Functional heterogeneity of the fucoxanthins and fucoxanthin-chlorophyll proteins in diatom cells revealed by their electrochromic response and fluorescence and linear dichroism spectra

Abstract

In this work, by analyzing the electrochromic transient spectra, the 77 K fluorescence emission and excitation, as well as the linear dichroism (LD) and circular dichroism (CD) spectra of low-light (LL) and high-light (HL) grown Phaeodactylum tricornutum cells, we show that the fucoxanthins (Fx) and fucoxanthin-chlorophyll proteins (FCP) exhibit marked functional heterogeneity. Electrochromic transients reveal that LL and HL cells differ substantially in their relative contents of two Fx forms, which absorb at 501 and 550 nm; they exhibit distinct LD signals but are CD silent. Fluorescence emission and excitation spectra at 77K reveal that although both forms efficiently transfer excitation energy to Chl a, the red form feeds somewhat more energy to photosystem II than to photosystem I. Similar data obtained in Cyclotella meneghiniana cells suggest that the heterogeneity of the FCP pool, with different Fx, forms plays a role in the regulation of energy utilization in FCP-containing organisms.

Macro-organization and structural flexibility of the photosynthetic pigment system in diatoms

“…The same arrangement was found in brown algae (Pascal et al 1998). The localization of the other Fx molecules is not determined yet; it is speculated that they are situated at the periphery of the complex, close to helix 2 (Premvardhan et al 2010). The heterogeneity of the Fx molecules within FCP complexes is also corroborated by Stark spectroscopy measurements of isolated FCPs .…”

“…Two of the eight Fx molecules are supposed to be similarly arranged as the central luteins in LHCII, in the close vicinity of the Chl a molecules, and probably play a role in stabilizing the FCP complex (Premvardhan et al 2010). The same arrangement was found in brown algae (Pascal et al 1998).…”

“…Resonance Raman spectroscopy investigations revealed the presence of 5-6 Fx molecules in one FCP monomer (Premvardhan et al 2010). …”

Structure and Functional Heterogeneity of Fucoxanthin-Chlorophyll Proteins in Diatoms

“…The structural association of FCPs with PSII remains unresolved. Only some data obtained in C. meneghiniana , points to FCPb complexes being more closely associated with PSI, and FCPa serving PSII (Szabó et al 2010 ;Veith and Büchel 2007 ). This severe lack of precise knowledge concerning protein associations also extends to the overall arrangement of the complexes in the thylakoid membranes.…”

“…2.3b ) in FCPa as well as in FCPb (Premvardhan et al 2008(Premvardhan et al , 2009. The existence of differently absorbing Fx was also demonstrated in whole cells using electrochromic shift measurements (Szabó et al 2010 ). In carotenoids in general, due to their symmetry, absorption from the ground state to the lowest-energy singlet S 1 state (2 1 A g ) is symmetry-forbidden.…”