From Reef to Rubble: the threat posed by Marine Heatwaves

Written by: Gus Fordyce

Reefs are more than just their biological communities. They are towering walls reaching for the sun, gentle slopes leading to the depths and expansive flats stretching to the horizon. The diverse range of shapes and structures, big and small, on coral reefs is a fundamental aspect of what makes them beautiful and invaluable. The complexity of their physical architecture influences the way reefs recover, the biomass of herbivorous fish and algal competition, and the provisioning of ecosystem services such as tourism and coastal protection. While we have long appreciated the threat posed by ocean acidification to the persistence of the physical reef, our new study demonstrates that increasingly severe marine heatwaves can rapidly degrade reef building blocks – coral skeletons. 

How does a marine heatwave degrade rock? We asked this question when we received coral fragment samples taken during the 2016 mass mortality event on the Great Barrier Reef. The tissue was almost falling off the oddly brittle coral skeletons. These skeletons were then dissolving unusually quickly in EDTA used for histological preparations. Then Chasing Coral came out and we saw the Lizard Island timelapse. The distressingly rapid overgrowth of algae seemed to us to be accompanied by a loss in structural complexity. But this was happening in 6 to 8 weeks; usually we measure these changes over years. So we set out to test and identify the possible processes that could be underlying this. We ran an 8-week experiment on Heron Island in the Great Barrier Reef, complete with four 600L tanks and and almost 300 fragments across two species. These poor fragments were blasted with severe heat stress, which we based off that recorded around Lizard Island in 2016.

We looked at the integrity of coral skeletons throughout this experiment and recorded rates of dissolution 5 times higher than the otherwise highest published rates observed by Catalina Reyes-Nivia during combined 3 °C warming and 1010 µatm pCO₂. It seemed that severe marine heatwaves were causing unprecedented rates of dissolution in a matter of weeks. This manifested itself as a 100% increase in internal skeletal porosity, a 12% decrease in skeletal hardness and a clear loss of the otherwise complex and symmetrical corallite structure. 

The cause is a little known group of microbes living in darkness inside coral skeletons. Within a week of coral mortality, the newly exposed coral skeleton was covered in microalgae that had emerged from within like a mild horror movie. These linked up with colonising microbes to form a thick, fleshy biofilm that encased the coral. These microbial photosynthetic bioeroders quickly dissolved the skeleton and produced large oxygen bubbles that were trapped within the biofilm and acted as a sort of flotation device. You can imagine my surprise to wake up and find bits of coral floating around the tanks at 6 am. This community was dominated by the eukaryotic algae that are now the focus of my PhD and thus, my endless frustration.

Bioerosion by photosynthetic microbes is nothing new. We have known about this weird bunch for over 50 years; and yet our knowledge is in no way near that of their tissue-dwelling, dinoflagellate neighbours that absorb almost all of the incoming light in healthy corals. Some argue that they are secondary symbionts and indeed evidence is building in favour of this. But here we show their more sinister side. A combination of warm water, high light and possibly elevated nutrients from the rotting coral tissue allows them to crumble coral skeletons in a matter of weeks. To fully answer the questions surrounding this process, we need to understand the perpertrators. But studying microbes that inhabit living rocks is, to say the least, a pain.

More broadly, the results of this study should update our understanding and expectations of how marine heatwaves affect coral reefs. In this instance it goes beyond coral bleaching and mortality; the reef itself begins to fall apart. In the first publication of my thesis, we argue for the inclusion of different criteria to include these extreme events – their outcomes are distinct and not always well represented by how we currently measure heat stress events. As we continue to falter in our global action to combat climate change, it is becoming ever more important that we constantly update our understanding and predictions of how coral reefs are responding. This is the latest in a long list of impacts that climate change is having upon coral reef ecosystems, and may be irreversible within our lifetimes without concerted action.