Contradictive effects of light on diarrhetic shellfish toxin production-depending on photosynthetic status and photoprotection ability of Prorocentrum lima strains

Introduction

Diarrhetic shellfish toxins (DST), including okadaic acid (OA) and its analogues, are potent inhibitors of protein phosphatases and can be transported via food chain, potentially leading to human poisoning. Their biosynthesis in Prorocentrum and Dinophysis genera is light-dependent; however, it can be inhibited by high or ultraviolet radiation (UV) in our recent study.

Methods

Based on the hypothesis that photosynthetic status is relevant to DST production, two Prorocentrum lima strains (SD4, light-tolerant; DS4G4, light-sensitive) were exposed to different light conditions (12–400 μmol photons m⁻² s⁻¹, with/without UV-A) for 30 days.

Results

Initially, photosynthetic activity (F_v/fm) was inhibited by high light/UV-A, then recovered. Notably, the DST production rate (R_tox) decreased exponentially with increasing light on day 6, more significantly in strain DS4G4. During photoacclimation (day 6 – day 18), R_tox increased and attained a maximum (R_max), which in all cases was exponentially increased with light intensity. Under PAR conditions, the half-light saturation intensity (I_k) for R_max of OA did not surpass 25 μmol photons m⁻² s⁻¹ in both strains, highlighting the relatively low light requirements for DST production. Additionally, on day 6, chlorophyll a production rate (R_{Chl a}) demonstrated different correlations with R_tox under low and high light.

Conclusion

DST production is tightly associated with photosynthetic status and strain-specific photoprotection ability. The inhibition of DST by light is an inevitable phenomenon, and the underlying relationship between toxin production and photoinhibition process was further explored and discussed.