Tephra-mediated manganese cycling shapes coral responses to coastal sedimentation

Terrestrial runoff from tropical volcanic islands impacts coral reefs by increasing turbidity and sedimentation. During explosive volcanic eruptions, large amounts of fragmented volcanogenic rock (tephra) are deposited, exacerbating sediment runoff for long periods of time. Nevertheless, tephra is an important, yet underestimated, source of the essential trace metal manganese (Mn), which promotes coral photosynthesis. Here, we show Mn leached from pristine and remobilised tephra increases resilience to sedimentation stress. Using coral culture experiments, microcolonies of Stylophora pistillata exposed to four tephra samples all showed rapid and sustained increases in photosynthetic efficiency (Φ PSII , rETR, P n and P gross ), even under reduced light conditions. Photosynthetic efficiency is logarithmically correlated to seawater Mn concentration, with large increases at values < 3 µg Mn L − 1 , and negligible changes at values > 10 µg Mn L − 1 . Tephra exposure has a crucial role in coastal Mn cycling and potentially benefits stressed corals following environmental disturbances.