Interactive effects of high seawater temperature and the PS II herbicide Irgarol 1051 on photosynthetic efficiency of one species of the Acropora tenuis [sensu lato] complex (Scleractinia: Acroporidae)

Among herbicides, Irgarol 1051 causes the strongest inhibition of maximum effective quantum yield (∆F/Fm′) in symbiotic dinoflagellates of corals. In this study, interactive effects of heat stress and Irgarol (1.0 μg/L) were investigated using aquaculture-reared Acropora sp., a species in the Acropora tenuis [sensu lato] complex in southern Japan. Two 7-d experiments were conducted with 27.5 °C as the temperature control and 30.0 or 32.0 °C as heat stress treatments. Coral colour, evaluated by red–green–blue values, and ∆F/Fm′ of symbiotic dinoflagellates in coral were monitored daily. In the 27.5 and 30.0 °C treatments, coral colour remained stable throughout the experiment; whereas, corals began to whiten at 32.0 °C in both control and Irgarol treatments. ∆F/Fm′ decreased from day 1 in both Irgarol treatments under control (27.5 °C) and heat stress conditions (30.0 and 32.0 °C). In the control treatment, heat stress of 32.0 °C induced a decrease in ∆F/Fm′, but less than the decrease in ∆F/Fm′ during Irgarol exposure. ∆F/Fm′ did not decrease in the temperature control and 30.0 °C heat stress treatments. A significant interactive decrease in ∆F/Fm′ was observed on day 1 at both 30.0 and 32.0 °C. Thus, at 30.0 and 32.0 °C, heat stress was considered to accelerate the ∆F/Fm′ reduction at the early stage of Irgarol exposure in aquaculture-reared Acropora sp. Compared to our previous study, the present study suggests that the response of corals in the A. tenuis [sensu lato] complex to Irgarol exposure differs depending on the genetic differences, such as species level.