Color constancy investigated via partial hue-matching.

Each hue is believed to be made up of the four component hues (yellow, blue, red, and green). A hue consisting of just one component hue is called unitary (or unique). A new technique--partial hue-matching--has been used to reveal the component and unitary hues for a sample of 32 Munsell papers, which were illuminated by neutral, yellow, blue, green, and red lights and assessed by four normal trichromatic observers. The same set of four component hues has been found under both the neutral and the chromatic illuminations for all of the observers. On average, more than 87% of the papers containing a particular component hue under the neutral illumination also have this component hue when lit by the chromatic lights. However, only a quarter of the papers perceived as unitary under the neutral illumination continues being perceived as unitary under all of the chromatic illuminations. In other words, most unitary colors shift along the hue circle due to change in an illuminant's chromaticity. Still, this shift of unitary colors is relatively small: On average, it does not exceed one Munsell hue step.

INFLUENCE OF DIFFERENT LIGHT INTENSITIES AND DIFFERENT IRON NUTRITION ON THE PHOTOSYNTHETIC APPARATUS IN THE DIATOM CYCLOTELLA MENEGHINIANA (BACILLARIOPHYCEAE)(1).

The diatom Cyclotella meneghiniana Kütz. (SAG 1020-a) was cultured under high-light (HL) and low-light (LL) conditions with either high (12 µM) or low (1 µM) iron in the media. Changes in cell morphology, especially cell volume and chloroplast size, were observed in cells grown under low iron. In contrast, HL had a much stronger influence on the photosynthetic apparatus. PSII function was unimpaired under lowered iron supply, but its quantum efficiency and reoxidation rate were reduced under HL conditions. As reported before, HL induced changes in antenna polypeptide composition. Especially the amount of Fcp6, an antenna protein related to LI818 and known to be involved in photoprotection, was increased under HL but was significantly reduced under lowered iron. The diatoxanthin content correlated with the amount of Fcp6 in isolated FCPa antenna complexes and was thus increased under HL and reduced under low iron as well. While the diatoxanthin (Dt) content of whole cells was enhanced under HL, no decrease was observed under lowered iron supply, ruling out the possibility that the decreased amounts in FCPa were due to a hampered diadinoxanthin de-epoxidase activity under these conditions. Thus, diatoxanthin not bound to FCPa has to be responsible for protection under the slight reduction in iron supply used here.

Pigment organization in fucoxanthin chlorophyll a/c(2) proteins (FCP) based on resonance Raman spectroscopy and sequence analysis.

Chlorophylls (Chls)-a and -c(2) are identified and characterized in fucoxanthin chlorophyll-a/c(2) protein (FCP) complexes in the trimeric (FCPa(trim)) and oligomeric (FCPb(olig)) forms of FCP from the diatom Cyclotella meneghiniana using resonance Raman (RR) spectroscopy. Importantly, two different Chl-c(2)s are identified in both FCPa(trim) and FCPb(olig) from their signature ring-breathing modes at approximately 1360 cm(-1). In addition, the C13(1)-keto carbonyl peaks indicate the presence of more than four Chl-a's in both FCP complexes and are broadly classified into three groups with strong, medium and weak external hydrogen bonds. Together, they provide the strongest spectroscopic evidence so far that there may be up to double the number of pigments previously estimated at 4Fx:4Chl-a:1Chl-c(2) per FCP monomer. Careful analysis of the protein sequences also strongly support the higher pigment content by showing that at least six Chl-a, and one Chl-b, binding sites found in LHCII are retained in the FCPs. The relative enhancement of the RR bands for 406.7 versus 413.1 nm further allows some distinction of blue- versus red-absorbing Chl-a's, respectively. Further differences between the Chls in FCPb(olig) and FCPa(trim) are present in the amino-acid sequences and the RR signals. Information about the Chl-binding sites, complemented by information about the structures and interactions of the Chls are used to characterize their local environments, and assign pigment locations (and functions) in FCPb(olig) and FCPa(trim), which along with the earlier characterization of the carotenoids (J. Phys. Chem. B. 112 (2009) 12565-12574) provide a first (global) framework for pigment organization in FCP.

Oligomerization and pigmentation dependent excitation energy transfer in fucoxanthin-chlorophyll proteins.

