Coral and algae relationship the key to understanding bleaching

14 May 2019

Genomic data is helping IMB researchers Dr Cheong Xin Chan and Raúl González-Pech understand the mutually beneficial (simbiotic) relationship between coral and the algae living inside them.

Coral bleaching occurs when the symbiotic relationship between coral and algae breaks down. Corals under environmental stress, for example from warming waters, disconnect from their algae partners, losing the energy source the algae had provided.

“Most studies to date have been about the corals and not the algae. Little is known about the molecular mechanisms underlying their symbiotic relationship. How can we understand the break-up if we don’t understand the relationship in the first place?” Dr Chan said.

Looking for algae genes to enhance resilience

The algae that live within coral are dinoflagellates from the family Symbiodiniaceae. Dinoflagellates are phytoplankton—tiny, photosynthetic organisms that manufacture their own food harvesting energy from sunlight. They are very diverse; some are toxic, causing the harmful algal blooms known as ‘red tides’, while others provide bioluminescence, or grow in sea ice, and many are free-living.

The researchers are using genome studies to complement ecological studies, looking for genes that enhance resilience in the algae and help their coral partners adapt to the environmental shifts that climate change creates.

The golden algae of Symbiodinium. Credit: Dr Cheong Xin Chan

Algal genomes can be complicated

“Nothing is straightforward with these algae—they have some of the weirdest genomes we’ve ever seen,” Mr González-Pech said. “Their genomes are about half the size of the human genome. In a human cell, DNA is organised into 23 pairs of chromosomes. When we look at these algal cells under a microscope, their DNA is so tightly packed that we still don’t know exactly how many chromosomes they have.”

“In our earlier analysis of the genomes, we found that these algae may be capable of sexual reproduction, which may enhance their capacity to adapt to the environment. We now want to compare the genomics of the symbiotic and free-living species and understand how the differences in their genomes correspond to differences in their lifestyles. This will give us more clues as about their symbiotic relationship with coral.”

Previous studies of other cell residents were based on bacteria or parasites, so researchers predicted these algae would have a similar evolutionary path, but now genetic data shows that these algae operate very differently to other cell inhabitants.

Studying DNA to answer fundamental questions

“Dinoflagellates are vital for the survival of Australia’s Great Barrier Reef—we can use genomic information to address fundamental questions of what makes these algae successful symbiotic partners in the coral reefs, for example, how these algae contribute to heat-resistance in certain corals more than others,” Dr Chan said.

“We have sequenced nine of these algal genomes in four years—the sequencing technology has come so far and allowed us to be the most prolific lab for generating dinoflagellate genomic data.”

“Understanding this relationship between algae and coral is one of the most challenging problems. Here at IMB, we are motivated by the most complex puzzles.”

The research, in collaboration with Emeritus Professor Mark Ragan at IMB and Professor Debashish Bhattacharya at Rutgers University, USA, was published in Trends in Ecology & Evolution and supported by two grants funded by the Australian Research Council