Properties of photosystem I antenna protein complexes of the diatom Cyclotella meneghiniana

Abstract

Analysis of photosystem I (PSI) complexes from Cyclotella meneghiniana cultured under different growth conditions led to the identification of three groups of antenna proteins, having molecular weights of around 19, 18, and 17 kDa. The 19-kDa proteins have earlier been demonstrated to be more peripherally bound to PSI, and their amount in the PSI complexes was significantly reduced when the iron supply in the growth medium was lowered. This polypeptide was almost missing, and thus the total amount of fucoxanthin-chlorophyll proteins (Fcps) bound to PSI was reduced as well. When treating cells with high light in addition, no further changes in antenna polypeptide composition were detected. Xanthophyll cycle pigments were found to be bound to all Fcps of PSI. However, PSI of high light cultures had a significantly higher diatoxanthin to diadinoxanthin ratio, which is assumed to protect against a surplus of excitation energy. PSI complexes from the double-stressed cultures (high light plus reduced iron supply) were slightly more sensitive against destruction by the detergent treatment. This could be seen as a higher 674-nm emission at 77 K in comparison to the PSI complexes isolated from other growth conditions. Two major emission bands of the Fcps bound to PSI at 77 K could be identified, whereby chlorophyll a fluorescing at 697 nm was more strongly coupled to the PSI core than those fluorescing at 685 nm. Thus, the build up of the PSI antenna of several Fcp components enables variable reactions to several stress factors commonly experienced by the diatoms in vivo, in particular diatoxanthin enrichment under high light and reduction of antenna size under reduced iron conditions.

Fig. 1.

Protein composition of photosystem I complexes. (A) Results of separating complexes from the differently grown cultures using sucrose density centrifugation [low light plus 12 μmol iron (LL(+)), LL plus 1 μmol iron (LL(–)), and high light plus 1 μmol iron (HL(–))]: for attribution of bands see Veith et al. (2009). (B) Signal intensities from immunoblots of complexes from cells grown under LL(+) (black), LL(–) (grey), and HL(–) (white), developed using α-cmFCP: identical amounts of chlorophyll a (5 μg each) were loaded and preparations from all three cultures were run on two blots each. (C) One example of the original blots and the results using the α-lhcf1-11 antibody.

Fig. 2.

Pigment composition of the photosystem I complexes. Ratios of the pigments specific for fucoxanthin-chlorophyll protein polypeptides [chlorophyll (Chl) c, fucoxanthin (Fx), diadinoxanthin and diatoxanthin (Dd + Dt)] are shown for complexes isolated from cells grown under low light plus 12 μmol iron [LL(+), black], LL plus 1 μmol iron [LL(–), grey], and high light plus 1 μmol iron [HL(–), white]. Values are means ± standard deviations from two independent preparations each.

Fig. 3.

(A) Fluorescence emission spectra at 77 K of photosystem I complexes from low light plus 12 μmol iron [(LL(+), dotted line], LL plus 1 μmol iron [LL(–), solid line], and high light plus 1 μmol iron [HL(–), dashed line] cells excited at 440 nm and normalized to identical 717-nm emission. (B) Excitation spectra at 77 K of complexes from LL(–) cells, recorded at 674 nm (dashed line), 686 nm (dotted line), and 717 nm (solid line). (C) A fit of the emission spectra with five Gaussians is shown exemplarily for complexes from LL(–): the original spectrum is shown with circles, the fit is demonstrated as a solid line, and the single Gaussians as dotted lines. Residuals (lower panel) were plotted on the same axis but shifted by –0.1.

Fig. 4.

Areas under the single Gaussians, calculated using the fits shown in Fig. 3 and the parameters given in Table 1, depicted for photosystem I complexes from low light plus 12 μmol iron [LL(+), black], LL plus 1 μmol iron [LL(–), grey], and high light plus 1 μmol iron [HL(–), white] cultures. (A) Sum of the areas of the 674-, 685-, and 697-nm bands normalized to the area of the 717-nm band. (B) Relative contribution of the three emitters to the short wavelength emission. (C) Relative contribution of the 685- and 697-nm bands: sum of the areas set to one. Values are means ± standard deviations from two independent preparations each.