The ultrafast carotenoid to chlorophyll a energy transfer dynamics of the isolated fucoxanthin-chlorophyll proteins FCPa and FCPb from the diatom Cyclotella meneghiniana was investigated in a comprehensive study using transient absorption in the visible and near infrared spectral region as well as static fluorescence spectroscopy. The altered oligomerization state of both antenna systems results in a more efficient energy transfer for FCPa, which is also reflected in the different chlorophyll a fluorescence quantum yields. We therefore assume an increased quenching in the higher oligomers of FCPb. The influence of the carotenoid composition was investigated using FCPa and FCPb samples grown under different light conditions and excitation wavelengths at the blue (500nm) and red (550nm) wings of the carotenoid absorption. The different light conditions yield in altered amounts of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin. Since no significant dynamic changes are observed for high light and low light samples, the contribution of the xanthophyll cycle pigments to the energy transfer is most likely negligible. On the contrary, the observed dynamics change drastically for the different excitation wavelengths. The analyses of the decay associated spectra of FCPb suggest an altered energy transfer pathway. For FCPa even an additional time constant was found after excitation at 500nm. It is assigned to the intrinsic lifetime of either the xanthophyll cycle carotenoids or more probable the blue absorbing fucoxanthins. Based on our studies we propose a detailed model explaining the different excitation energy transfer pathways in FCPa.

Carotenoid structures and environments in trimeric and oligomeric fucoxanthin chlorophyll a/c2 proteins from resonance Raman spectroscopy.

Resonance Raman (RR) spectroscopy is used to characterize the structures and environments of the carotenoid fucoxanthin (Fx), the primary light harvester in the membrane-intrinsic fucoxanthin chlorophyll a/c2 proteins(FCP) from the diatom Cyclotella meneghiniana, thereby building on the findings from Stark spectroscopy and calculations (J. Phys. Chem. B 2008, 112 (37), 11838-11853). Solvent-dependent effects on the RR bands of isolated Fx and the xanthophyll-cycle carotenoid, diadinoxanthin (Ddx), are studied to better characterize the protein-specific environmental factors that affect their geometry and spectral signatures. In addition, excitation-wavelength-dependent (441.6-570 nm) changes in the RR bands of the nu1 and nu 3 modes,as well as the conjugated C8 carbonyl stretch, allow the identification of 5-6 in both the trimeric (FCPatrim)and oligomeric (FCPbolig) forms of FCP. These Fx's are broadly classified into two each of high (Fxblue) and low (Fxred) energy, and 1-2 of intermediate (Fxgreen) energy that are allied to their location and function in the protein. The CdC stretching frequencies (nu 1), which indicate conjugation over at least 7 double bonds, and the low intensity of the nu 4 C-H bending modes attest to their planar all-trans conformations both in the protein and in solution, with the protein-bound Fxred's exhibiting signs of nonlinearity. Additionally, rededge excitation of Fx in solution, and in the FCPs, exhibits the effect of mixing between the two lowest energy, 21Ag--like and 11Bu+-like, excited states, which underpins the high light-harvesting and energy transfer efficiency of the Fxred's. RR spectra also reveal differences between FCPatrim and FCPbolig complexes,such as the greater prevalence of Ddx in FCPbolig. Importantly, the identification of 5-6 Fx's per FCP monomer suggests that there may be more than the four Fx's previously assumed per FCP monomer, or else there is definite heterogeneity in Fx structures and/or binding sites.

Subunit composition and pigmentation of fucoxanthin-chlorophyll proteins in diatoms: evidence for a subunit involved in diadinoxanthin and diatoxanthin binding.

Two different fucoxanthin-chlorophyll protein complexes (FCP) were purified from the centric diatom Cyclotella meneghiniana and characterized with regard to their polypeptide and pigment composition. Whereas the oligomeric FCPb complex is most probably composed of fcp5 gene products, the trimeric FCPa has subunits encoded by fcp1-3 and fcp6/7. The amount of the latter polypeptide is enhanced when FCPa is isolated from algae grown under HL conditions. This increase in Fcp6/7 polypeptides is accompanied by an increase in the pool of xanthophyll cycle pigments, diadinoxanthin and diatoxanthin, and a concomitant decrease in fucoxanthin content. In addition, the de-epoxidation ratio, i.e., the amount of diatoxanthin in relation to the pool of xanthophyll cycle pigments, is increased by a factor of 2. With regard to fluorescence yield, HL FCPa was quenched in comparison to LL FCPa. This is in accordance with the larger amount of diatoxanthin that is bound, which is supposed to act as a quencher like zeaxanthin in higher plants. Thus, we conclude that the enhanced content of diatoxanthin in FCPa plays a protective role, which is paralleled by a weakened light harvesting function due to a smaller amount of fucoxanthin.