Artificial membranes as model systems to investigate the temperature adaptation of diatoms
Final Report Abstract
This project was a cooperation with Prof. Dr. Kazimierz Strzalka, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, Krakow. In the beginning of the project the adaptation of diatom thylakoid membranes to low temperatures was analysed. It was found that Thalassiosira pseudonana thylakoids isolated from cultures grown at 12°C contained significantly higher amounts of MGDG in comparison to membranes purified from cultures grown at 20°C while the concentration of DGDG was decreased. This can be interpreted as a way to maintain the membrane in a fluid state at lower temperatures because the non-bilayer lipid MGDG induces a higher membrane fluidity compared to the bilayer lipid DGDG. In line with the results from the lipid analysis is the observation that the decrease of the growth temperature from 20 to 12°C caused a decrease of the saturated (16:0 and 24:0) and an increase of the unsaturated fatty acids, especially EPA (20:5). The presence of decosahexenoic acid (22:6) was only detected at lower temperatures. Increasing contents of long-chain, highly-unsaturated fatty acids within the thylakoid membrane lipids can also be seen as a way to maintain the membrane fluidity at lower growth temperatures. Laurdan fluorescence and EPR spectroscopy, using two different spin probes, was then used to determine the membrane fluidity of isolated thylakoid membranes from T. pseudonana grown at moderate or low temperatures. These measurements showed an increased membrane fluidity in the hydrophobic region of the membranes prepared from the cultures grown at the low temperature. This result is in good agreement with the higher content of unsaturated long-chain fatty acids and the increased MGDG to DGDG ratio in the thylakoids prepared from these cultures. With respect to the dynamics of diatom thylakoid membranes an important part of the project dealt with the impact of the diadinoxanthin (DD) diatoxanthin (Dt) cycle, one of the important photoprotection mechanisms of diatoms, on the fluidity of the membranes. Again with the use of Laurdan fluorescence and EPR spectroscopy we found that the conversion of DD to Dt leads to a significant decrease of the fluidity of the hydrophobic interior of the thylakoid membrane. This increased rigidity is stable and can be differentiated from a dynamic effect which affects both the hydrophobic part of the membrane, where the fatty acid chains are located, and the more hydrophilic outer membrane parts with the lipid head groups. Effects of Dt on these two membrane parts can be seen when the de-epoxidation reaction is at its fullest, i.e. when the strongest conversion of DD to Dt is taking place. Under conditions of a stable DD and Dt concentration this dynamic effect is not observed and only a stable decrease of the membrane fluidity of the membrane interior can be detected in the presence of high Dt concentrations. Another important topic of the project concerned the regulation of structural changes of the main light-harvesting complexes of diatom thylakoids, the FCPs. Here we found that Dt leads to strong aggregation of FCPs. The aggregation is connected with a strong quenching of the FCP fluorescence and a shift of the fluorescence emission maximum from 680 nm to 705 nm. The shift of the FCP fluorescence emission to a wavelength of around 705 to 710 nm is also observed in NPQ measurements of intact diatom cells and is characteristic for the establishment of NPQ quenching site Q1 in the recent NPQ models. The thylakoid lipids also play an important role in the aggregation/disaggregation of FCPs. Here we observed that the galactolipids MGDG and DGDG stabilize aggregates of FCPs which is in line with our results on LHCII of higher plants. However, in contrast to higher plant LHCII we did not observed FCP disaggregation in the presence of the negatively charged lipids SQDG or PG. We concluded that the diatom FCPs are insensitive to the negative effects of these anionic lipids which is most probably related to the significantly higher concentrations of SQDG and PG in the diatom thylakoid membranes compared to those of higher plants and green algae.
Publications
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(2017) Influence of thylakoid membrane lipids on the structure of aggregated light-harvesting complexes of the diatom Thalassiosira pseudonana and the green alga Mantoniella squamata. Physiologia plantarum 160 (3) 339–358
Schaller-Laudel, Susann; Latowski, Dariusz; Jemioła-Rzemińska, Małgorzata; Strzałka, Kazimierz; Daum, Sebastian; Bacia, Kirsten; Wilhelm, Christian; Goss, Reimund
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(2014). Influence of pH, Mg2+, and lipid composition on the aggregation state of the diatom FCP in comparison to the LHCII of vascular plants. Photosynth Res 119: 305–317
Schaller S, Richter K, Wilhelm C, Goss R
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(2015) The diadinoxanthin diatoxanthin cycle induces structural rearrangements of the isolated FCP antenna complexes of the pennate diatom Phaeodactylum tricornutum. Plant Physiol Biochem 96: 364-376
Schaller-Laudel S, Volke D, Redlich M, Kansy M, Hoffmann R, Wilhelm C, Goss R
