Record high oceanic temperatures triggered an unprecedented global-scale coral bleaching event from 2014 to 2017, which was the longest-lasting and most severe. More than 75% of the world’s tropical reefs – the most diverse ecosystems in the world – were exposed to bleaching-level heat stress and many corals perished. But different coral taxa responded differently to the high sea surface temperatures, depending on the location of reefs – some recovered quickly while others died.
Indian coral reefs are poorly studied compared with those in the Caribbean or the Great Barrier Reef, for example, and as a result, many lack baseline data. A study published earlier this year revealed how coral taxa in Palk Bay, off the Tamil Nadu coast, responded to the severe summer bleaching event of 2016. The branching coral genus Acropora was the most susceptible to bleaching while Favites emerged as the most resistant. And post bleaching, Acropora shifted to hosting a type of symbiont of microalgae that is more heat-resistant, found the researchers, in what comprised the first observation of coral symbiont diversity in an Indian reef.
Many reef-building corals normally house microalgae from the family Symbiodiniaceae, also known as zooxanthellae, inside their tissues. Algae are responsible for giving corals their colour and are referred to as symbionts because they form a partnership with corals where both of them benefit: corals provide algae shelter and nutrients and in return, algae produce food for corals by performing photosynthesis.
But this fragile relationship breaks down when prolonged high sea surface temperatures trigger corals to expel their symbiotic microalgae – a process known as bleaching –making them vulnerable to other stressors, which could eventually kill them. Apart from heat stress caused by warming oceans, coral reef ecosystems are also exposed to local stressors from human activities such as the discharge of waste, coastal runoff, coastal development, and destructive fishing practices, among others.
For the study, the research team surveyed coral colonies in March, May and July, i.e. before, during and after the summer bleaching event of 2016 in the shallow fringing reefs of the Palk Bay region – one near Mandapam and two near Rameswaram Island –noting their bleaching susceptibility and mortality.
The team found differential bleaching susceptibility among coral taxa. In May, the temperature reached a maximum of 33.3 degrees Celsius and among the 508 coral colonies surveyed, 53.8% were bleached. They observed the highest bleaching susceptibilities in the reef-building stony corals of the genus Acropora – 86% – followed by Porites – 65%. In contrast, there were no notable bleaching incidences in Favites, another reef-building stony coral genus that emerged as the most resistant among other coral genera.
“This pattern has already been recorded in many geographical locations including the Great Barrier Reef and Caribbean reefs,” said lead author T Thinesh, who carried out the study when he was a post-doctoral researcher at Pondicherry University and is currently a post-doctoral research associate at Suganthi Devadason Marine Research Institute, Tuticorin. “In India, the observed susceptibility in Acropora is consistent with studies from reefs in the Gulf of Mannar and the Andamans, which found the highest susceptibility in Acropora genera.”
In July, the team recorded 20.9% mortality among all the colonies surveyed and 36.5% of the bleached colonies were dead. Acropora experienced the highest mortality at 48.2%, followed by Porites at 31.9%, Favia at 34.5%, Platygyra at 33.3% and Goniostrea at 30.8%. As with bleaching, the team did not observe any mortality in Favites.
Notably, the mortality rates of corals in Palk Bay appear to be much lower than those in other locations such as in South East Asia, the Maldives, and the Great Barrier Reef where mortality was around 50% in 2016 according to past studies, write the authors.
Boosting coral adaption
There are a diverse range of microalgae symbionts some of which are better at tolerating heat stress than others. After bleaching, some corals have been found to shift to hosting more heat-tolerant symbionts such as those of Symbiodinium clade D –now classified as the genus Durusdinium – as a strategy to adapt and increase their resilience to future heat stress and subsequent bleaching. So the researchers compared the dominant symbionts in five colonies of four dominant coral genera before and after bleaching.
What they found was that while the coral genera Favia, Favites, and Symphyllia were dominated by Durusdinium – formerly clade D – symbionts, Acropora, which had previously hosted the genus Cladocopium – formerly clade C – switched to Durusdinium symbionts during recovery. Symbionts from the newly assigned genus Durusdinium –clade D – are considered to be the most heat-tolerant among all clades and this study expands their geographical distribution.
“Changes in symbiont diversity from Cladocopium to Durusdinium suggest and support the hypothesis that corals in Palk Bay are adapting to rising temperatures by choosing favorable symbionts, explained Thinesh, adding that although clade D symbionts help corals to adapt to high temperatures, they have some demerits too. “Studies found lower calcification rates in corals that hosted clade D symbionts.”
“This is an important and useful piece of work, and one of just a handful of studies in South Asian waters that have attempted to understand the mechanism of bleaching responses,” said Rohan Arthur, a scientist at the Mysuru-based Nature Conservation Foundation, who was not involved in the study. Although “we have known for a while now that coral are able to shift their symbiont assemblages towards more bleaching-resistant clades [typically clade D],” Arthur noted that documenting this transition in Indian coral is a valuable first.
“As climate change is changing the stakes for tropical coral reefs, we need many more studies of this nature to explore the cellular responses of coral as they adapt to a rapidly changing environment,” added Arthur. “Paradoxically, the very laws meant to protect coral [hard coral are Schedule I species under the Wildlife Protection Act of 1972] significantly limit how much we can study them.”
Ramesh Chattagadda, a postdoctoral fellow at the Andaman Nicobar Center for Ocean Science and Technology, National Institute of Ocean Technology, who was also not connected to the study said that the role of Cladocopium and Durusdinium in Acropora corals from the Palk Bay region needs to be demonstrated further.
The mechanisms by which Durusdinium symbionts provide tolerance to heat is unclear but a recent study suggests that the heat-tolerant species Durusdinium trenchii hosted by Acropora shows a different membrane lipid composition from the heat-sensitive Cladocopium C3.
More detailed investigations on symbiont diversity and their distribution should be carried out in all reefs during bleaching to understand the responses of coral communities, said Thinesh, explaining that this will help in categorising potential winners and losers in the face of rising heat stress due to climate change. “Such information will enable us to select potential coral genera for coral transplantation efforts that are being undertaken worldwide, including in India, to repopulate degraded reefs.”
This article first appeared on Mongabay.