Photosynthetic sea slugs inflict protective changes to the light reactions of the chloroplasts they steal from algae

Abstract

7Sacoglossan sea slugs are able to maintain functional chloroplasts inside their own cells, and 8 mechanisms that allow preservation of the chloroplasts are unknown. We found that the slug Elysia 9 timida inflicts changes to the photosynthetic light reactions of the chloroplasts it steals from the alga 10 Acetabularia acetabulum. Working with a large continuous laboratory culture of both the slugs (>500 11 individuals) and their prey algae, we show that the plastoquinone pool of slug chloroplasts remains 12 oxidized, which can suppress reactive oxygen species formation. Slug chloroplasts also rapidly build up a 13 strong proton motive force upon a dark-to-light transition, which helps them to rapidly switch on 14 photoprotective non-photochemical quenching of excitation energy. Finally, our results suggest that 15 chloroplasts inside E. timida rely on flavodiiron proteins as electron sinks during rapid changes in light 16 intensity. These photoprotective mechanisms are expected to contribute to the long-term functionality 17 of the chloroplasts inside the slugs. 18 2014) and E. chlorotica (Rumpho et al., 2011), but still today most research is conducted on animals 55 caught from the wild. We have grown the sea slug E. timida and its prey Acetabularia in our lab for 56 several years ( Figure 1). As suggested by Schmitt et al. (2014), E. timida is an attractive model organism 57 for photosynthetic sea slugs because it is easy to culture with relatively low costs ( Figure 1E). A constant 58 supply of slugs has opened a plethora of experimental setups yet to be tested, one of the more exciting 59 ones being the case of red morphotypes of both E. timida and Acetabularia ( Figure 1C,D). Red 60 morphotypes of E. timida and Acetabularia were first described by González-Wangüemert et al. (2006) 61 and later shown to be due to accumulation of an unidentified carotenoid during cold/high-light 62 acclimation of the algae that were then eaten by E. timida (Costa et al., 2012). The red morphotypes 63 provide a visual proof that the characteristics of the kleptoplasts inside E. timida can be modified by 64 acclimating their feedstock to different environmental conditions. 65 4 We optimized a completely new set of biophysical methods to study photosynthesis in the sea slugs and 66 found differences in photosynthetic electron transfer reactions between E. timida and Acetabularia 67 grown in varying culture conditions. The most dramatic differences between the slugs and their prey 68 were noticed in PSII electron transfer of the red morphotype E. timida ( Figure 1C) and Acetabularia 69 ( Figure 1D). In addition to measuring chlorophyll a fluorescence decay kinetics, we also measured 70 fluorescence induction kinetics, PSI electron transfer and formation of proton motive force during dark 71 to light transition. Our results suggest that dark reduction of the plastoquinone (PQ) pool, a reserve of 72 central electron carriers of the photosynthetic electron transfer chain, is weak in the slugs compared to 73 the algae, and that a ...