seawater flowing over reefs by shifting it toward a higher pH and higher aragonite saturation state (Ωa). Reduction of CO2 in seawater using bubble streams with low CO2 partial pressure builds on the principle that CO2 in air equilibrates with CO2 dissolved in seawater. The addition of strong bases may increase pH directly. The addition of powdered limestone has been proposed as a mechanism to enhance CO2 uptake by the global ocean, and further as an avenue for limiting ocean acidification. Accelerated weathering of limestone is a variant of the approach, but it involves the use of CO2 to create a local environment of low pH around a calcium carbonate (CaCO3) source. Electrochemical splitting of CaCO3 may also increase alkalinity, which can help to elevate Ωa. While bubble stripping carries little risk due to its reliance on predominantly air injection, the introduction of chemicals into the reef environment carries unknown risks. Scale, logistics, resources, and infrastructure represent major constraints.
Seagrass meadows and macroalgal beds can act as OA interventions by drawing down CO2 concentrations and elevating Ωa in shallow-water environments on or adjacent to coral reefs. Feasibility and efficacy are location-dependent because local processes including oceanography,
geomorphology, bathymetry, and currents interacting with benthic communities collectively drive seawater biogeochemistry. Whereas CO2 levels decline during the day due to photosynthesis, they would increase at night due to respiration. Macroalgal management is likely to be preferred in coastal, nutrient-rich coral reef waters while coastal, shallow-water environments are better habitats for seagrasses. Use of seagrasses and macroalgae have varying benefits and risks. Seagrass meadows are vulnerable to ocean warming and cyclones whereas macroalgae are generally more resilient. Additionally, seagrass meadows have high conservation value, while macroalgae are generally an indicator of degraded reef state.
Table S.1 contains a summary of the different types of coral reef interventions included in this report. Current feasibility, potential scale, limitations, and risks are estimated on the basis of current knowledge, research, or deployment, and are interrelated. For example, in most cases there is limited capacity to scale up the feasibility of most interventions to the global level without incurring increased risk.
Identifying Versus Creating Resilience
Some corals show broad tolerance for environmental stresses, can inhabit a strong mosaic of environments, and can be associated with a diverse array of symbionts and microbes. Such variation in tolerance across populations of a species represents capacity for adaptation via natural selection. A strong component of increasing the adaptive capacity of coral reefs is to map these adaptations, understand their function in the holobiont, and use them as potential targets for enhancing population viability or for further genetic manipulation. Finding natural adaptive capacity for heat tolerance or disease resistance, for example, and using it in programs of coral outplanting or managed breeding represents a feasible, scalable approach that can potentially be undertaken in the near term on multiple species. Interventions that focus on augmenting such natural resilience may have low barriers to implementation. While not risk free, if such tolerant variants can be found locally for multiple species, then there are fewer risks than, for example, genomic manipulation or long-distance relocation. However, it is not certain that natural levels of stress resistance will prove adequate to protect corals across the extreme conditions that might occur with future climate changes. Therefore, it may be necessary to generate unprecedented genetic changes. Genomic manipulation of corals or symbionts is just beginning and faces a number of research hurdles before it can become operational.
