δ65Cu as biomarker for the photophysiological state of a symbiotic coral

Explosive volcanic eruptions are capable of producing large amounts of ash, that affect the surrounding ecosystems. Once the ash comes in contact with seawater, the metal salts coating the ash particles quickly dissolve, releasing essential trace metals into the environment. Moderate ash exposure increases the concentrations of several essential metals in the coral host tissue and their algal endosymbionts. As a result, the photosythetic activity of algal symbionts increases, leading to healthier corals and suggesting that ash has a fertilizing effect on symbiotic corals. This study aims to investigate how the duration of ash exposure and the ash concentration affect the photophysiological state of corals and whether metal concentrations and stable isotope ratios can provide insight into the underlying biological processes.

 

Microcolonies of the scleractinian coral Stylophora pistillata were grown under controlled laboratory conditions, including pH, temperature and irradiance. The corals were divided into various tanks under four different conditions: a control group without ash exposure and three ash-exposure treatments (3.75 g ash/week for three weeks, 7.5 g ash/week for three weeks and  7.5 g ash/week for six weeks). These conditions were chosen to evaluate the effects of different amounts of ash and exposure durations on coral responses. Throughout the experiment, various photophysiological parameters were monitored, including photosynthesis and respiration rates, as well as the photosynthetic efficiency (measured by e.g. relative electron transfer rate and F_v/F_m). At the end of the experiment, Cu, Fe, and Zn concentrations and isotopic compositions (δ⁵⁶Fe, δ⁶⁵Cu and δ⁶⁶Zn) were measured on the symbionts and tissues of three nubbins per tank.

         

Volcanic ash exposure enhanced the coral photosynthetic activity, although trace metal concentrations and isotope ratios didn’t change between the exposure conditions. The effect was also independent of the intensity or duration of exposure. However, δ⁶⁵Cu levels in the coral host correlated almost perfectly with the photosynthetic parameters; corals with lighter δ⁶⁵Cu demonstrated better photosynthetic performance. We propose that the δ⁶⁵Cu serves as an indicator of the photochemical efficiency and may be linked to the antioxidant capacity of the coral host to mitigate oxidative stress, with stress likely increasing with long-term exposure. Understanding physiologically-induced metallomic responses following ash exposure improves the understanding of ecosystem resilience and collapse.