Just as different humans handle stress differently, corals handle it differently. Even coral colonies of the same species that grow next to each other vary in their tolerance to stressors such as heat.
In research published today, we have discovered surprising new evidence of variable heat tolerance in corals. These differences become increasingly important as the world’s oceans warm.
The world’s fourth global mass bleaching event was declared earlier this year. The Great Barrier Reef has suffered mass bleaching five times since 2016 – most recently last summer. The declaration comes after the world’s hottest year on record.
To keep the world’s coral reefs healthy and functioning, global carbon emissions must be reduced dramatically to slow the rate at which the oceans are warming. As humanity works toward that goal, interventions can buy time for corals to survive in their warming environment.
what we did
The heat tolerance of corals can be measured by analysing their responses to higher water temperatures. Our research involved measuring the bleaching threshold of more than 500 colonies of tabulate corals, acropora hyacinthus,
acropora hyacinthus It is a common coral that forms “tables” of small branches. This species is ecologically important and highly sensitive to heat waves, making it a prime candidate for conservation.
The coral’s distinctive color is provided by algae living inside its tissues. The algae also provide most of the coral’s nutrition. When water temperatures get too high for too long, the coral expels the algae, causing it to bleach and starve.
While at sea, we visited 17 reefs for scuba diving and exploring acropora hyacinthusWe then brought samples of these corals aboard a research vessel to conduct experiments.
Our specially designed portable experiment system contained 12 tanks set at four different temperatures. Coral fragments were placed in each tank and subjected to short-term heat stress at different temperatures.
Next, we measured the amount of pigment left in the coral fragments, which directly corresponds to the amount of algae left in the coral cells.
We then determined each coral’s bleaching threshold – in other words, the temperature at which the coral’s colour drops to 50% of its healthy level. This helped us understand how much variation exists and where the most heat-tolerant colonies live.
So what did we find? Under our experiments, the amount of pigment retained at high temperatures varied from 3% to 95%. This means that at high temperatures, some coral colonies bleached completely while others were not affected.
Of the 17 reefs we studied, 12 had colonies with bleaching thresholds in the top 25%. This means that heat-tolerant corals can be found in most of the reefs we sampled.
nature versus nurture
Corals handle stress differently for two reasons: nature and nurture.
Each coral has a unique “nature” or genetic makeup that can influence its heat tolerance. Our results suggest that corals found throughout the Great Barrier Reef may have unique genetic resources that are important for recovery and adaptation.
However, certain aspects of the marine environment can promote or hinder corals’ response to heat stress. These include water temperature, nutrient conditions, and symbiotic algae living inside coral tissue.
We found that corals living in warmer areas, such as the northern Great Barrier Reef, can tolerate higher water temperatures. However, because the water in these areas is so warm, the corals are already close to their temperature limit.
Corals in the southern Great Barrier Reef cannot tolerate as high temperatures as their northern neighbours. Our findings show that these corals can tolerate warmer temperatures than corals further north.
These tolerance patterns may influence which corals survive marine heatwaves.
Giving our reefs a future
Our findings have potentially important implications for the ability of corals to adapt to warming seas under climate change.
The results could also inform reef restoration and conservation efforts. For example, heat-tolerant native corals could be selectively bred to produce offspring that are better suited to warmer waters.
The success of such programs depends on the extent to which the genetic makeup of corals controls their tolerance to heat. Therefore, the next step in this research is to investigate these genetic differences.
Selective breeding trials are already underway using the most heat-tolerant corals identified in this study.
Reducing greenhouse gas emissions is imperative when it comes to protecting our coral reefs. However, interventions such as selective breeding can be useful complements to give coral reefs a better future.
Melissa Naugle, PhD Candidate in Coral Ecology, Southern Cross University; Emily Howells, Senior Research Fellow in Marine Biology, Southern Cross University, and Line Kay Bay, Research Program Director, Australian Institute of Marine Science
This article is republished from The Conversation under a Creative Commons license. Read the original article.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
