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

Abstract

The photosystem II (PSII) herbicide diuron is commonly used within catchments that drain into the Great Barrier Reef lagoon, where it potentially reduces photosynthesis in Symbiodinium, 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 tenuis and A. valida, and a brooding coral, Pocillopora damicornis, were exposed to 0 (controls), 1.0 (low) and 10 (moderate) µg l–1 diuron 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–1 and 75% at 10 µg l–1 diuron compared to control corals. A. valida and P. damicornis were both sensitive to chronic diuron-induced photoinhibition, becoming severely bleached (loss of Symbiodinium spp. and/or reductions in light-harvesting pigments), especially at 10 µg l–1 diuron. At this moderate concentration, A. valida sustained both partial and full colony mortality. A. tenuis was 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 in A. valida and both A. valida and P. damicornis were unable to spawn or planulate following long-term exposures to 10 µg l–1 diuron. 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.