Toxicity thresholds of nine herbicides to coral symbionts (Symbiodiniaceae)

Over 30 herbicides have been detected in catchments and waters of the Great Barrier Reef (GBR) and their toxicity to key tropical species, including the coral endosymbiotic algae Symbiodiniaceae, is not generally considered in current water quality guideline values (WQGVs). Mutualistic symbionts of the family Symbiodiniaceae are essential for the survival of scleractinian corals. We tested the effects of nine GBR-relevant herbicides on photosynthetic efficiency (ΔF/F_m') and specific growth rate (SGR) over 14 days of cultured coral endosymbiont Cladocopium goreaui (formerly Symbiodinium clade C1). All seven Photosystem II (PSII) herbicides tested inhibited ΔF/F_m' and SGR, with toxicity thresholds for SGR ranging between 2.75 and 320 µg L^-1 (no effect concentration) and 2.54-257 µg L^-1 (EC₁₀). There was a strong correlation between EC₅₀s for ΔF/F_m' and SGR for all PSII herbicides indicating that inhibition of ΔF/F_m' can be considered a biologically relevant toxicity endpoint for PSII herbicides to this species. The non-PSII herbicides haloxyfop and imazapic did not affect ΔF/F_m' or SGR at the highest concentrations tested. The inclusion of this toxicity data for Symbiodiniaceae will contribute to improving WQGVs to adequately inform risk assessments and the management of herbicides in tropical marine ecosystems.