The Smell of Coral

Stony gardens of shifting light
sway vibrantly beneath the waves;
rainforests of the sea,
whose motely splendour
permeates still waters
with a pale and delicate aroma.
Bouquets that blossom
in the gauzy depths,
then wilt in the heat
of unwelcome advances.
The chemical richness
of this subtle scent
drowned out by the stench
of our planetary halitosis.

This poem is inspired by recent research, which has found that the chemical signals that are emitted by coral reefs provide an indicator of their relative health.

Increasing heat stress due to anthropogenic climate change is causing coral reef decline, with severe heat stress acting as the main precursor to large-scale bleaching, the outbreak of disease, and eventually death. For example, recent mass bleaching events, driven by heat stress, have caused a 50% loss of Great Barrier Reef corals over the past 25 years. These ecosystem-scale disruptions are typically accompanied by changes in the emissions of biogenic volatile organic compounds (BVOCs), i.e. organic chemicals that are produced by the corals as a result of biological activity. These compounds represent the end products of many biological processes and can therefore be directly linked to the health of an ecosystem. For example, stress events can lead to the creation of specific BVOCs to support tolerance to harm and can also result in changes to the amount that are produced. These compounds are also tied to the ways in which corals communicate with other species as well as their antimicrobial defence against pathogens. Understanding when these compounds are emitted and how they change in response to heat stress is therefore vital to better understanding the effect that humans are having on coral reefs.

In this new study, researchers investigated how these gaseous emissions are altered by heat stress events that are driving rapid deterioration of coral reef ecosystems. By observing species found in the southern Great Barrier Reef near Heron Island in northeast Queensland (Australia), the abundance and chemical diversity of the coral’s gas emissions were found to fall significantly during heat stress experiments. This study is the first to explore the overall chemical ‘smell’ of healthy and stressed corals, revealing that heat stress dramatically decreases the chemical diversity and quantity of these BVOCs, which could further impact their capacity to cope with increasing temperatures. The discovery of a loss of these smells under heat stress driven by ocean warming is yet more evidence that these reefs will continue to change dramatically, unless something is done to urgently address climate change.