Manganese supplementation enhances cnidarian–dinoflagellate symbiosis under thermal stress

Manganese (Mn) is an essential trace element for all photosynthetic life, playing an integral role in their photosystems, metabolism, and antioxidant activity. For corals, most studies focus on the potential toxicity of Mn at high concentrations (e.g. >700 µg L-1). However, there has been less exploration on beneficial, biologically relevant levels of Mn. Combining promtomics, ICP, and PAM fluorometry, we evaluate how Mn supplementation at increasing concentrations (0.5, 4.8, 11.4, 15.6 µg L-1) alters the physiology and proteome of the model cnidarian, Exaiptasia diaphana, when subjected to ambient (26 ˚C) and elevated (32 ˚C) temperatures. We demonstrate that Mn from 4.8 to 15.6 µg L-1 mitigates thermal stress to E. diaphana, resulting in reduced photochemical damage and symbiont expulsion. Derived photobiology and proteomics data contributes to a mechanistic model for how Mn reduces thermal susceptibility, supporting the viability of Mn additions to enhance the protective capacities of photosynthetic cnidarians during heatwaves. Manganese supplementation at biologically relevant levels enhances cnidarian-dinoflagellate symbiosis in Exaiptasia daiphana. mitigating photochemical damage, symbiont loss and revealing mechanistic links in cnidarian thermal tolerance.