Timing and method of manganese supplementation effects thermal resilience of Acropora millepora

As coral reefs continue to decline globally, a toolbox of new interventions such as nutritional supplementation are being considered. Low level supplementation of the trace element manganese (Mn) has increasingly been shown to enhance the resilience of photosynthetic Cnidarians to thermal stress, however, questions around the timing of Mn delivery relative to a thermal stress event remain unanswered. Here we explore how the timing of Mn additions influences the response of the common reef building coral Acropora millipora to thermal stress. Coral health was assessed using a variety of different metrics including photochemical efficiency, oxygen dynamics, elementomics and symbiont densities. A. millepora displayed significantly improved thermal tolerance when exposed to either intermittent pulse (19.34 µg L^-1 every second day) dosing or constant (4.24 µg L^-1) supply of dissolved Mn from 7 days before heat stress exposure (32 °C). Pulse dosed corals maintained higher photosynthetic efficiency and photosynthesis to respiration rates, as well as increased Symbiodiniaceae densities compared to thermally stressed corals without Mn supplementation. Elemental analysis revealed that both coral tissue and Symbiodiniaceae in Mn-treated corals accumulated significantly more Mn than non-Mn treated corals. Collectively, our results suggest that intermittent (pulse) Mn exposure prior to the onset of thermal stress may provide the greatest support for thermal resilience in corals. This study advances understanding on how essential elements like Mn could be strategically delivered as an active intervention to enhance coral tolerance during periods of thermal stress.