Volcanic eruptions, although magnificent to observe, put human infrastructure and environmental ecosystems close to them at high risk of danger. Explosive eruptions can eject kilotons of rock fragments, glass, and minerals into the atmosphere. Volcanic ash, composed of fine particles with a diameter below 2 mm, can travel long distances and accumulate in large quantities on land and in the ocean. Ash deposition especially affects sessile organisms that cannot escape the imminent threat. Among these organisms are corals. 

 

Skeleton-secreting corals are engineers of the coral reefs, which represent arguably the largest bioconstructions on Earth. Through the process of biomineralization, reef-building corals precipitate calcium carbonate skeleton that archives the environmental history in which the corals were formed. Corals live in partnership with tiny, photosynthesizing organisms. These organisms help corals get up to 95%¹ of their daily energy from sunlight, which supports their growth and survival. 

We initially wanted to see if coral skeletons could record past volcanic eruptions, because tropical corals grow quickly and might capture these events in their structure. To achieve this, we conducted a six-week ash exposure experiment at the Centre Scientifique de Monaco using cultured microcolonies of the branching coral Stylophora pistillata. The corals were reared in tanks with controlled light intensity, temperature, and pH and maintained under two different conditions: a control condition and an experimental condition in which corals were exposed to volcanic ash from the 2021 eruption of the La Soufrière volcano on St. Vincent. A moderate ash concentration was chosen to minimize shading and prevent coral smothering, as this study aimed to develop an understanding of the principles of dilute volcanic ash exposure to corals. Throughout the experimental runtime, physiological parameters (photosynthesis rates, photosynthetic efficiency, skeletal growth rate) were monitored. After the experiment, the coral soft tissue was analyzed for metal concentration, total protein content, chlorophyll content, and symbiont density. 

 

After being exposed to volcanic ash for several days, the corals showed significantly enhanced photosynthetic efficiency, with increased oxygen production and improvements in their ability to capture sunlight for energy, compared to non-exposed corals under similar environmental conditions. Additionally, these corals appeared healthier, with vibrant colors and fuller flesh, thanks to higher levels of chlorophyll in their symbiotic partners. During the experiment, corals exposed to ash experienced accelerated skeletal growth, with microcolonies growing on average twice as fast as those in the control group by the end of the experiment. 

 

Upon examining the metal content, we discovered significantly higher concentrations of certain trace metals such as chromium, manganese, and iron in both the coral host and its symbiotic partners. These metals are essential for various metabolic and enzymatic processes within the coral. Iron and manganese, in particular, play critical roles in photosynthesis and are often limited in ocean environments. The leaching of metals from volcanic ash thus provides corals with important micronutrients, overcoming these limitations, and thereby enhancing certain biological functions and overall coral health. The chosen concentration of ash was found to have a fertilizing effect on the coral microcosm. 

 

Most people don’t think of volcanic eruptions as beneficial for coral reefs. However, our study—the first to look at this interaction in a lab—shows that moderate volcanic ash exposure could enhance coral health. While more research is needed to confirm these results in natural reef environments, our results indicate that the metal supply derived from volcanic ash leaching might bolster coral health and may help mitigate the impacts of external stressors such as climate change-induced disturbances. 

 

Our study sheds light on the complex interactions between geological events and marine ecosystems. Understanding the role of volcanic ash in shaping coral reef dynamics offers new avenues for coral conservation and management strategies in an era of environmental change.