[en] The eukaryotic tree
bears numerous taxa that have lost the photosynthetic capacities of their
ancestors. In the presence of an alternative source of energy the need for photosynthesis
declines and the electron transport machinery can be partially to completely dropped over
generations. Despite its large distribution across Eukaryotes, this shift to heterotrophism is
still poorly understood, particularly its onset . To explore this topic, we examined
Symbiodinium microadriaticum , which belongs to dinoflagellates, a phylum where the loss
of photosynthesis has occurred particularly frequently.
G
lucose requiring mutant strains were isolated from S microadriaticum cultivated in
conditions supplemented with glucose and amino acids. These strains were identified as
lack ing chloroplastic genes encoding proteins essential for photosynthesis . We focused on
the physiology of two specific strains, deficient either for PSBI or PSBE proteins which are
anticipated to play a role in the assembly or repair of photosystem II (PSII).
Our
findings reveal that the psbI strain exhibits limited growth i n minimal medium while its
growth was rescued in supplemented medium. The absence of PSBI result s in a decreased
activity of PSII , as indicated by reduced maximum photosynthetic yield and relative electron
transport rate through PSII , wh ereas the activity of PSI remains similar to that of the wild
type strain. Conversely, t he psbE strain displays a more pronounced phenotype, relying
entirely on glucose and amino acid supplementation for grow t h and carrying a completely
inactive PSII. However , sustained activity of PSI was also observed in this strain. The
im balance between PSII and PSI was corroborated in both strains by analysis of the
electrochromic shift. This PSII independent PSI activity suggests that these strains leverage
a circular electron flow around PSI leading to the establishment of a proton gradient across
the thylakoid membran e.
In conclusion,
the analysis of these mutant strains suggests that surviv al of Symbiodinium
microadriaticum is achievable with a weakened or inactive PSII in the presence of an
external energy source and with a fully functional PSI. Beside their evolutionary significance ,
these strains re present a valuable opportunity to conduct comprehensive stud ies on the
photosynthetic complexes within th is model organism.
