Remobilized volcanogenic sediments boost photophysiological activity in three symbiotic reef-building coral species.

Coral reefs that surround volcanic islands are continually influenced by run-off carrying remobilized volcanogenic sediments. Although unaltered volcanic ash is known to supply trace metals that enhance coral holobiont productivity, the impacts of long-term exposure to weathered tephra, a far more likely scenario in island settings, are less understood. To assess possible effects of tephra exposure on coral physiology, we conducted controlled culture experiments with three scleractinian corals (Stylophora pistillata, Turbinaria reniformis and Pocillopora damicornis) exposed to weekly additions of 50 g of untreated tephra, collected from the volcanic island of St. Vincent, and its leachate over a six-week period. All exposed colonies experienced optimized photosynthetic light-harvesting processes, while most other physiological parameters were not affected. Remobilized tephra supplies hormetic concentrations of water soluble trace metals, with high-resolution inductively coupled plasma mass spectrometer (HR-ICP-MS) analyses showing significant increases in seawater trace metal (V, Mn, Fe, Co, Ni, Cu, Cd and Pb) concentrations. This highlights tephra's previously underrecognized contribution to the island mass effect affecting corals.