Photoinhibition from chronic herbicide exposure reduces reproductive output of reef-building corals

The photosystem II (PSII) herbicide diuron is commonly used within catchments that drain into the Great Barrier Reef lagoon, where it potentially reduces photosynthesis inSymbiodinium, the dinoflagellate symbiont associated with reef corals. Here we examine the importance of energy (carbohydrates) derived from photosynthesis to the gametogenesis of corals following long-term, experimental exposures to diuron. Two broadcast spawning corals,Acropora tenuisandA. valida, and a brooding coral,Pocillopora damicornis, were exposed to 0 (controls), 1.0 (low) and 10 (moderate) μg l–1diuron treatments for 2 to 3 mo prior to spawning or planulation. Diuron caused photoinhibition in each species, with pulse amplitude modulation (PAM) fluorometery recording consistent declines in effective quantum yields of 20% at 1.0 μg l–1and 75% at 10 μg l–1diuron compared to control corals.A. validaandP. damicorniswere both sensitive to chronic diuron-induced photoinhibition, becoming severely bleached (loss ofSymbiodiniumspp. and/or reductions in light-harvesting pigments), especially at 10 μg l–1diuron. At this moderate concentration,A. validasustained both partial and full colony mortality.A. tenuiswas more resistant to these concentrations of diuron, and neither bleached nor sustained partial mortality in any of the treatments. We found 2.5-to 5-fold reductions in total lipid content (coral tissue, oocytes and planulae) for the 3 species in the presence of diuron, indicating significant use of storage lipid to meet nutritional demands under conditions of chronic photoinhibition. Polyp fecundity was reduced by 6-fold inA. validaand bothA. validaandP. damicorniswere unable to spawn or planulate following long-term exposures to 10 μg l–1diuron. This is the first study to investigate the sub-lethal effects of chronic herbicide-induced photoinhibition on symbiotic corals and provides evidence of a link between reduced energy acquisition due to PSII photoinhibition and reduced reproductive output in zooxanthellate corals.