MEPS 284:133-145 (2004)  -  doi:10.3354/meps284133

ABSTRACT: Explants of the hard coral Seriatopora hystrix were exposed to sublethal concentrations of the herbicide diuron DCMU (N’-(3,4-dichlorophenyl,-N,N-dimethylurea)) and the heavy metal copper. Pulse amplitude modulated (PAM) chlorophyll fluorescence techniques were used to assess the effects on the photosynthetic efficiency of the algal symbionts in the tissue (in symbio), and chlorophyll fluorescence and counts of symbiotic algae (normalised to surface area) were used to assess the extent of coral bleaching. At 30 µg DCMU l^-1, there was a reduction in both the maximum effective quantum yield (ΔF/F_m’) and maximum potential quantum yield (F_v/F_m) of the algal symbionts in symbio. Corals subsequently lost their algal symbionts and discoloured (bleached), especially on their upper sunlight-exposed surfaces. At the same DCMU concentration but under low light (5% of growth irradiance), there was a marked reduction in ΔF/F_m’ but only a slight reduction in F_v/F_m and slight loss of algae. Loss of algal symbionts was also noted after a 7 d exposure to concentrations as low as 10 µg DCMU l^-1 under normal growth irradiance, and after 14 d exposure to 10 µg DCMU l^-1 under reduced irradiance. Collectively the results indicate that DCMU-induced bleaching is caused by a light-dependent photoinactivation of algal symbionts, and that bleaching occurs when F_v/F_m (measured 2 h after sunset) is reduced to a value of less than ca. 0.6. Elevated copper concentrations (60 µg Cu l^-1 for 10 h) also induced a rapid bleaching in S. hystrix but without affecting the quantum yield of the algae in symbio. Tests with isolated algae indicated that substantially higher concentrations (300 µg Cu l^-1 for 8 h) were needed to significantly reduce the quantum yield. Thus, copper-induced bleaching occurs without affecting the algal photosynthesis and may be related to effects on the host (animal). It is argued that warm-water bleaching of corals resembles both types of chemically induced bleaching, suggesting the need for an integrated model of coral bleaching involving the effect of temperature on both host (coral) and algal symbionts.

KEY WORDS: Coral · Coral bleaching · Symbiotic dinoflagellate · Diuron · Copper · Antifouling paint · Herbicide