Probiotics and selective breeding help coral survive the heat

Climate change is increasing the temperatures of the oceans, which disrupts the relationship between corals and their symbiotic photosynthetic algae and causes corals to bleach, in some cases eventually leading to their death. Two new studies have now proposed novel methods of helping coral to survive warmer temperatures.

Beneficial bacteria

An international research team, led by the Federal University of Rio de Janeiro (UFRJ), proposed that manipulating the coral microbiome with beneficial microorganisms for corals (BMC) might enhance stress tolerance. A probiotic cocktail of beneficial bacteria could be administered in advance of a predicted heatwave to help corals recover from high sea temperatures, the team suggested.

To test this, the team selected six beneficial bacterial strains isolated from the coral Mussismilia hispida and used them to inoculate experimental cultures of M. hispida. At the same time, the corals were exposed to a heat stress treatment, with the temperature increasing to 30°C over the course of 10 days before dropping back down to 26°C. The researchers monitored the health of the corals and measured microbial diversity and metabolic parameters in treatments with and without probiotics or heat stress, with the results published in the journal Science Advances.

Initially, there was no difference — corals with and without probiotics reacted similarly at the peak temperature and both bleached. “But then after we dropped the temperature, we observed a plot twist from the group that we treated with probiotic BMC,” said lead author Erika Santoro, who is now a postdoc at King Abdullah University of Science and Technology (KAUST). “That was a really nice surprise.”

Probiotic treatment improved the response and recovery of the corals after the heat stress event, boosting survival from 60–100%. The researchers interpret this as BMC helping the holobiont to mitigate the effects of ‘post-heat stress disorder’ and thus restructure the physiological and metabolic profiles.

The research also describes some of the protective molecular mechanisms. During the recovery period, BMC-treated corals had lower expression of genes involved in apoptosis and cellular reconstruction, and increased expression of thermal stress protection genes. BMC treatment also altered the profile of the microbiome through incorporating some of the beneficial bacteria as well as other changes in the population structure.

BMCs provide a valuable tool to cope with the consequences of climate change, but Santoro emphasised that it is no silver bullet. “Using a probiotic is an effective tool to help corals deal with the heat stress, but we also have to consider other interventions,” she said. These include “protection and conservation, everyone being more aware and, most importantly, decreasing greenhouse gas emissions and trying to change our resource use. Corals will need all of these interventions.”

Selective breeding

Meanwhile, a global research team led by Southern Cross University (SCU) has found that some coral populations harbour greater genetic variation for heat tolerance than has previously been realised, and thus have the potential to adapt to warming oceans.

Dr Emily Howells, a Senior Research Fellow at the National Marine Science Centre at SCU, has been investigating standing genetic variation for heat tolerance in Australian coral populations. She said, “Our previous work revealed that corals in an extreme environment have an exceptionally high heat tolerance, in part due to genetic adaptation of the coral animal.

“In this study, we wanted to test whether we could transfer these beneficial genetic variants to a comparatively heat-sensitive coral population living in milder oceanic conditions.”

The researchers cross-bred corals from the thermally extreme Persian Gulf with those of the same species from the milder Indian Ocean and measured the performance of many families of their offspring. The team found that heat tolerance increased by up to 84% when Indian Ocean mothers were bred with Persian Gulf fathers and was, on average, equivalent to the heat tolerance of offspring with both parents from the Persian Gulf.

“This was an impressive result, because while we were expecting to see some enhancement of heat tolerance, the signal was much stronger than we anticipated from the genetic contribution of the fathers alone as maternal effects also contribute to offspring tolerance,” Dr Howells said.

Genome-wide sequencing of the coral families confirmed these findings by revealing that genetic variants positively associated with heat survival were predominantly inherited from Persian Gulf parents. The researchers also deployed the offspring to the Indian Ocean site and found that cross-bred corals with Persian Gulf fathers survive equally as well as native Indian Ocean purebred corals, but both had higher survival than non-native Persian Gulf purebred corals.

An unexpected finding of the study was that genetic variation was not restricted to the Persian Gulf and was also present at low levels in the cooler Indian Ocean populations. Dr Howells said, “One of the most exciting results of the study was that a small number of Indian Ocean fathers produced offspring with surprisingly high heat tolerance, and these fathers had some of the same genetic variants associated with heat tolerance that were prevalent in the Persian Gulf.

“Such standing genetic variation is essential for natural selection to occur in warming environments.”

These results demonstrate that corals can be selectively bred for enhanced heat tolerance using corals from populations in extreme or warmer environments which have a higher proportion of heat-tolerant genetic variants due to local adaptation. Dr Howells said further testing is required before the method can be implemented in intervention and restoration programs, and noted that “the most important actions to enhance the resilience of all coral populations are those that limit the magnitude of climate change”.

Image caption: A close-up showing the detail in a Platygyra coral. Image credit: Emily Howells.

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