Ultraviolet screening by slug tissue and tight packing of plastids protect photosynthetic sea slugs from photoinhibition
O_LIOne of the main unsolved questions regarding photosynthetic sea slugs is how the slug plastids handle photoinhibition of Photosystem II. Photoinhibition has not been studied in detail in these animals although resilience against photoinhibition might obviously explain the longevity of plastids inside animal cytosol. C_LIO_LILight response and action spectrum of photoinhibition were measured from the slug Elysia timida and its prey alga Acetabularia acetabulum. Plastid packing in the slugs and algae was compared with spectroscopic and microscopic methods. The importance of plastid concentration was also estimated by measuring photoinhibition from starved slugs. C_LIO_LICompared to A. acetabulum, E. timida is highly resistant against photoinhibition. The resilience of the slugs is even more pronounced in the UV-region, as the slug tissue screens UV radiation. The plastids in the slug tissue are tightly packed, and the outer plastids protect the inner ones from photoinhibition. C_LIO_LIThe sea slug E. timida protects its plastids from photoinhibition by screening UV radiation and packing the plastids tightly in its tissues. Both mechanisms enhance the longevity of the plastids in slug cytosol and ameliorate the need for repair of photoinhibited Photosystem II. C_LI