The cnidarian group includes corals, jellyfish, and sea anemones. But corals, as we know them, are not simply animals: they are ecological units encompassing the cnidarian host and its associated dinoflagellate algae (Symbiodinaceae), microeukaryotes, viruses, bacteria, and archaea. 

Extreme temperatures disrupt the metabolic relationship between the coral host and symbionts, causing bleaching and, if prolonged, death of the holobiont. Responses to acute warming among corals do indeed depend, in part, on genetic factors in both the coral host and its symbionts.

Currently, some simplistic media articles suggest using Red Sea corals, which apparently possess exceptional thermal tolerance, in other geographies. A first solution would indeed be the natural selection of corals that can adapt to thermal stress, but the strategy would not come without problems: apart from logistical hurdles and possible issues with symbionts, this one-dimensional approach could lead to the loss of vulnerable coral individuals, decreasing biodiversity.

In their recent academic endeavour, 28 researchers suggested how to deal with coral adaptation measures in a more balanced way: a comprehensive research agenda catalysed by large-scale, multi-institutional hubs increasing experimental scope and statistical power. The collaboration, which should involve governments and research organisations, aims at gathering long-term data spanning coral generations.

In their paper published on March 30, they did indeed underline that members of a holobiont differ greatly in terms of transmission mode, and generation time: coral lifespans can vary between a few years and centuries, whereas members of the associated microbiome typically live hours to days. Corals’ adaptation measures require these complex ecological units to remain in balance.

As adult host thermal tolerance variation is a heritable trait, thermal adaptation can happen through sexual propagation or through translocations: translocated corals can transmit tolerant symbionts to neighbouring conspecifics. 

Scientists did indeed reveal a natural variation in thermal tolerance, a spread of Symbiodinaceae from a translocated host to a local host, and a possible proliferation of tolerant algal symbionts during heatwaves. Interaction between corals is indeed the first possible solution. 

“Interspecific hybridisation can generate offspring with fitness equal to or greater than that of the parental species”, the 28 researchers added in the paper.

For the “between corals” strategy, though, the research lists three sets of problems: estimates of vital rates are limited, thermal tolerance estimates are not necessarily related to vital rates, and information needed to predict the effect of assisted evolution is lacking.

Within corals, conditioning and inoculation with heat-tolerant Symbiodinaceae can enhance thermal tolerance. Indeed, artificial selection applies to corals and to their symbionts, too. Inoculating the holobiont with beneficial, artificially selected bacteria is another possible solution.

Yet, the paper explains that the spatiotemporal dynamics of holobiont associations are poorly understood, that differences in dispersal among holobiont members constrain the effectiveness of assisted evolution, and that the contribution of different holobiont members to trade-offs among traits is poorly understood. “Evidence for the persistence over years of these effects in the wild is still lacking”, explains the paper.

A third set of actions concerns the cells within corals, as scientists have already identified the single genes associated with thermal tolerance. Selective breeding and the enhancement of Symbiodinaceae’s tolerance are other primary examples of assisted evolution solutions.

“Selective breeding experiments have demonstrated that heat-tolerant crosses can produce first-generation offspring capable of withstanding an additional 1- degree heating week (DHW) of thermal stress compared with less-tolerant crosses. Greater gains could be achieved through successive rounds of selection or by targeting parents from warmer or more variable thermal environments,” said the researchers in the paper “Accelerating coral assisted evolution to keep pace with climate change”.

Also in this case, though, there are knowledge gaps: the genetic basis of thermal tolerance in corals remains unresolved, estimates of additive genetic variance are rare and underpowered, and estimates of genetic covariance between traits are lacking, the paper explains.

A 1.5 °C warming above preindustrial levels could cause 75–90% of reefs to experience lethal heat stress.

“If global warming is limited to 2 °C above preindustrial levels, some Pacific reefs will probably still experience marine heatwaves exceeding 16 DHWs (the intensity of the most extreme heatwave recorded to date) up to four times per decade by mid-century. In the more likely 2.7 °C warming scenario, these heatwaves could occur every 2 years in the second half of the century. Under a 3.6 °C rise, marine heatwaves exceeding 16 DHWs would occur in 9 out of 10 years, reaching up to 50 DHWs by the end of the century,” wrote the researchers. 

The precise rates at which coral populations can adapt to thermal stress are unknown.

Conservation programs that combine assisted evolution with natural adaptation are unlikely to increase thermal tolerance quickly and broadly enough to offset the escalating thermal stress in most climate change scenarios, says the academic paper published in Nature Reviews Biodiversity.

“If research and development of methods of assisted evolution are accelerated and deployed at scale, thermal tolerance might be enhanced sufficiently in the short window of opportunity (between 10 and 20 years depending on the climate change scenario and ecoregion)”.

Possible solutions to systemic problems, therefore, require a quick, international, systemic approach. 

The researchers highlight the importance of broad collaborative efforts centred on a selected group of species, without advocating for specific species.

“We propose experiments measuring vital rates across holobiont members of the same coral species, sourced from multiple populations,” reads the paper, suggesting that the method could validate laboratory assays in identifying tolerant corals.

The researchers should then, through coordinated efforts, study how manipulated genets can affect holobionts population dynamics, through different sensors: satellites, drones, cameras, and autonomous underwater vehicles.

The data would then be used to estimate the size and scope of the intervention.

The researchers proposed to prioritize extensive genetic samples across different varieties to understand cryptic species and reproductive barriers.

The researchers also said that the international community should pull resources together to study how tolerant algal symbionts disperse during their free-living stage, also as a function of the host genetics. 

The researchers underlined four priorities for holobiont thermal tolerance: use quantitative genetic approaches to assess how fast thermal tolerance can evolve; leverage genotype-by-environment interactions to optimize coral fitness; produce thermally enhanced, genetically diverse broodstock that thrive across environments; harness genomic prediction to identify heat-tolerant corals.