Novel structural aspect of the diatom thylakoid membrane: lateral segregation of photosystem I under red-enhanced illumination

Abstract

Spatial segregation of photosystems in the thylakoid membrane (lateral heterogeneity) observed in plants and in the green algae is usually considered to be absent in photoautotrophs possessing secondary plastids, such as diatoms. Contrary to this assumption, here we show that thylakoid membranes in the chloroplast of a marine diatom, Phaeodactylum tricornutum, contain large areas occupied exclusively by a supercomplex of photosystem I (PSI) and its associated Lhcr antenna. These membrane areas, hundreds of nanometers in size, comprise hundreds of tightly packed PSI-antenna complexes while lacking other components of the photosynthetic electron transport chain. Analyses of the spatial distribution of the PSI-Lhcr complexes have indicated elliptical particles, each 14 × 17 nm in diameter. On larger scales, the red-enhanced illumination exerts a significant effect on the ultrastructure of chloroplasts, creating superstacks of tens of thylakoid membranes.

Figure 1. An example of an intact membrane patch isolated from thylakoid membranes of P. tricornutum grown under red-enhanced illumination. Scale bar corresponds to 200 nm.

Figure 2. Protein composition analysis of the membrane patches isolated from red-light grown P. tricornutum.

Gels are representative of three replicates.

Figure 3. Absorbance spectra of PSI-only membrane patches from P. tricornutum.

(A) – Absorption spectrum, Q_y maximum at 680 nm; (B) – Flash-induced absorbance difference spectrum, in presence of 20 μM PMS and 20 mM ascorbate. Maximum of bleaching was at 704 nm.

Figure 4

(a) Black – experimental pair-correlation function (pcf) computed from the positions of ~1500 particles occupying the total membrane area of 0.3 μm² of 11 separate membrane patches; grey – the pcf computed from simulation of distribution of 1500 elliptical particles of axes 14 and 17 nm representing the PSI-Lhcr complex. A section of a simulated PSI-Lhcr patch is shown in panel b.

Figure 5. Comparison of ultrastructural changes induced by the prolonged cultivation under red-enhanced illumination in the cells of P. tricornutum.

(a,b) – Two representative examples of daylight culture cells; (c,d) – Examples of the red-light cells; The DL cells show the typical stacked thylakoid membranes grouped by three (box 1). Compare the greatly increased number of homogeneously stacked thylakoids (“superstacks”) in the chloroplast of the RL cells in D, (box 2). To illustrate the size of the PSI-Lhcr membrane patches, such as the one in Fig. 1, a 400 nm scale bar is placed in the panel c.