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

•The sea anemone Exaiptasia pallida is suitable, for phylum Cnidaria as a test species.•PAM fluorometry measures photosynthetic efficiency effects of herbicides on in-hospite zooxanthellae.•A mean 48-h EC50 (effective concentration, 50%) of 8 μg/L for diuron was estimated.•12-d exposures at 10-200 μg/L diuron have greatest effect on EQY within the first 48 h.•12nd exposure to < 1 μg/L diuron showed the lowest EQYs, and recovery to baseline.

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 μg/L of diuron was estimated, however atrazine elicited a much lower toxicity. Twelve-day exposures to 10-200 μg/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 μg/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 μg/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.