Salt Stress Induces Paramylon Accumulation and Fine-Tuning of the Macro-Organization of Thylakoid Membranes in Euglena gracilis Cells

Abstract

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.

Mixotrophic Cultivation Optimization of Microalga Euglena pisciformis AEW501 for Paramylon Production

“…Studies have shown that although salt stress might inhibit the growth of E. gracilis , it could significantly promote the accumulation of paramylon [ 17 ]. In the present study, NH 4 Cl was the only nitrogen source for the accumulation of paramylon in E. pisciformis cells under the mixotrophic mode.…”

“…The mechanisms leading to the increase in E. pisciformis AEW501 paramylon content under mixotrophic cultivation were speculated. Studies have shown that although salt stress might inhibit the growth of E. gracilis , it could significantly promote the accumulation of paramylon [ 17 ]. In the present study, although E. pisciformis cells with the supplementation of NH 4 Cl could accumulate plenty of paramylon granules, cell growth was significantly inhibited compared to the control group.…”

“…Previous research reported that platinum electrode electrical treatment under 10 mA current intensity on E. gracilis cells had a significant effect on paramylon accumulation [ 16 ]. Additionally, salt stress could induce paramylon accumulation in E. gracilis cells [ 17 ]. E. gracilis could achieve higher paramylon production in a co-culture system of E. gracilis and bacterium [ 18 , 19 , 20 ].…”

Small-Angle Neutron Scattering for Studying Lipid Bilayer Membranes

“…One example of where SANS has been used to study intact cells is in investigations of the thylakoid membranes, which are part of the photosynthetic apparatus of plants [ 257 , 258 , 259 , 260 , 261 , 262 , 263 , 264 , 265 , 266 , 267 ]. Thylakoid membranes organize into stacks, and thus they give rise to diffraction features in SANS patterns that can be resolved from the remainder of the signal of the leaf of the plant or cell that is being measured.…”

“…Thylakoid membranes organize into stacks, and thus they give rise to diffraction features in SANS patterns that can be resolved from the remainder of the signal of the leaf of the plant or cell that is being measured. SANS studies have been performed on cyanobacteria [ 257 , 259 , 260 , 263 , 264 ], diatoms [ 258 ], algae [ 261 , 267 ], and higher plants [ 262 , 265 , 266 ]. The impact of environmental stressors, such as salt [ 267 ], selenium [ 265 ] and herbicide [ 264 ], on the structure of the membranes has also been studied.…”