Background

Coral bleaching is a process in which corals expel symbiotic algae (zooxanthellae) living in their tissues and their photosynthetic pigments, resulting in the coral turning white. Bleaching is a stress response that may occur in response to changes in temperature, salinity, and light. As zooxanthellae provide up to 90% of a coral's energy, bleached corals are vulnerable to starvation, decreasing reproductive capacity and increading mortality rates. Driven by global warming, sea surface temperatures around Australia have increased by over 1℃ since 1900, putting corals under heat stress and resulting in more intense and frequent bleaching events. Over the past seven years, four mass bleaching events have occurred on the Great Barrier Reef (GBR), with 73% of reefs in the marine park experiencing prevalent bleaching (>10% of coral cover) during the most recent event in 2023-2024.

Given coral reefs support 25% of marine species, their destruction as a result of bleaching poses a significant risk to ocean ecology and biodiversity. Coral bleaching also has pervasive social impacts. In 2015-16, the GBR was estimated to contribute 64,000 jobs and $6.4 billion to the Australian economy, 90% of which was from tourism. Reefs damaged by bleaching lose many features that underpin the aesthetic appeal on which reef tourism is based, and surveys suggest the GBR is at risk of losing one million visitors annually if severe bleaching continues. Furthermore, the diminution of coral reefs disproportionately affects First Nations peoples, whose cultural and spiritual ties to the reefs span millennia.

Many environmental factors associated with mass bleaching can induce increased production of reactive oxygen species (ROS) by zooxanthellae and their host coral. While, below some ROS concentration threshold, the coral’s antioxidant defences can compensate for the destructive capacity of ROS, high ROS concentrations may cause oxidative damage. It has been hypothesised that this then triggers signalling cascades ending with a breakdown of host–algae symbiosis, supported by studies showing that exogenous antioxidants mitigate bleaching.

Solution

We aim to limit future coral bleaching events by neutralising excess ROS produced when coral are under heat stress. Our system, CORA, achieves this through the use of an ROS-detoxifying catalase, which we express in host coral probiotics. Catalase expression will be regulated by a redox-sensitive promoter so that it is only induced when ROS levels are high, minimising the likelihood of these enzymes disrupting normal bacterial functioning. We test the efficacy of this strategy through experiments that monitor ROS levels with fluorescent probes.

In the future, we plan to incorperate a chemically inducible CRISPR kill switch mechanism into the bacterial genome to act as a failsafe in case of unintended ecological outcomes and to prevent the escape of mutant probiotics outside controlled areas.

We also aim to design hardware for the implementation of our engineered probiotics to ensure localisation to reefs. This will include fabricating novel biofilms with coral biocompatibility, including working with emergent biomaterials for reef regeneration. Incorporation of our engineered organism into these coral scaffolds will enhance restoration efforts, by providing robust responses to abiotic stresses.

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