Volcanic ash is a significant source of micronutrients including iron (Fe), copper (Cu), and zinc (Zn) in oligotrophic tropical waters. These bioactive metals enhance primary productivity, influencing local and global biogeochemical cycles. This study e10xplores how volcanic ash exposure affects trace metal uptake and photophysiological response, and how redox-sensitive metal stable isotope measurements in the tissues of the scleractinian coral Stylophora pistillata can provide crucial information on coral health. Controlled coral culture experiments were conducted in which coral nubbins were exposed to varying intensity and duration of volcanic ash. Throughout the experiment, coral symbionts showed enhanced photosynthetic performance irrespective of intensity or duration of ash exposure. Stable isotopes, such as δ⁶⁵Cu and δ56Fe, in the coral tissue are marked by systematic variations, not associated with intensity or duration of ash exposure. Instead, we suggest biologically modulated redox-sensitive fractionation associated with ash exposure, linked to the coral host's oxidative stress state. This is evidenced by significant correlations between δ⁶⁵Cu in coral hosts and photophysiology, with lighter Cu isotope ratios associated with higher photosynthetic performances. Hence, we propose that δ65Cu, and more generally redox-sensitive isotopic ratios (i.e. δ56Fe), in coral hosts serve as an indicator of the physiological state of symbiotic corals.