TABLE S.1 Overview of Interventions Examined in This Report
|Intervention||What It Is||Current Feasibility||Potential Scale||Limitations||Risks|
|Genetic and Reproductive Interventions|
|Managed Selection||Creating increased frequency of existing tolerance genes||In laboratory and at small local scales||Local reef scale; potentially transgenerational||Needs large populations||Decrease in genetic variation|
|Managed Breeding: Supportive Breeding||Enhancing population size by captive rearing and release||Success with some species at small scales||Local reef population; potentially transgenerational||Depends on sufficient population sampling and recruitment success of released individuals||Decrease in genetic variation|
|Managed Breeding: Outcrossing Between Populations||Introducing diversity from other populations through breeding||Demonstrated in laboratory for a few species||Local reef population; potentially transgenerational||Requires transport of gametes or colonies across distances and field testing across generations||Outbreeding depression; native genotypes may be swamped|
|Managed Breeding: Hybridization Between Species||Creation of novel genotypes through breeding||Demonstrated in laboratory for a few species||Local reef population; potentially transgenerational||Limited ability to create hybrids; requires testing for fertility and fitness||Outbreeding depression; competition with native species|
|Gamete and Larval Capture and Seeding||Collection and manipulation in the field and laboratory and release into the wild||Feasible at local scales||Laboratory to local reef scale; potentially transgenerational||Site-specific reproductive timing, recruitment success can be poor||Limited genetic diversity; selection for laboratory versus field success|
|Coral Cryopreservation||Frozen storage of gametes and other cells for later use and transport||Feasibility is high for sperm, and growing for other tissue types||Materials can be transported globally||Requires excess gametes, larvae, or tissues||Long-term survival uncertain; genetic variation reflects only current conditions|
|Genetic Manipulation: Coral||Altering coral genes for new function||Technically feasible for larvae||Would occur in laboratory; can be self-perpetuating||Gene targets and cellular raw material unidentified, long lead time to roll out to reefs||Might alter wrong genes; unknown risks|
|Genetic Manipulation: Symbionts||Altering symbiont genes for new function||Not yet feasible||Would occur in laboratory; can be self-perpetuating||Technology not established; gene targets and cellular raw material unidentified||Might alter wrong genes; kill target cells; unknown risks|
|Pre-exposure||Using stress exposure to make colonies more tolerant||In laboratory and small-scale field trials||Local reef scale; may be temporary or transgenerational||Difficult to scale up beyond local||Could be detrimental if applied incorrectly|
|Algal Symbiont Manipulation||Changing algal symbionts to more tolerant types||Observed after bleaching events; demonstrated in laboratory||Individual coral colony or large spawning events; unknown longevity||Difficult to scale; easier for some coral species than others||Ecological tradeoffs, e.g., slower growth|
|Microbiome Manipulation||Maintaining/increasing abundance of the native or new beneficial microbes||Demonstrated in laboratory and nursery facilities for limited coral species||Locations on reefs to reef scale; applied at times of stress||Reef-wide delivery mechanisms are lacking; lack of known beneficial microbes; little understanding of direct or indirect effects||Potential to increase deleterious microbes, decrease beneficial ones|
|Antibiotics||Adding antibiotics to control pathogenic microbes||Used in aquaculture and demonstration in small-scale field trials||Laboratory, aquarium, and colonies on reef; requires repeated application||Lack of specificity to target pathogens limits effectiveness||Promote antibiotic resistance in deleterious microbes; destabilization of native beneficial microbiomes|
|Intervention||What It Is||Current Feasibility||Potential Scale||Limitations||Risks|
|Phage Therapy||Adding phage viruses to control pathogenic microbes||Demonstrated in laboratory experiments||Local reef scale; potential to self-propagate||Lack of identified target coral pathogens||Undesirable gene transfers across microbial populations; impact on beneficial microbes|
|Antioxidants||Reducing cellular oxidative damage derived from stress using chemical treatments||Demonstrated in some laboratory experiments||Laboratory only; requires repeated application||Little understanding of direct or indirect effects||May affect other reef species|
|Nutritional Supplementation||Using nutrients to improve fitness and increase stress tolerance||Regular use in coral research and aquaculture||Laboratory and aquarium; requires repeated application||Poor understanding of balanced coral diets; reef-wide delivery mechanisms are lacking||Shifts carbon, nitrogen, and phosphate balance and may benefit coral competitors|
|Coral Population and Community Interventions|
|Managed Relocation: Assisted Gene Flow||Increasing abundance of stress-tolerant genes or colonies within population range||Technically feasible with information gaps regarding successful methods||Regional reef scale; can be permanent||Uncertain maintenance of stress tolerance over time||Moving nontarget genes; ecological tradeoffs|
