The effects of salt stress condition on the growth, morphology, photosynthetic performance, and paramylon content were examined in the mixotrophic, unicellular, flagellate Euglena gracilis. We found that salt stress negatively influenced cell growth, accompanied by a decrease in chlorophyll (Chl) content. Circular dichroism (CD) spectroscopy revealed the changes in the macro-organization of pigment-protein complexes due to salt treatment, while the small-angle neutron scattering (SANS) investigations suggested a reduction in the thylakoid stacking, an effect confirmed by the transmission electron microscopy (TEM). At the same time, the analysis of the thylakoid membrane complexes using native-polyacrylamide gel electrophoresis (PAGE) revealed no significant change in the composition of supercomplexes of the photosynthetic apparatus. Salt stress did not substantially affect the photosynthetic activity, as reflected by the fact that Chl fluorescence yield, electron transport rate (ETR), and energy transfer between the photosystems did not change considerably in the salt-grown cells. We have observed notable increases in the carotenoid-to-Chl ratio and the accumulation of paramylon in the salt-treated cells. We propose that the accumulation of storage polysaccharides and changes in the pigment composition and thylakoid membrane organization help the adaptation of E. gracilis cells to salt stress and contribute to the maintenance of cellular processes under stress conditions.

Department of Plant Anatomy ELTE Eötvös Loránd University Budapest Hungary

Division for Biotechnology Bay Zoltán Nonprofit Ltd for Applied Research Szeged Hungary

Doctoral School of Biology University of Szeged Szeged Hungary

European Spallation Source ESS ERIC Lund Sweden

Faculty of Science University of Ostrava Ostrava Czechia

Institute for Solid State Physics and Optics Wigner Research Centre for Physics Eötvös Loránd Research Network Budapest Hungary

Institute of Plant Biology Biological Research Centre Eötvös Loránd Research Network Szeged Hungary

Laboratory for Neutron Scattering and Imaging Paul Scherrer Institute Villigen PSI Villigen Switzerland

Neutron Scattering Division Oak Ridge National Laboratory Oak Ridge TN United States

Szilak Laboratory Ltd Szeged Hungary

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