Okadaic acid inhibits cell growth and photosynthetic electron transport in the alga Dunaliella tertiolecta

Okadaic acid (OA), which is produced by several dinoflagellate species, is a phycotoxin known to induce a decrease of biomass production in phytoplankton. However, the mechanisms of OA cytotoxicity are still unknown in microalgae. In this study, we exposed the green microalga Dunaliella tertiolecta to OA concentrations of 0.05 to 0.5 μM in order to evaluate its effects on cell division, reactive oxygen species production and photosynthetic electron transport. After 72 h of treatment under continuous illumination, OA concentrations higher than 0.10 μM decreased culture cell density, induced oxidative stress and inhibited photosystem II electron transport capacity. OA effect in D. tertiolecta was strongly light dependent since no oxidative stress was observed when D. tertiolecta was exposed to OA in the dark. In the absence of light, the effect of OA on culture cell density and photosystem II activity was also significantly reduced. Therefore, light appears to have a significant role in the toxicity of OA in microalgae. Our results indicate that the site of OA interaction on photosynthetic electron transport is likely to be at the level of the plastoquinone pool, which can lead to photo-oxidative stress when light absorbed by the light-harvesting complex of photosystem II cannot be dissipated via photochemical pathways. These findings allowed for a better understanding of the mechanisms of OA toxicity in microalgae.

► Exposition of Dunaliella tertiolecta to okadaic acid in light conditions results in reactive oxygen species formation. ► Inhibition of photosystem II is dependent on oxidative stress and effects of okadaic acid on the plastoquinone pool. ► Oxidative stress and inhibition of photosynthesis increase okadaic acid effect on cell density in light conditions. ► Okadaic acid induces toxicity in algae via both light-dependent and light-independent mechanisms.