Sacoglossan sea slugs are able to maintain functional chloroplasts inside their own
cells, and mechanisms that allow preservation of the chloroplasts are unknown. We found that the
slug Elysia timida induces changes to the photosynthetic light reactions of the chloroplasts it steals
from the alga Acetabularia acetabulum. Working with a large continuous laboratory culture of both
the slugs (>500 individuals) and their prey algae, we show that the plastoquinone pool of slug
chloroplasts remains oxidized, which can suppress reactive oxygen species formation. Slug
chloroplasts also rapidly build up a strong proton-motive force upon a dark-to-light transition,
which helps them to rapidly switch on photoprotective non-photochemical quenching of excitation
energy. Finally, our results suggest that chloroplasts inside E. timida rely on oxygen-dependent
electron sinks during rapid changes in light intensity. These photoprotective mechanisms are
expected to contribute to the long-term functionality of the chloroplasts inside the slugs.