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07 April 2019 02:00

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Great Barrier Reef: Mass decline in 'coral babies', scientists say

"It's quite a head scratcher," says Waldan Kwong, a University of British Columbia postdoctoral research fellow and lead author for a team of scientists that have found an organism that can produce chlorophyll but does not engage in photosynthesis that may provide clues as to how to protect the world's coral reefs in the future. The peculiar organism is dubbed 'corallicolid' because it is found in 70 per cent of corals around the world and may provide clues as to how to protect endangered coral reefs in the future. "This is the second most abundant cohabitant of coral on the planet and it hasn't been seen until now," says Patrick Keeling, a University of British Columbia botanist and senior researcher overseeing the study published in Nature. They are an apicomplexan, part of a vast group of parasites that have a cellular compartment called a plastid, which is the part of plant and algal cells where photosynthesis takes place. More than a decade ago, photosynthetic algae related to apicomplexans were discovered in healthy corals, indicating they might have evolved from benign photosynthesizing organisms attached to corals before turning into the parasites we know today.

Ecological data showed that coral reefs contain several apicomplexans, but corallicolids, the most most common one, had not been studied until now. The organism has revealed a new puzzle: not only does it have a plastid, but it contains all four plastid genes used in chlorophyll production. Image copyright NETTE WILLIS/ARC CENTRE CORAL REEF STUDIES Image caption The Great Barrier Reef suffered mass bleaching events in 2016 and 2017 The number of new corals on Australia's Great Barrier Reef has plunged by 89% since unprecedented bleaching events in 2016 and 2017, scientists say. The events, which damaged two-thirds of the world's largest reef system, are now being blamed for triggering a collapse in coral re-growth last year. "Dead corals don't make babies," said lead author Prof Terry Hughes, from Queensland's James Cook University. It measured how many adult corals along the reef had survived following the mass bleaching events, and the number of new corals that had been produced. "Across the length of the Great Barrier Reef, there was an average 90% decline from historical [1990s] levels of recruitment," co-author Prof Andrew Baird told the BBC. The study highlights the link between coral vulnerability and rising sea temperatures resulting from sustained global warming, and recommends increased international action to reduce carbon emissions. Coral bleaching is caused by rising temperatures and occurs when corals under stress drive out the algae - known as zooxanthellae - that give them colour. Image copyright ARC COE FOR CORAL REEF STUDIES/ TORY CHASE Image caption Scientists measured the number of coral "babies" in 2018 The researchers said coral replenishment could recover over the next five to 10 years if there were no future bleaching events. The Great Barrier Reef is struggling to create new coral. Scientists at James Cook University just published a study that shows a shocking decrease in the number of baby coral last year, leading to uncertainty about the future of the reef system. The study revealed that new coral declined by a shocking 89 percent because of large bleaching events in 2016 and 2017 — which were caused by climate change. The last bleaching happened in 2017, and scientists counted how many coral survived the crisis and how many new coral sprung up in 2018. Not only were the numbers extremely low compared to historical counts, but the types of new coral being produced are different as well. According to The Guardian, scientists are worried about the health of the reef, especially if it experiences another bleaching event in the next decade. The Great Barrier Reef would probably recover just fine if it weren't for the threat of future bleaching. Despite the negative outlook, scientists believe the Great Barrier Reef can still recover. If the reef recovers, there is also worry that it will be unable to sustain those numbers against additional bleaching events. Hopefully, the Great Barrier Reef will not witness any bleaching in the near future, so it can withstand the effects of climate change and fully flourish. Coral, the partnership between an animal from the Anthozoa group and a microbial alga called Symbiodinium, is an archetypal model of symbiosis. Writing in Nature, Kwong et al.2 challenge this simple binary model of coral symbiosis by identifying a third player in the association. The challenge, therefore, is to map the DNA sequences that identify these microbes to physical cells, and to uncover the biology of such organisms. Two such types of mystery DNA sequence, called ARL-V (apicomplexan-related lineage-V) and type-N, have been consistently found in samples from coral ecosystems5. Phylogenetic trees that map how the organisms containing ARL-V and type-N DNA are related to known microbes suggest that these organisms belong to the Apicomplexa. Many apicomplexan parasites live in the dark, but they contain the vestige of a plastid6, a DNA-containing structure found in plant and algal cells that is required for photosynthesis. But how do the elusive microbes that contain ARL-V and type-N DNA fit into this picture, and what can they tell us about coral ecosystems and the evolutionary history of the Apicomplexa? Kwong and colleagues were intrigued by the apparent association of the ARL-V/type-N DNA signature with corals and related species (Fig. 1). This pattern of localization of ARL-V/type-N cells in the coral is distinct from that of Symbiodinium algae, indicating that the newly identified symbionts participate in an anatomically separate interaction with the anthozoan animal. Kwong et al.2 identified microorganisms belonging to the Apicomplexa group in many related symbiotic associations, such as anemones (a) and corals (b). The corallicolids have retained genes encoding molecules that synthesize chlorophyll, the pigment that absorbs energy from light to enable photosynthesis. In contrast to the situation in all known photosynthetic eukaryotes (species that carry their DNA in a nucleus), the genes that encode the photosystem proteins in corallicolids might be part of the nuclear genome, instead of being part of the plastid genome. The challenge now is to identify the role of the extra parties in corals, lichens and many other symbioses, and to rethink the roles of the better-known partners in light of the new evidence. Do the new players provide some important nutritional12 or protective service for the symbiosis? "I think that's what most people probably think, that you would see more bleaching in places where it's warmer year-round, and that was one of my assumptions as well," says Deron Burkepile, an associate professor in the University of California, Santa Barbara, ecology, evolution, and marine biology department and coauthor of the study in Nature Communications. Many corals host a symbiotic algae whose photosynthesis provides the animal with extra energy. However, when the coral is under stress, it often expels this colorful algae, revealing its white skeleton underneath in a process known as bleaching. The researchers combined field observations of coral bleaching at 3,351 sites across 81 countries from 1998 through 2017 and found it was less severe in the low tropics (close to the equator) and in regions with naturally high surface temperature variability. On the other hand, corals used to high levels of variability may cope better with warming events as well, given they are used to a range of temperatures. "But coral reefs are not out of the woods," Burkepile says. "They are under extreme threat in the near-term—the next few decades to century—from climate change." As carbon emissions continue to rise, we're likely to continue to see the same trend in increasing frequency and intensity of coral bleaching, he notes. The new study points out places where less bleaching than expected occurs. "Our work will help identify the places in the world where maybe we can put extra effort into conserving some of the bright spots in the world's coral reefs." The models scientists currently use to predict bleaching events are pretty accurate, but they work on large scales. Scientists didn't design them to predict the extent of bleaching on the level of individual reefs or islands. They suspect that reefs that had more severe bleaching events faced many of these other stressors in addition to high temperatures. Citizen scientist data made the research possible, Donovan says.