Adult corals are uniquely more sensitive to manganese than coral early-life stages.

Manganese is an essential element and generally considered to be one of the least toxic metals to aquatic organisms, with chronic effects rarely seen at concentrations below 1000 µg/L. Anthropogenic activities lead to elevated concentrations of manganese in tropical marine waters. Limited data suggests that manganese is more acutely toxic to adult than to early life-stages of scleractinian corals in static renewal tests. However, to enable the inclusion of sufficient sensitive coral data in species sensitivity distributions to derive water quality guideline values (GVs) for manganese, we determined the acute toxicity of manganese to the adult scleractinian coral, Acropora muricata, in flow-through exposures. The 48-h EC50 was 824 µg Mn/L (based on time-weighted average, measured, dissolved manganese). The endpoint was tissue sloughing, a lethal process by which coral tissue detaches from the coral skeleton. Tissue sloughing was unrelated to superoxidase dismutase activity in coral tissue, and occurred in the absence of bleaching, i.e., toxic effects were observed for the coral host, but not for algal symbionts. We confirm that adult scleractinian corals are uniquely sensitive to manganese in acute exposures at concentrations 10 to 340 times lower than those reported to cause acute or chronic toxicity to coral early life stages, challenging the traditional notion that early life stages are more sensitive than mature organisms. This article is protected by copyright. All rights reserved. Environ Toxicol Chem 2023;00:0-0. © 2023 SETAC.