Photoinactivation in Kleptoplasts

The enigmatic association of photosynthetically active chloroplasts from algae and some
sacoglossan sea slugs, called functional kleptoplasty, is a functional unique system of
photosymbioses observed in metazoans. Besides the specific adaptations of the slugs
necessary to incorporate andmaintain the plastids, the organelles need to ensure optimal
photosynthesis. Photoprotective mechanisms in the plastids, namely the xanthophyll
cycle (XC) and the high-energy dependent quenching (qE) part of the non-photochemical
quenching (NPQ), and the repair of the D1 protein of Photosystem II (PSII) are considered
crucial for kleptoplast longevity in the slugs. Here, we studied the sea slugs Elysia viridis
fed with the naturally occurring XC and qE-deficient Bryopsis hypnoides, and E. timida fed
on Acetabularia acetabulum, an alga that possesses both mechanisms. The aim of the
study was to understand (i) whether qE remains active after ingestion of kleptoplasts by
E. timida, (ii) how different light intensities affect the photosynthetic activity of kleptoplasts
with and without photoprotection mechanisms, and (iii) if the kleptoplasts are able to
repair photodamaged D1 protein. With regard to NPQ, freshly incorporated kleptoplasts
responded to different light stress in the same manner as the chloroplasts in their native
host algae. Even after 3 weeks of incorporation the qE part of NPQ was present in
the kleptoplasts of E. timida. However, the presence of the qE component of NPQ
did not prevent the kleptoplasts from significant PSII photoinactivation under high light
intensities. This is probably due to the fact that the kleptoplasts have a reduced PSII
repair capacity, despite plastid encoded repair mechanisms in every sacoglossan food
source. Hence, photoprotective mechanisms are probably not a key factor explaining
kleptoplast longevity in Sacoglossa sea slugs.