An atypical antenna complex is involved in state transitions in Euglena gracilis

The secondary green microalga Euglena gracilis is an important species for diverse biotechnological applications and evolutionary studies. Yet, its photosynthetic traits are not detailed as it is for other photosynthetic model organisms. We aimed to study the spectroscopic properties of the antenna system of E. gracilis and its participation in the state transitions (ST), a common regulatory mechanism of photosynthesis.
Cultures of E. gracilis grown photo-autotrophycally in low light (50 μmol photons m-2s-1) were exposed to high light intensity (HL; 450 μmol photons m-2s-1) for 48 hours. Transition to state 1 was induced by illuminating dark acclimated cells with far-red light (720-730 nm), and in vivo chlorophyll a fluorescence was used to monitor changes in the maximum fluorescence value (Fm). Room temperature (RT) fluorescence emission spectra and absorption spectra were recorded. Thylakoid membranes were also extracted from the two cultures to perform high resolution clear native electrophoresis.
No state transitions could be observed in HL cells following Fm changes. The HL treatment caused a decrease of the chlorophyll a to b ratio, a lower absorbance in the far-red region (peak at 695 nm), and loss of RT fluorescence emission band peaking at 705 nm. These spectroscopic changes were accompanied by the specific loss of a 250 kDa antenna complex enriched in chlorophyll a, while abundance of typical LHCII trimers was not altered. The spectroscopic properties of the isolated antenna complex corresponded to the difference observed between the control and HL treated cells.
We conclude that the atypical antenna component which absorbs in the far-red region and is responsible of energy diverting to its low energy chlorophylls, is needed to perform ST and may be pivotal for light energy regulation in Euglena gracilis under different light conditions.