New research published recently in Coral Reefs, a quarterly peer-reviewed scientific journal, documents a shallow-water coral reef in Great Abaco that can tolerate extreme warming ocean temperatures. During a research expedition led by the Perry Institute of Marine Science, scientists from the Shedd Aquarium in Chicago, SECORE International, the University of South Florida and Middlebury College examined a coral reef off Abaco that is able to thrive in conditions in which other coral reefs struggle to survive. The study uncovered genetic differences between two adjacent reefs, both dominated by mountainous star coral (Orbicella faveolata), which may have allowed certain corals to survive through a coral bleaching event in 2015. Their results may help inform strategies to protect and restore coral reefs in the face of rising sea temperatures from climate change.
For The Bahamas, which has more coral reef area than any other nation in the region, conserving coral reefs is vital to the country’s economy, culture and livelihoods.
“Coral reefs are part of the fabric of The Bahamas, harboring important fishery resources that feed the nation, are vital to the economy and protect the coastal zone where homes and tourism is concentrated,” said Craig Dahlgren, Ph.D., executive director of the Perry Institute for Marine Science. “As our oceans warm from climate change, we must continue to study invaluable and fragile corals to help them persist. This new research is an essential piece of the coral conservation puzzle.”
Previous research describes climate change as the primary driver of coral reef declines across the globe. Increased sea temperatures of even about 1 degree Celsius (less than 2 degrees Fahrenheit) above usual conditions can induce coral bleaching – the collapse of a vital symbiosis between corals and microalgae, during which stressed corals expel the beneficial algae. Bleaching can result in coral death and broader ecosystem degradation. For example, coral cover has decreased by over 80 percent on Caribbean reefs since 1970.
“We know that warmer sea temperatures associated with climate change are dramatically impacting reefs globally, so with our research partners, we are working hard and working quickly to determine which corals could survive,” said Ross Cunning, Ph.D., research biologist at Shedd Aquarium. “Identifying thermally tolerant reefs like the one in Great Abaco in The Bahamas is a high research priority for coral scientists and will help us determine where we should focus our conservation and reef restoration efforts.”
Adaptive genes and helpful microorganisms, such as the microalgae and bacteria that live in the corals’ tissues, are known to influence corals’ tolerance to heat. Four groups of algae from the family Symbiodiniaceae, which commonly live within Caribbean corals, are known to provide varying levels of thermal tolerance to the corals they live with. However, knowledge gaps persist.
“There’s a lot that we don’t know about how genetics, symbiotic microorganisms and environmental factors work together to influence coral bleaching,” said Katie Parker, research assistant at Shedd Aquarium. “We set out for answers that will help us understand how some corals might survive extreme temperatures, while others cannot, even though they’re the same species.”
To investigate, PIMS researchers and partners looked to corals that are thriving in an extreme, marginal environment. Mermaid Reef in Great Abaco experienced warming up to 33 degrees Celsius (91.4 degrees Fahrenheit) in 2015, beyond the temperature expected to cause bleaching. However, at this location, mountainous star corals did not bleach. Yet, extensive bleaching of this same species of corals was observed at Sandy Cay Reef (about 18 kilometers or 11 miles south of Mermaid Reef) despite maximum temperatures of only 32 degrees Celsius (89.6 degrees Fahrenheit).
Comparing data from both locations, the researchers discovered that heat-tolerant mountainous star corals at Mermaid Reef were genetically identical and exclusively hosted one algae group (Durusdinium), which is known to increase thermal tolerance by about 1 to 1.5 degrees Celsius. A more diverse bacterial community was found in the tissues of these corals, which may provide the corals with additional defense to heat and other stressors. Meanwhile, the mountainous star corals that bleached at Sandy Cay Reef were more genetically diverse and they hosted a variety of algal symbiont communities.
“The corals we studied in Mermaid Reef were presumably better suited to handle warmer water temperatures because this shallow, inshore reef regularly experiences high temperature and high light conditions, which may have selected for the hardiest corals and symbiotic algae,” said Parker. “Further study of reefs like these may help us better understand how corals acclimate and adapt to ongoing climate change.”
The study suggests that certain coral communities with adaptive genetic and microbial defenses to heat stress may survive and even thrive as sea temperatures rise.
“We are hopeful that through selective breeding and propagation, heat-tolerant corals like the ones at Mermaid Reef may help optimize conservation and restoration efforts for reefs at risk,” said Cunning. “But these interventions can only buy a little more time; we have to stop ocean warming in order to ensure a future with healthy coral reefs.”