Symbiont-specific responses in foraminifera to the herbicide diuron

The effects of the photosystem II (PSII) herbicide diuron was assessed on thirteen tropical foraminifera hosting diatom, dinoflagellate, red or green algae endosymbionts. Inhibition of photosynthesis (reduced Δ F / F m ′ ) by diuron depended on both symbiont type and test ultrastructure, with greatest sensitivity observed for diatom- and chlorophyte-hosting species (24 h IC25 2.5–4 μg L−1). Inhibition kinetics was slow (24–48 h until maximum inhibition) in comparison with corals, suggesting structural differences may influence herbicide uptake and transport. Although foraminifera were generally less sensitive to direct effects of diuron (inhibition Δ F / F m ′ ) than other marine phototrophs, damage to PSII (reduction Fv/Fm) occurred at concentrations lower than observed for other organisms (24 h IC25 3–12 μg L−1). Damage to PSII was highly light dependent and occurred at very low light intensities indicating limited photoprotective capacity. The high diversity, widespread occurrence and relative sensitivity make foraminifera good bioindicator organisms to evaluate phytotoxic stress on coral reefs.