|Managed Relocation: Assisted Migration||Moving stress-tolerant or diverse genes or colonies just outside species’ range||Technically feasible with information gaps regarding project design||Regional reef scale; can be permanent||Uncertain maintenance of stress tolerance and persistence over time between locations||Moving nontarget genes, species, and microbes; ecological tradeoffs|
|Managed Relocation: Introduction to New Areas||Moving stress-tolerant or diverse genes or colonies to new regions||Untested though technically feasible with information gaps regarding project design||Global movement impacting individual reef scale; can be permanent||Uncertain maintenance of stress tolerance and persistence over time between locations||High risk of moving nontarget genes, species, and microbes; ecological tradeoffs|
|Shading: Atmospheric||Sky brightening to relieve light and heat stress||Untested||Local to regional scale; temporary||Needs appropriate atmospheric conditions and technology||Altered light regimes; aerosol (salt) deposition|
|Shading: Marine||Reducing sunlight to relieve light and heat stress||Operational at small scales||Sites within reefs; temporary||Retention and advection limit application||Altered light regimes; plastic pollution|
|Mixing of Cool Water||Pumping cool water onto reef to reduce heat stress||Small-scale field tests with unknown efficacy||Local reef scale; temporary||Energetically costly or impossible to scale up||Altered physical and chemical (pH, nutrients) regimes|
|Abiotic Ocean Acidification Interventions||Reducing CO2 levels chemically||Effective in small-scale laboratory experiments||Sites within reefs depending on environmental setting; requires consistent input||Costly to scale up chemical quantities||Impact of chemicals on environment|
|Seagrass Meadows and Macroalgal Beds||Reducing daytime CO2 levels biologically||Some efficacy shown in field measurements||Local reefs depending on environmental setting; long-term benefit||Limited environmental settings; need to remove macroalgae||Detritus; altered nutrient loads; competition from macroalgae; increased CO2 at night|
A key feature of any intervention scheme for coral reefs is the movement of coral colonies to areas where they are needed to support reef resilience. Whether new adaptive capacity is found on native reefs or generated in the laboratory, the most tolerant corals are likely to be a subset of the population. If these corals survive, then the expectation is that their tolerance is heritable and will spread. Supporting the spread of tolerant types can take several forms. First, promoting propagation and breeding can support local stress-resistant populations so that their offspring can seed other reefs. Second, moving stress-tolerant colonies to adjacent reefs can help them pass their heat tolerance to future offspring in a wider location. Third, long-distance movement of tolerant corals from laboratories or warm water regions can potentially build thermal resistance in new or depleted areas. No known long-distance introduction of corals has been done purposefully. Movement of local stress-resistant colonies over short distances likely has relatively low risks and costs, and has the best scope for upscaling. Movement of laboratory-grown coral colonies to target sites and movement of corals across large distances carry greater risks and costs.
The Value of Diversity
Coral reef ecosystems are built on diversity at the levels of species, genotypes, phenotypes, habitat, ecosystem functions, symbioses, and interactions at both macroscopic and microbial levels. Diverse populations have greater scope for adaptation and are likely to maintain abilities to respond to other stressors besides heat. Diversity supports key coral reef ecosystem services including fisheries and recreation. Interventions that focus on single species, genotypes, or symbionts may be important milestones in developing intervention technology and rescuing corals at these scales in the short term. However, sustaining coral reef ecosystems that will be exposed to a diversity of stressors will require multispecies approaches and consideration of the broad suite of both biological and ecological processes that underpin ecosystem resilience.
Interventions that target a particular resilience trait may necessitate a tradeoff. For example, symbionts that are naturally more heat tolerant, such as those in Durusdinium, impart greater heat tolerance to their coral hosts but may result in slower coral growth rates, reduced reproductive output, and greater disease susceptibility. Additionally, reducing the diversity of genotypes through genetic interventions reduces the ability to
adapt via natural selection to future stresses. Multiple stressors are often associated with coral declines, and the inability to respond to multiple stressors is a risk to reef persistence. Interventions that reduce the light incidence may reduce photosynthetic activity of coral and other nearby organisms such as seagrasses. Artificial upwelling of cool water may lead to both nutrient and CO2 enrichment from deeper waters.
