Changes in the colour and photosynthetic efficiency of the hermatypic coral Acropora tenuis exposed to Irgarol 1051 at 30 °C seawater temperature

Besides global warming, coral reefs face various anthropogenic threats, including watershed-based and marine-based contaminants. Recently, Irgarol 1051, a photosystem II herbicide, has been introduced as a “booster” agent in copper-based antifouling paints after global ban of tributyltin (TBT) in January 2008. Irgarol is considered to pose a potential risk to corals. In this study, a clone of Acropora tenuis separated from the colony collected from Sesoko Island, Okinawa, southern Japan, was exposed to 1.0 μ g L −1 Irgarol at 27.5 °C and 30.0 °C (higher than the temperature in the warmest month in southern Japan) for 7 d. The coral colour was converted to RGB values, ranging from 0 (darkest) to 255 (brightest). The RGB values under both temperature treatments slightly decreased in the control (no herbicide exposure), but those in both 27.5 °C and 30.0 °C Irgarol treatments increased after day 3. The maximum effective quantum yield ( Δ F / Fm ′) slightly increased in both 27.5 °C and 30.0 °C control treatments, but decreased in 27.5 °C and 30.0 °C Irgarol treatments after day 1. The exposure to 1.0 μ g L −1 Irgarol induced significant differences in RGB values and Δ F / Fm ′ compared with those in the control at the same temperature. Regarding the temperature difference (27.5 °C vs. 30.0 °C), no significant differences in RGB values and Δ F / Fm ′ were detected in both control and Irgarol treatments. Our results revealed no significant combined effect of Irgarol exposure and 30.0 °C on A. tenuis. These findings suggest that 30.0 °C is a normally inhabitable environmental temperature for A. tenuis. The bleaching of A. tenuis is more likely caused by the exposure to 1.0 μ g L −1 Irgarol than by the exposure to high seawater temperature of 30.0 °C.