Toxicity estimates for diuron and atrazine for the tropical marine cnidarian Exaiptasia pallida and in-hospite Symbiodinium spp. using PAM chlorophyll-a fluorometry

Effective ecotoxicological risk assessments for herbicides in tropical marine environments are restricted by a lack of toxicity data, sensitive test methods and endpoints for relevant species, and this requires rectification. The symbiotic sea anemone Exaiptasia pallida is a suitable test species, representing the phylum Cnidaria and allowing for assessments of toxicological responses of both the animal host and in-hospite Symbiodinium spp. Pulse amplitude modulated (PAM) chlorophyll-a fluorometry is recognised as a valuable ecotoxicological tool, and here newly-developed test methods are presented using PAM fluorometry to measure herbicide effects on photosynthetic efficiency of in-hospite Symbiodinium spp. Additionally, measurements on healthy laboratory reared E. pallida provide baseline data demonstrating the normal effective quantum yield (EQY) and the maximum electron transport rate (ETRm) for Symbiodinium spp. in the absence of herbicide stress. Concentration dependant reductions in the EQY and ETRm occurred during diuron and atrazine exposures; a mean 48-h EC50 (effective concentration; 50%) of 8 pg/L of diuron was estimated, however atrazine elicited a much lower toxicity. Twelve-day exposures to 10-200 pg/L diuron showed that the greatest EQY effect occurred during the first 48 h, with little subsequent change. However, longer exposures to the lowest diuron treatment (1 pg/L) showed the lowest EQYs after 96 h followed by recovery to control levels within 12 d. Furthermore, asexual reproduction was inhibited during 12-d exposures to diuron, and 12-d EC50 values of 100 and 132 pg/L were estimated to inhibit successful reproduction of pedal lacerates and juveniles by 50% respectively. This study provides much needed data contributions to species sensitivity curves for development of diuron and atrazine water quality guidelines in tropical marine environments.