The Complex Holobiont
Corals and their algal and microbial symbionts are a unit that responds uniquely to stress depending on the coral and symbiont genomes and the mix of microbes that live on and within the colony. Alteration of symbiont communities is known to increase heat tolerance for some corals, but with very different levels of empirical evidence. Manipulating each of these poses very different barriers to implementation, different levels of permanence, and different needs for technology development. They also impose different risks. Parallel efforts in native gene discovery, physiological testing, genetic manipulation, and selective breeding will be important investments.
The spatial and temporal scales on which interventions must operate depend on conservation goals, usually related to maintaining a certain level of local diversity and/or ecosystem services. To date, most interventions have operated at experimental or local scales, impacting a limited number of individuals. Some have the potential to be produced and applied at reef scales, including atmospheric shading or application of probiotics, antibiotics, antioxidants, and nutritional supplementation. However, delivering these interventions with specificity, reduced risk, and at the required scale still has significant knowledge barriers. Others rely on large-scale efforts, at least at first, to achieve results beyond the individual. This encompasses efforts that require relocation or managed breeding in the laboratory and outplanting.
On the temporal scale, the effect of an intervention may be either permanent or self-perpetuating across generations, or it may be temporary, requiring either continuous or periodic reapplication during times of stress. Genetic interventions are intended to perpetuate themselves to future generations (unless they are limited to an epigenetic response), although it is likely that a degree of captive breeding and release could continue to be necessary. Physiological interventions affecting individual coral holobionts are generally not permanent and are unlikely to convey resilience to future generations. Managed relocation of coral individuals, if successful, has the potential to remain permanent.
Engineering the Local Environment
Although the increase in average long-term ocean temperatures chronically stresses corals, bleaching events result in acute impacts that are concentrated in the summer period of weeks or months. These acute reactions to abnormally high temperatures might be reduced by transient, local manipulation of the heat or light environment. Furthermore, ocean acidification may become a chronic and significant impact on corals in the future. Potential engineering solutions to these problems are being explored, but none are ready to be deployed on anything but an experimental scale. Additionally, the spatial scale at which they will ultimately have impact is a lingering question. Nevertheless, the ability to deploy this type of transient protection in the future may be important to protect high-value reef environments on local scales.
CONSIDERATIONS FOR IMPLEMENTATION
The interventions discussed in this report have not been implemented beyond experimental scales in the field, if at all, making their efficacy and impacts uncertain. Adaptive management can help account for and resolve key uncertainties in management practices that have uncertain results, and thus is important for assessing the readiness of interventions for implementation at meaningful scales and their ability to meet conservation goals. Careful planning and monitoring of interventions, including the development of model-based expectations, can ensure that projects maximize learning to enhance benefits and reduce risk.
These interventions have varying degrees and likelihoods of benefits and risks. They alter the environment with consequences that cannot completely be foreseen given the current state of knowledge. While adaptive management provides a structured way of improving understanding of these benefits and risks, even this cannot be implemented without the decision to deploy these interventions in the ocean at least at an experimental scale. The task for this report is to synthesize current knowledge and lay the groundwork for informed decisions about conserving coral reefs under climate change. The remainder of the committee’s task, to be documented in a subsequent report, is to provide a framework for evaluating the relative risks and benefits of implementing these interventions. Additionally, the committee will develop a decision pathway to guide progress of these interventions from the research phase to implementation, when and where appropriate. Such a framework can be used to identify intervention strategies for which the consequences and costs may be justified. While it is not the committee’s task to consider the social, policy, legal, and ethical considerations of implementing these approaches, these will be important to decision makers as well.