Modulation of the multilamellar membrane organization and of the chiral macrodomains in the diatom Phaeodactylum tricornutum revealed by small-angle neutron scattering and circular dichroism spectroscopy

Abstract

Diatoms possess effective photoprotection mechanisms, which may involve reorganizations in the photosynthetic machinery. We have shown earlier, by using circular dichroism (CD) spectroscopy, that in Phaeodactylum tricornutum the pigment-protein complexes are arranged into chiral macrodomains, which have been proposed to be associated with the multilamellar organization of the thylakoid membranes and shown to be capable of undergoing light-induced reversible reorganizations (Szabó et al. Photosynth Res 95:237, 2008). Recently, by using small-angle neutron scattering (SANS) on the same algal cells we have determined the repeat distances and revealed reversible light-induced reorganizations in the lamellar order of thylakoids (Nagy et al. Biochem J 436:225, 2011). In this study, we show that in moderately heat-treated samples, the weakening of the lamellar order is accompanied by the diminishment of the psi-type CD signal associated with the long-range chiral order of the chromophores (psi, polymer or salt-induced). Further, we show that the light-induced reversible increase in the psi-type CD is associated with swelling in the membrane system, with magnitudes larger in high light than in low light. In contrast, shrinkage of the membrane system, induced by sorbitol, brings about a decrease in the psi-type CD signal; this shrinkage also diminishes the non-photochemical quenching capability of the cells. These data shed light on the origin of the psi-type CD signal, and confirm that both CD spectroscopy and SANS provide valuable information on the macro-organization of the thylakoid membranes and their dynamic properties; these parameters are evidently of interest with regard to the photoprotection in whole algal cells.

Chloroplast remodeling during state transitions in Chlamydomonas reinhardtii as revealed by noninvasive techniques in vivo

“…Therefore, the light-induced increase of the RD of thylakoid membranes observed in the WT is not linked to qE, the energy-dependent nonphotochemical quenching mechanism. Variations in the SANS profiles in the diatom Phaeodactilum tricornutum are sensitive to changes in the osmolality of the medium (12). We observed the same phenomenon in Chlamydomonas, where both peaks gradually and reversibly shifted to higher Q values also upon gradually increasing the sorbitol concentration from 0 to 300 mM (Fig.…”

“…Earlier experiments performed on cyanobacterial and diatom cells (11,12,18) have shown that light exposure induces darkreversible changes in the thylakoid-membrane ultrastructure, affecting both the Bragg peaks and the high-Q peak, without inducing ST. To verify the existence of this phenomenon in Chlamydomonas, we performed experiments on the stt7-9 mutant, which is unable to perform STs (20). In dark-adapted stt7-9 cells, a strong Bragg peak at 0.033 Å -1 (RD = 190 Å) was found (Fig.…”

“…It requires neither staining, nor chemical, nor low-temperature fixation of the sample and therefore can be performed under physiologically relevant conditions in living cells, which can be illuminated during the measurements. In a series of SANS experiments on isolated thylakoids, cyanobacteria, and algae, repeat distances (RDs) of membranes were determined (11)(12)(13), and small (<20 Å) but well-discernible light-induced reversible RD changes were revealed on time scales of several minutes (11,12). CD spectroscopy is another noninvasive technique, which provides information on the organization of the pigment systems in hierarchically organized molecular assemblies.…”

Organization and Flexibility of Cyanobacterial Thylakoid Membranes Examined by Neutron Scattering

“…We attributed these distances to the thylakoid membrane system due to the repeating nature of the concentric thylakoid membrane layers and considered it unlikely that the membrane components of the cell envelope are able to contribute to the observed peaks. Recent work by Nagy et al (8,12) has successfully applied SANS to photosynthetic systems. However, their pioneering study has left many open questions.…”

“…SANS can also be used to characterize periodically organized biological membrane systems on smaller length scales by observing diffraction from regularly spaced structural features separated by ϳ1-200 nm (11). Both the accessible length scales and the ability to examine individual components of biological systems using contrast matching have led to the application of SANS in studies of cyanobacteria, eukaryotic algae, plant thylakoids (8,12), and mammalian mitochondria (13).…”